title: The Transport Layer Security (TLS) Protocol Version 1.3 abbrev: TLS docname: draft-ietf-tls-tls13-latest category: std updates: 4492, 5705, 6066, 6961 obsoletes: 5077, 5246, 5746
ipr: pre5378Trust200902 area: General workgroup: keyword: Internet-Draft
stand_alone: yes pi: rfcedstyle: yes toc: yes tocindent: yes sortrefs: yes symrefs: yes strict: yes comments: yes inline: yes text-list-symbols: -o*+ docmapping: yes author:
ins: E. Rescorla
name: Eric Rescorla
organization: RTFM, Inc.
email: ekr@rtfm.com
normative: RFC2104: RFC2119: RFC2434: RFC3447: RFC5280: RFC5869: RFC6066: RFC6655: RFC7539: RFC7748: RFC7919: I-D.irtf-cfrg-eddsa:
AES: title: Specification for the Advanced Encryption Standard (AES) date: 2001-11-26 author: org: National Institute of Standards and Technology seriesinfo: NIST: FIPS 197 SHS: title: Secure Hash Standard date: 2012-03 author: org: National Institute of Standards and Technology, U.S. Department of Commerce seriesinfo: NIST: FIPS PUB 180-4 X690: title: "Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)" date: 2002 author: org: ITU-T seriesinfo: ISO/IEC: 8825-1:2002 X962: title: "Public Key Cryptography For The Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)" date: 1998 author: org: ANSI seriesinfo: ANSI: X9.62 DH: title: "New Directions in Cryptography" author: - ins: W. Diffie - ins: M. Hellman date: 1977-06 seriesinfo: IEEE Transactions on Information Theory, V.IT-22 n.6
informative: RFC4086: RFC4279: RFC4346: RFC4366: RFC4492: RFC4506: RFC4507: RFC4681: RFC5054: RFC5077: RFC5081: RFC5116: RFC5246: RFC5746: RFC5764: RFC5878: RFC5929: RFC6176: RFC6091: RFC6520: RFC7301: RFC7230: RFC7250: RFC7366: RFC7465: RFC7568: RFC7627: RFC7685:
DSS: title: "Digital Signature Standard, version 4" date: 2013 author: org: National Institute of Standards and Technology, U.S. Department of Commerce seriesinfo: NIST: FIPS PUB 186-4 ECDSA: title: "Public Key Cryptography for the Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)" author: org: American National Standards Institute date: 2005-11 seriesinfo: ANSI: ANS X9.62-2005 FI06: title: "Bleichenbacher's RSA signature forgery based on implementation error" author: - name: Hal Finney date: 2006-08-27 target: https://www.ietf.org/mail-archive/web/openpgp/current/msg00999.html
GCM: title: "Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC" date: 2007-11 author: ins: M. Dworkin seriesinfo: NIST: Special Publication 800-38D PKCS6: title: "PKCS #6: RSA Extended Certificate Syntax Standard, version 1.5" author: org: RSA Laboratories date: 1993-11 PKCS7: title: "PKCS #7: RSA Cryptographic Message Syntax Standard, version 1.5" author: org: RSA Laboratories date: 1993-11 RSA: title: "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems" author: - ins: R. Rivest - ins: A. Shamir - ins: L. M. Adleman date: 1978-02 seriesinfo: Communications of the ACM: v. 21, n. 2, pp. 120-126. SSL2: title: "The SSL Protocol" author: name: Kipp Hickman org: Netscape Communications Corp. date: 1995-02-09 SSL3: title: The SSL 3.0 Protocol author: - ins: A. Freier org: Netscape Communications Corp. - ins: P. Karlton org: Netscape Communications Corp. - ins: P. Kocher org: Netscape Communications Corp. date: 1996-11-18 TIMING: title: "Remote timing attacks are practical" author: - ins: D. Boneh - ins: D. Brumley seriesinfo: USENIX: Security Symposium date: 2003 X501: title: "Information Technology - Open Systems Interconnection - The Directory: Models" date: 1993 seriesinfo: ITU-T: X.501 IEEE1363: title: "Standard Specifications for Public Key Cryptography" date: 2000 author: org: IEEE seriesinfo: IEEE 1363 PSK-FINISHED: title: "Revision 10: possible attack if client authentication is allowed during PSK" date: 2015 target: https://www.ietf.org/mail-archive/web/tls/current/msg18215.html author: - ins: C. Cremers - ins: M. Horvat - ins: T. van der Merwe - ins: S. Scott SLOTH: title: "Transcript Collision Attacks: Breaking Authentication in TLS, IKE, and SSH" author: - ins: K. Bhargavan - ins: G. Leurent seriesinfo: Network and Distributed System Security Symposium (NDSS 2016) date: 2016 AEAD-LIMITS: title: "Limits on Authenticated Encryption Use in TLS" author: - ins: A. Luykx - ins: K. Paterson target: http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf date: 2016 CK01: title: "Analysis of Key-Exchange Protocols and Their Use for Building Secure Channels" author: - ins: R. Canetti - ins: H. Krawczyk seriesinfo: Proceedings of Eurocrypt 2001 date: 2001 BBFKZG16: title: "Downgrade Resilience in Key-Exchange Protocols" author: - ins: K. Bhargavan - ins: C. Brzuska - ins: C. Fournet - ins: M. Kohlweiss - ins: S. Zanella-Beguelin - ins: M. Green seriesinfo: Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2016 date: 2016 DOW92: title: “Authentication and authenticated key exchanges” author: - ins: W. Diffie - ins: P. van Oorschot - ins: M. Wiener seriesinfo: Designs, Codes and Cryptography data: 1992 SIGMA: title: "SIGMA: the 'SIGn-and-MAc' approach to authenticated Diffie-Hellman and its use in the IKE protocols" author: - ins: H. Krawczyk seriesinfo: Proceedings of CRYPTO 2003 date: 2003 CHSV16: title: "Automated Analysis and Verification of TLS 1.3: 0-RTT, Resumption and Delayed Authentication" author: - ins: C. Cremers - ins: M. Horvat - ins: S. Scott - ins: T. van der Merwe seriesinfo: Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2016 date: 2016 FGSW16: title: "Key Confirmation in Key Exchange: A Formal Treatment and Implications for TLS 1.3" author: - ins: M. Fischlin - ins: F. Guenther - ins: B. Schmidt - ins: B. Warinschi seriesinfo: Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2016 date: 2016 LXZFH16: title: "Multiple Handshakes Security of TLS 1.3 Candidates" author: - ins: X. Li - ins: J. Xu - ins: D. Feng - ins: Z. Zhang - ins: H. Hu seriesinfo: Proceedings of IEEE Symposium on Security and Privacy (Oakland) 2016 date: 2016 FW15: title: "Factoring RSA Keys With TLS Perfect Forward Secrecy" author: - ins: Florian Weimer org: Red Hat Product Security date: 2015-09 --- abstract
This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. --- middle
DISCLAIMER: This is a WIP draft of TLS 1.3 and has not yet seen significant security analysis.
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH The source for this draft is maintained in GitHub. Suggested changes should be submitted as pull requests at https://github.com/tlswg/tls13-spec. Instructions are on that page as well. Editorial changes can be managed in GitHub, but any substantive change should be discussed on the TLS mailing list.
The primary goal of TLS is to provide a secure channel between two communicating peers. Specifically, the channel should provide the following properties:
-
Authentication: The server side of the channel is always authenticated; the client side is optionally authenticated. Authentication can happen via asymmetric cryptography (e.g., RSA {{RSA}}, ECDSA {{ECDSA}}) or a pre-shared symmetric key.
-
Confidentiality: Data sent over the channel is not visible to attackers.
-
Integrity: Data sent over the channel cannot be modified by attackers.
These properties should be true even in the face of an attacker who has complete control of the network, as described in {{?RFC3552}}. See {{security-analysis}} for a more complete statement of the relevant security properties.
TLS consists of two primary components:
-
A handshake protocol ({{handshake-protocol}}) that authenticates the communicating parties, negotiates cryptographic modes and parameters, and establishes shared keying material. The handshake protocol is designed to resist tampering; an active attacker should not be able to force the peers to negotiate different parameters than they would if the connection were not under attack.
-
A record protocol ({{record-protocol}}) that uses the parameters established by the handshake protocol to protect traffic between the communicating peers. The record protocol divides traffic up into a series of records, each of which is independently protected using the traffic keys.
TLS is application protocol independent; higher-level protocols can layer on top of TLS transparently. The TLS standard, however, does not specify how protocols add security with TLS; how to initiate TLS handshaking and how to interpret the authentication certificates exchanged are left to the judgment of the designers and implementors of protocols that run on top of TLS.
This document defines TLS version 1.3. While TLS 1.3 is not directly compatible with previous versions, all versions of TLS incorporate a versioning mechanism which allows clients and servers to interoperably negotiate a common version if one is supported.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 {{RFC2119}}.
The following terms are used:
client: The endpoint initiating the TLS connection.
connection: A transport-layer connection between two endpoints.
endpoint: Either the client or server of the connection.
handshake: An initial negotiation between client and server that establishes the parameters of their transactions.
peer: An endpoint. When discussing a particular endpoint, "peer" refers to the endpoint that is remote to the primary subject of discussion.
receiver: An endpoint that is receiving records.
sender: An endpoint that is transmitting records.
session: An association between a client and a server resulting from a handshake.
server: The endpoint which did not initiate the TLS connection.
(*) indicates changes to the wire protocol which may require implementations to update.
draft-17
-
Remove the 0-RTT Finished, resumption_context, and replace with a psk_binder field in the PSK itself (*)
-
Restructure PSK key exchange negotiation modes (*)
-
Add max_early_data_size field to TicketEarlyDataInfo (*)
-
Add a 0-RTT exporter and change the transcript for the regular exporter (*).
-
Merge TicketExtensions and Extensions registry. Changes ticket_early_data_info code point (*).
-
Replace Client.key_shares in response to HRR (*)
-
Explicitly allow predicting ClientFinished for NST.
-
Clarify conditions for allowing 0-RTT with PSK.
draft-16
-
Revise version negotiation (*)
-
Change RSASSA-PSS and EdDSA SignatureScheme codepoints for better backwards compatibility (*)
-
Move HelloRetryRequest.selected_group to an extension (*)
-
Clarify the behavior of no exporter context and make it the same as an empty context.(*)
-
New KeyUpdate format that allows for requesting/not-requesting an answer. This also means changes to the key schedule to support independent updates (*)
-
New certificate_required alert (*)
-
Forbid CertificateRequest with 0-RTT and PSK.
-
Relax requirement to check SNI for 0-RTT.
draft-15
-
New negotiation syntax as discussed in Berlin (*)
-
Require CertificateRequest.context to be empty during handshake (*)
-
Forbid empty tickets (*)
-
Forbid application data messages in between post-handshake messages from the same flight (*)
-
Clean up alert guidance (*)
-
Clearer guidance on what is needed for TLS 1.2.
-
Guidance on 0-RTT time windows.
-
Rename a bunch of fields.
-
Remove old PRNG text.
-
Explicitly require checking that handshake records not span key changes.
draft-14
-
Allow cookies to be longer (*)
-
Remove the "context" from EarlyDataIndication as it was undefined and nobody used it (*)
-
Remove 0-RTT EncryptedExtensions and replace the ticket_age extension with an obfuscated version. Also necessitates a change to NewSessionTicket (*).
-
Move the downgrade sentinel to the end of ServerHello.Random to accommodate tlsdate (*).
-
Define ecdsa_sha1 (*).
-
Allow resumption even after fatal alerts. This matches current practice.
-
Remove non-closure warning alerts. Require treating unknown alerts as fatal.
-
Make the rules for accepting 0-RTT less restrictive.
-
Clarify 0-RTT backward-compatibility rules.
-
Clarify how 0-RTT and PSK identities interact.
-
Add a section describing the data limits for each cipher.
-
Major editorial restructuring.
-
Replace the Security Analysis section with a WIP draft.
draft-13
-
Allow server to send SupportedGroups.
-
Remove 0-RTT client authentication
-
Remove (EC)DHE 0-RTT.
-
Flesh out 0-RTT PSK mode and shrink EarlyDataIndication
-
Turn PSK-resumption response into an index to save room
-
Move CertificateStatus to an extension
-
Extra fields in NewSessionTicket.
-
Restructure key schedule and add a resumption_context value.
-
Require DH public keys and secrets to be zero-padded to the size of the group.
-
Remove the redundant length fields in KeyShareEntry.
-
Define a cookie field for HRR.
draft-12
-
Provide a list of the PSK cipher suites.
-
Remove the ability for the ServerHello to have no extensions (this aligns the syntax with the text).
-
Clarify that the server can send application data after its first flight (0.5 RTT data)
-
Revise signature algorithm negotiation to group hash, signature algorithm, and curve together. This is backwards compatible.
-
Make ticket lifetime mandatory and limit it to a week.
-
Make the purpose strings lower-case. This matches how people are implementing for interop.
-
Define exporters.
-
Editorial cleanup
draft-11
-
Port the CFRG curves & signatures work from RFC4492bis.
-
Remove sequence number and version from additional_data, which is now empty.
-
Reorder values in HkdfLabel.
-
Add support for version anti-downgrade mechanism.
-
Update IANA considerations section and relax some of the policies.
-
Unify authentication modes. Add post-handshake client authentication.
-
Remove early_handshake content type. Terminate 0-RTT data with an alert.
-
Reset sequence number upon key change (as proposed by Fournet et al.)
draft-10
-
Remove ClientCertificateTypes field from CertificateRequest and add extensions.
-
Merge client and server key shares into a single extension.
draft-09
-
Change to RSA-PSS signatures for handshake messages.
-
Remove support for DSA.
-
Update key schedule per suggestions by Hugo, Hoeteck, and Bjoern Tackmann.
-
Add support for per-record padding.
-
Switch to encrypted record ContentType.
-
Change HKDF labeling to include protocol version and value lengths.
-
Shift the final decision to abort a handshake due to incompatible certificates to the client rather than having servers abort early.
-
Deprecate SHA-1 with signatures.
-
Add MTI algorithms.
draft-08
-
Remove support for weak and lesser used named curves.
-
Remove support for MD5 and SHA-224 hashes with signatures.
-
Update lists of available AEAD cipher suites and error alerts.
-
Reduce maximum permitted record expansion for AEAD from 2048 to 256 octets.
-
Require digital signatures even when a previous configuration is used.
-
Merge EarlyDataIndication and KnownConfiguration.
-
Change code point for server_configuration to avoid collision with server_hello_done.
-
Relax certificate_list ordering requirement to match current practice.
draft-07
-
Integration of semi-ephemeral DH proposal.
-
Add initial 0-RTT support.
-
Remove resumption and replace with PSK + tickets.
-
Move ClientKeyShare into an extension.
-
Move to HKDF.
draft-06
-
Prohibit RC4 negotiation for backwards compatibility.
-
Freeze & deprecate record layer version field.
-
Update format of signatures with context.
-
Remove explicit IV.
draft-05
-
Prohibit SSL negotiation for backwards compatibility.
-
Fix which MS is used for exporters.
draft-04
-
Modify key computations to include session hash.
-
Remove ChangeCipherSpec.
-
Renumber the new handshake messages to be somewhat more consistent with existing convention and to remove a duplicate registration.
-
Remove renegotiation.
-
Remove point format negotiation.
draft-03
-
Remove GMT time.
-
Merge in support for ECC from RFC 4492 but without explicit curves.
-
Remove the unnecessary length field from the AD input to AEAD ciphers.
-
Rename {Client,Server}KeyExchange to {Client,Server}KeyShare.
-
Add an explicit HelloRetryRequest to reject the client's.
draft-02
-
Increment version number.
-
Rework handshake to provide 1-RTT mode.
-
Remove custom DHE groups.
-
Remove support for compression.
-
Remove support for static RSA and DH key exchange.
-
Remove support for non-AEAD ciphers.
This document defines several changes that optionally affect implementations of TLS 1.2:
-
A version downgrade protection mechanism is described in {{server-hello}}.
-
RSASSA-PSS signature schemes are defined in {{signature-algorithms}}.
An implementation of TLS 1.3 that also supports TLS 1.2 might need to include changes to support these changes even when TLS 1.3 is not in use. See the referenced sections for more details.
The cryptographic parameters of the session state are produced by the TLS handshake protocol, which a TLS client and server use when first communicating to agree on a protocol version, select cryptographic algorithms, optionally authenticate each other, and establish shared secret keying material. Once the handshake is complete, the peers use the established keys to protect application layer traffic.
A failure of the handshake or other protocol error triggers the termination of the connection, optionally preceded by an alert message ({{alert-protocol}}).
TLS supports three basic key exchange modes:
-
Diffie-Hellman (both the finite field and elliptic curve varieties),
-
A pre-shared symmetric key (PSK), and
-
A combination of a symmetric key and Diffie-Hellman.
{{tls-full}} below shows the basic full TLS handshake:
Client Server
Key ^ ClientHello
Exch | + key_share*
| + pre_shared_key_modes*
v + pre_shared_key* -------->
ServerHello ^ Key
+ key_share* | Exch
+ pre_shared_key* v
{EncryptedExtensions} ^ Server
{CertificateRequest*} v Params
{Certificate*} ^
{CertificateVerify*} | Auth
{Finished} v
<-------- [Application Data*]
^ {Certificate*}
Auth | {CertificateVerify*}
v {Finished} -------->
[Application Data] <-------> [Application Data]
+ Indicates extensions sent in the
previously noted message.
* Indicates optional or situation-dependent
messages that are not always sent.
{} Indicates messages protected using keys
derived from handshake_traffic_secret.
[] Indicates messages protected using keys
derived from traffic_secret_N
{: #tls-full title="Message flow for full TLS Handshake"}
The handshake can be thought of as having three phases (indicated in the diagram above):
-
Key Exchange: Establish shared keying material and select the cryptographic parameters. Everything after this phase is encrypted.
-
Server Parameters: Establish other handshake parameters (whether the client is authenticated, application layer protocol support, etc.).
-
Authentication: Authenticate the server (and optionally the client) and provide key confirmation and handshake integrity.
In the Key Exchange phase, the client sends the ClientHello ({{client-hello}}) message, which contains a random nonce (ClientHello.random); its offered protocol versions; a list of symmetric cipher/HKDF hash pairs; some set of Diffie-Hellman key shares (in the "key_share" extension {{key-share}}), a set of pre-shared key labels (in the "pre_shared_key" extension {{pre-shared-key-extension}}) or both; and potentially some other extensions.
The server processes the ClientHello and determines the appropriate cryptographic parameters for the connection. It then responds with its own ServerHello, which indicates the negotiated connection parameters. [{{server-hello}}]. The combination of the ClientHello and the ServerHello determines the shared keys. If (EC)DHE key establishment is in use, then the ServerHello contains a "key_share" extension with the server's ephemeral Diffie-Hellman share which MUST be in the same group as one of the client's shares. If PSK key establishment is in use, then the ServerHello contains a "pre_shared_key" extension indicating which of the client's offered PSKs was selected. Note that implementations can use (EC)DHE and PSK together, in which case both extensions will be supplied.
The server then sends two messages to establish the Server Parameters:
EncryptedExtensions. : responses to any extensions that are not required to determine the cryptographic parameters. [{{encrypted-extensions}}]
CertificateRequest. : if certificate-based client authentication is desired, the desired parameters for that certificate. This message is omitted if client authentication is not desired. [{{certificate-request}}]
Finally, the client and server exchange Authentication messages. TLS uses the same set of messages every time that authentication is needed. Specifically:
Certificate. : the certificate of the endpoint. This message is omitted if the server is not authenticating with a certificate. Note that if raw public keys {{RFC7250}} or the cached information extension {{?RFC7924}} are in use, then this message will not contain a certificate but rather some other value corresponding to the server's long-term key. [{{certificate}}]
CertificateVerify. : a signature over the entire handshake using the public key in the Certificate message. This message is omitted if the server is not authenticating via a certificate. [{{certificate-verify}}]
Finished. : a MAC (Message Authentication Code) over the entire handshake. This message provides key confirmation, binds the endpoint's identity to the exchanged keys, and in PSK mode also authenticates the handshake. [{{finished}}] {:br }
Upon receiving the server's messages, the client responds with its Authentication messages, namely Certificate and CertificateVerify (if requested), and Finished.
At this point, the handshake is complete, and the client and server may exchange application layer data. Application Data MUST NOT be sent prior to sending the Finished message. Note that while the server may send application data prior to receiving the client's Authentication messages, any data sent at that point is, of course, being sent to an unauthenticated peer.
If the client has not provided a sufficient "key_share" extension (e.g., it includes only DHE or ECDHE groups unacceptable or unsupported by the server), the server corrects the mismatch with a HelloRetryRequest and the client needs to restart the handshake with an appropriate "key_share" extension, as shown in Figure 2. If no common cryptographic parameters can be negotiated, the server MUST abort the handshake with an appropriate alert.
~~~
Client Server
ClientHello
+ key_share -------->
<-------- HelloRetryRequest
ClientHello
+ key_share -------->
ServerHello
+ key_share
{EncryptedExtensions}
{CertificateRequest*}
{Certificate*}
{CertificateVerify*}
{Finished}
<-------- [Application Data*]
{Certificate*}
{CertificateVerify*}
{Finished} -------->
[Application Data] <-------> [Application Data]
{: #tls-restart title="Message flow for a full handshake with mismatched parameters"}
Note: The handshake transcript includes the initial
ClientHello/HelloRetryRequest exchange; it is not reset with the new
ClientHello.
TLS also allows several optimized variants of the basic handshake, as
described in the following sections.
## Resumption and Pre-Shared Key (PSK) {#resumption-and-psk}
Although TLS PSKs can be established out of band,
PSKs can also be established in a previous session and
then reused ("session resumption"). Once a handshake has completed, the server can
send the client a PSK identity that corresponds to a key derived from
the initial handshake (see {{NewSessionTicket}}). The client
can then use that PSK identity in future handshakes to negotiate use
of the PSK. If the server accepts it, then the security context of the
new connection is tied to the original connection. In TLS 1.2 and
below, this functionality was provided by "session IDs" and
"session tickets" {{RFC5077}}. Both mechanisms are obsoleted in TLS
1.3.
PSKs can be used with (EC)DHE exchange in order to provide forward
secrecy in combination with shared keys, or can be used alone, at the
cost of losing forward secrecy.
{{tls-resumption-psk}} shows a pair of handshakes in which the first establishes
a PSK and the second uses it:
Client Server
Initial Handshake: ClientHello + key_share --------> ServerHello + key_share {EncryptedExtensions} {CertificateRequest*} {Certificate*} {CertificateVerify*} {Finished} <-------- [Application Data*] {Certificate*} {CertificateVerify*} {Finished} --------> <-------- [NewSessionTicket] [Application Data] <-------> [Application Data]
Subsequent Handshake: ClientHello + key_share* + psk_key_exchange_modes + pre_shared_key --------> ServerHello + pre_shared_key + key_share* {EncryptedExtensions} {Finished} <-------- [Application Data*] {Finished} --------> [Application Data] <-------> [Application Data]
{: #tls-resumption-psk title="Message flow for resumption and PSK"}
As the server is authenticating via a PSK, it does not send a
Certificate or a CertificateVerify message. When a client offers resumption
via PSK, it SHOULD also supply a "key_share" extension to the server as well
to allow the server to decline resumption and fall back to a full handshake,
if needed. The server responds with a "pre_shared_key" extension
to negotiate use of PSK key establishment and can (as shown here)
respond with a "key_share" extension to do (EC)DHE key
establishment, thus providing forward secrecy.
## Zero-RTT Data
With the Zero-RTT mode, clients can send data on their first flight
("early data") whereby the client uses a PSK either obtained
out-of-band or as a ticket (i.e., one with the "early_data_info"
extension) from an earlier exchange to authenticate to the server.
When clients use a PSK obtained out-of-band then the following
information MUST be provisioned to both parties:
* The PSK identity
* The cipher suite for use with this PSK
* The key exchange and authentication modes this PSK is allowed to be used with
* The Application-Layer Protocol Negotiation (ALPN) label(s)
* The Server Name Indication (SNI), if any is to be used
Note: only the first two of these need to be provisioned to use
an out-of-band PSK in 1-RTT mode.
As shown in {{tls-0-rtt}}, the Zero-RTT data is just added to the 1-RTT
handshake in the first flight. The rest of the handshake uses the same messages
as with a 1-RTT handshake with PSK resumption.
Client Server
ClientHello
+ early_data
+ key_share*
+ pre_shared_key_modes
+ pre_shared_key
(Application Data*)
(end_of_early_data) -------->
ServerHello
+ early_data
+ pre_shared_key
+ key_share*
{EncryptedExtensions}
{Finished}
<-------- [Application Data*]
{Finished} -------->
[Application Data] <-------> [Application Data]
* Indicates optional or situation-dependent
messages that are not always sent.
() Indicates messages protected using keys
derived from client_early_traffic_secret.
{} Indicates messages protected using keys
derived from handshake_traffic_secret.
[] Indicates messages protected using keys
derived from traffic_secret_N
{: #tls-0-rtt title="Message flow for a zero round trip handshake"}
[[OPEN ISSUE: Should it be possible to combine 0-RTT with the
server authenticating via a signature
https://github.com/tlswg/tls13-spec/issues/443]]
IMPORTANT NOTE: The security properties for 0-RTT data are weaker than
those for other kinds of TLS data. Specifically:
1. This data is not forward secret, as it is encrypted solely under
keys derived using the offered PSK.
2. There are no guarantees of non-replay between connections.
Unless the server takes special measures outside those provided by TLS,
the server has no guarantee that the same
0-RTT data was not transmitted on multiple 0-RTT connections
(See {{replay-time}} for more details).
This is especially relevant if the data is authenticated either
with TLS client authentication or inside the application layer
protocol. However, 0-RTT data cannot be duplicated within a connection (i.e., the server
will not process the same data twice for the same connection) and
an attacker will not be able to make 0-RTT data appear to be
1-RTT data (because it is protected with different keys.)
Protocols MUST NOT use 0-RTT data without a profile that defines its
use. That profile needs to identify which messages or interactions are
safe to use with 0-RTT. In addition, to avoid accidental misuse,
implementations SHOULD NOT enable 0-RTT unless specifically
requested. Special functions for 0-RTT data are RECOMMENDED to ensure
that an application is always aware that it is sending or receiving
data that might be replayed.
The same warnings apply to any use of the early exporter secret.
The remainder of this document provides a detailed description of TLS.
# Presentation Language
This document deals with the formatting of data in an external representation.
The following very basic and somewhat casually defined presentation syntax will
be used. The syntax draws from several sources in its structure. Although it
resembles the programming language "C" in its syntax and XDR {{RFC4506}} in
both its syntax and intent, it would be risky to draw too many parallels. The
purpose of this presentation language is to document TLS only; it has no
general application beyond that particular goal.
## Basic Block Size
The representation of all data items is explicitly specified. The basic data
block size is one byte (i.e., 8 bits). Multiple byte data items are
concatenations of bytes, from left to right, from top to bottom. From the byte
stream, a multi-byte item (a numeric in the example) is formed (using C
notation) by:
value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) |
... | byte[n-1];
This byte ordering for multi-byte values is the commonplace network byte order
or big-endian format.
## Miscellaneous
Comments begin with "/\*" and end with "\*/".
Optional components are denoted by enclosing them in "\[\[ \]\]" double
brackets.
Single-byte entities containing uninterpreted data are of type
opaque.
## Vectors
A vector (single-dimensioned array) is a stream of homogeneous data elements.
The size of the vector may be specified at documentation time or left
unspecified until runtime. In either case, the length declares the number of
bytes, not the number of elements, in the vector. The syntax for specifying a
new type, T', that is a fixed- length vector of type T is
T T'[n];
Here, T' occupies n bytes in the data stream, where n is a multiple of the size
of T. The length of the vector is not included in the encoded stream.
In the following example, Datum is defined to be three consecutive bytes that
the protocol does not interpret, while Data is three consecutive Datum,
consuming a total of nine bytes.
opaque Datum[3]; /* three uninterpreted bytes */
Datum Data[9]; /* 3 consecutive 3 byte vectors */
Variable-length vectors are defined by specifying a subrange of legal lengths,
inclusively, using the notation \<floor..ceiling\>. When these are encoded, the
actual length precedes the vector's contents in the byte stream. The length
will be in the form of a number consuming as many bytes as required to hold the
vector's specified maximum (ceiling) length. A variable-length vector with an
actual length field of zero is referred to as an empty vector.
T T'<floor..ceiling>;
In the following example, mandatory is a vector that must contain between 300
and 400 bytes of type opaque. It can never be empty. The actual length field
consumes two bytes, a uint16, which is sufficient to represent the value 400
(see {{numbers}}). On the other hand, longer can represent up to 800 bytes of
data, or 400 uint16 elements, and it may be empty. Its encoding will include a
two-byte actual length field prepended to the vector. The length of an encoded
vector must be an exact multiple of the length of a single element (e.g.,
a 17-byte vector of uint16 would be illegal).
opaque mandatory<300..400>;
/* length field is 2 bytes, cannot be empty */
uint16 longer<0..800>;
/* zero to 400 16-bit unsigned integers */
## Numbers
The basic numeric data type is an unsigned byte (uint8). All larger numeric
data types are formed from fixed-length series of bytes concatenated as
described in {{basic-block-size}} and are also unsigned. The following numeric
types are predefined.
uint8 uint16[2];
uint8 uint24[3];
uint8 uint32[4];
uint8 uint64[8];
All values, here and elsewhere in the specification, are stored in network byte
(big-endian) order; the uint32 represented by the hex bytes 01 02 03 04 is
equivalent to the decimal value 16909060.
## Enumerateds
An additional sparse data type is available called enum. Each definition is a
different type. Only enumerateds of the same type may be assigned or compared.
Every element of an enumerated must be assigned a value, as demonstrated in the
following example. Since the elements of the enumerated are not ordered, they
can be assigned any unique value, in any order.
enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te;
Future extension or additions to the protocol may define new values. Upon
Implementations need to be able to parse and ignore them unless their
definition states otherwise.
An enumerated occupies as much space in the byte stream as would its maximal
defined ordinal value. The following definition would cause one byte to be used
to carry fields of type Color.
enum { red(3), blue(5), white(7) } Color;
One may optionally specify a value without its associated tag to force the
width definition without defining a superfluous element.
In the following example, Taste will consume two bytes in the data stream but
can only assume the values 1, 2, or 4 in current version of protocol.
enum { sweet(1), sour(2), bitter(4), (32000) } Taste;
The names of the elements of an enumeration are scoped within the defined type.
In the first example, a fully qualified reference to the second element of the
enumeration would be Color.blue. Such qualification is not required if the
target of the assignment is well specified.
Color color = Color.blue; /* overspecified, legal */
Color color = blue; /* correct, type implicit */
For enumerateds that are never converted to external representation, the
numerical information may be omitted.
enum { low, medium, high } Amount;
For numerical values, the notation (floor..ceiling) is also allowed.
## Constructed Types
Structure types may be constructed from primitive types for convenience. Each
specification declares a new, unique type. The syntax for definition is much
like that of C.
struct {
T1 f1;
T2 f2;
...
Tn fn;
} [[T]];
The fields within a structure may be qualified using the type's name, with a
syntax much like that available for enumerateds. For example, T.f2 refers to
the second field of the previous declaration. Structure definitions may be
embedded. Anonymous structs may also be defined inside other structures.
### Variants
Defined structures may have variants based on some knowledge that is available
within the environment. The selector must be an enumerated type that defines
the possible variants the structure defines. There must be a case arm for every
element of the enumeration declared in the select. Case arms have limited
fall-through: if two case arms follow in immediate succession with no fields in
between, then they both contain the same fields. Thus, in the example below,
"orange" and "banana" both contain V2. Note that this piece of syntax was added
in TLS 1.2 {{RFC5246}}.
The body of the variant structure may be given a label for reference. The
mechanism by which the variant is selected at runtime is not prescribed by the
presentation language.
struct {
T1 f1;
T2 f2;
....
Tn fn;
select (E) {
case e1: Te1;
case e2: Te2;
case e3: case e4: Te3;
....
case en: Ten;
} [[fv]];
} [[Tv]];
For example:
enum { apple, orange, banana } VariantTag;
struct {
uint16 number;
opaque string<0..10>; /* variable length */
} V1;
struct {
uint32 number;
opaque string[10]; /* fixed length */
} V2;
struct {
select (VariantTag) { /* value of selector is implicit */
case apple:
V1; /* VariantBody, tag = apple */
case orange:
case banana:
V2; /* VariantBody, tag = orange or banana */
} variant_body; /* optional label on variant */
} VariantRecord;
## Constants
Typed constants can be defined for purposes of specification by declaring a
symbol of the desired type and assigning values to it.
Under-specified types (opaque, variable-length vectors, and structures that
contain opaque) cannot be assigned values. No fields of a multi-element
structure or vector may be omitted.
For example:
struct {
uint8 f1;
uint8 f2;
} Example1;
Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */
## Decoding Errors
TLS defines two generic alerts (see {{alert-protocol}}) to use upon failure to parse
a message. Peers which receive a message which cannot be parsed according to the syntax
(e.g., have a length extending beyond the message boundary or contain an out-of-range
length) MUST terminate the connection with a "decoding_error" alert. Peers which receive
a message which is syntactically correct but semantically invalid (e.g., a DHE share of p - 1)
MUST terminate the connection with an "illegal_parameter" alert.
# Handshake Protocol
The handshake protocol is used to negotiate the secure attributes
of a session. Handshake messages are supplied to the TLS record layer, where
they are encapsulated within one or more TLSPlaintext or TLSCiphertext structures, which are
processed and transmitted as specified by the current active session state.
%%% Handshake Protocol
enum {
hello_request_RESERVED(0),
client_hello(1),
server_hello(2),
new_session_ticket(4),
hello_retry_request(6),
encrypted_extensions(8),
certificate(11),
server_key_exchange_RESERVED(12),
certificate_request(13),
server_hello_done_RESERVED(14),
certificate_verify(15),
client_key_exchange_RESERVED(16),
finished(20),
key_update(24),
(255)
} HandshakeType;
struct {
HandshakeType msg_type; /* handshake type */
uint24 length; /* bytes in message */
select (Handshake.msg_type) {
case client_hello: ClientHello;
case server_hello: ServerHello;
case hello_retry_request: HelloRetryRequest;
case encrypted_extensions: EncryptedExtensions;
case certificate_request: CertificateRequest;
case certificate: Certificate;
case certificate_verify: CertificateVerify;
case finished: Finished;
case new_session_ticket: NewSessionTicket;
case key_update: KeyUpdate;
} body;
} Handshake;
Protocol messages MUST be sent in the order defined below (and
shown in the diagrams in {{protocol-overview}}).
A peer which receives a handshake message in an unexpected order
MUST abort the handshake with an "unexpected_message" alert.
Unneeded handshake messages are omitted, however.
New handshake message types are assigned by IANA as described in
{{iana-considerations}}.
## Key Exchange Messages
The key exchange messages are used to exchange security capabilities
between the client and server and to establish the traffic keys used to protect
the handshake and data.
### Cryptographic Negotiation
TLS cryptographic negotiation proceeds by the client offering the
following four sets of options in its ClientHello:
- A list of cipher suites which indicates the AEAD algorithm/HKDF hash
pairs which the client supports.
- A "supported_group" ({{negotiated-groups}}) extension which indicates the (EC)DHE groups
which the client supports and a "key_share" ({{key-share}}) extension which contains
(EC)DHE shares for some or all of these groups.
- A "signature_algorithms" ({{signature-algorithms}}) extension which indicates the signature
algorithms which the client can accept.
- A "pre_shared_key" ({{pre-shared-key-extension}}) extension which
contains a list of symmetric key identities known to the client and a
"psk_key_exchange_modes" ({{pre-shared-key-exchange-modes}})
extension which indicates the key exchange modes that may be used
with PSKs.
If the server does not select a PSK, then the first three of these
options are entirely orthogonal: the server independently selects a
cipher suite, an (EC)DHE group and key share for key establishment,
and a signature algorithm/certificate pair to authenticate itself to
the client. If there is overlap in the "supported_group" extension
but the client did not offer a compatible "key_share" extension,
then the server will respond with a HelloRetryRequest ({{hello-retry-request}}) message.
If the server selects a PSK, then it MUST also select a key
establishment mode from the set indicated by client's
"psk_key_exchange_modes extension (PSK alone or with (EC)DHE). Note
that if the PSK can be used without (EC)DHE then non-overlap in the
"supported_group" parameters need not be fatal.
The server indicates its selected parameters in the ServerHello as
follows:
- If PSK is being used then the server will send a
"pre_shared_key" extension indicating the selected key.
- If PSK is not being used, then (EC)DHE and certificate-based
authentication are always used.
- When (EC)DHE is in use, the server will also provide a
"key_share" extension.
- When authenticating via a certificate (i.e., when a PSK is not
in use), the server will send Certificate ({{certificate}}) and
CertificateVerify ({{certificate-verify}}) messages.
If the server is unable to negotiate a supported set of parameters
(i.e., there is no overlap between the client and server parameters),
it MUST abort the handshake and and SHOULD send either
a "handshake_failure" or "insufficient_security" fatal alert
(see {{alert-protocol}}).
### Client Hello
When this message will be sent:
> When a client first connects to a server, it is REQUIRED to send the
ClientHello as its first message. The client will also send a
ClientHello when the server has responded to its ClientHello with a
HelloRetryRequest. In that case, the client MUST send the same
ClientHello (without modification) except:
- If a "key_share" extension was supplied in the HelloRetryRequest,
replacing the list of shares with a list containing a single
KeyShareEntry from the indicated group.
- Removing the "early_data" extension ({{early-data-indication}}) if one was
present. Early data is not permitted after HelloRetryRequest.
- Including a "cookie" extension if one was provided in the
HelloRetryRequest.
Because TLS 1.3 forbids renegotiation, if a server receives a
ClientHello at any other time, it MUST terminate the connection.
If a server established a TLS connection with a previous version of TLS
and receives a TLS 1.3 ClientHello in a renegotiation, it MUST retain the
previous protocol version. In particular, it MUST NOT negotiate TLS 1.3.
A client that receives a TLS 1.3 ServerHello during renegotiation
MUST abort the handshake with a "protocol_version" alert.
Structure of this message:
%%% Key Exchange Messages
uint16 ProtocolVersion;
opaque Random[32];
uint8 CipherSuite[2]; /* Cryptographic suite selector */
struct {
ProtocolVersion legacy_version = 0x0303; /* TLS v1.2 */
Random random;
opaque legacy_session_id<0..32>;
CipherSuite cipher_suites<2..2^16-2>;
opaque legacy_compression_methods<1..2^8-1>;
Extension extensions<0..2^16-1>;
} ClientHello;
TLS allows extensions to follow the compression_methods field in an extensions
block. The presence of extensions can be detected by determining whether there
are bytes following the compression_methods at the end of the ClientHello. Note
that this method of detecting optional data differs from the normal TLS method
of having a variable-length field, but it is used for compatibility with TLS
before extensions were defined.
As of TLS 1.3, all clients and servers will send at least
one extension (at least "key_share" or "pre_shared_key").
legacy_version
: In previous versions of TLS, this field was used for version negotiation
and represented the highest version number supported by the client.
Experience has shown that many servers do not properly implement
version negotiation, leading to "version intolerance" in which
the server rejects an otherwise acceptable ClientHello with a version
number higher than it supports.
In TLS 1.3, the client indicates its version preferences in the
"supported_versions" extension ({{supported-versions}}) and this field MUST
be set to 0x0303, which was the version number for TLS 1.2.
(See {{backward-compatibility}} for details about backward compatibility.)
random
: 32 bytes generated by a secure random number generator.
See {{implementation-notes}} for additional information.
legacy_session_id
: Versions of TLS before TLS 1.3 supported a session resumption
feature which has been merged with Pre-Shared Keys in this version
(see {{resumption-and-psk}}).
This field MUST be ignored by a server negotiating TLS 1.3 and
SHOULD be set as a zero length vector (i.e., a single zero byte
length field) by clients which do not have a cached session ID
set by a pre-TLS 1.3 server.
cipher_suites
: This is a list of the symmetric cipher options supported by the
client, specifically the record protection algorithm (including
secret key length) and a hash to be used with HKDF, in descending
order of client preference. If the list contains cipher suites
the server does not recognize, support, or wish to use, the server
MUST ignore those cipher suites, and process the remaining ones as
usual. Values are defined in {{cipher-suites}}.
legacy_compression_methods
: Versions of TLS before 1.3 supported compression with the list of
supported compression methods being sent in this field. For every TLS 1.3
ClientHello, this vector MUST contain exactly one byte set to
zero, which corresponds to the "null" compression method in
prior versions of TLS. If a TLS 1.3 ClientHello is
received with any other value in this field, the server MUST
abort the handshake with an "illegal_parameter" alert. Note that TLS 1.3
servers might receive TLS 1.2 or prior ClientHellos which contain
other compression methods and MUST follow the procedures for
the appropriate prior version of TLS.
extensions
: Clients request extended functionality from servers by sending
data in the extensions field. The actual "Extension" format is
defined in {{hello-extensions}}.
{:br }
In the event that a client requests additional functionality using
extensions, and this functionality is not supplied by the server, the
client MAY abort the handshake. Note that TLS 1.3 ClientHello messages
MUST always contain extensions, and a TLS 1.3 server MUST respond to
any TLS 1.3 ClientHello without extensions or with data following
the extensions block with a "decode_error"
alert. TLS 1.3 servers may receive TLS 1.2 ClientHello messages
without extensions. If negotiating TLS 1.2, a server MUST check that
the message either contains no data after legacy_compression_methods
or that it contains a valid extensions block with no data following.
If not, then it MUST abort the handshake with a "decode_error" alert.
After sending the ClientHello message, the client waits for a ServerHello
or HelloRetryRequest message.
### Server Hello
When this message will be sent:
> The server will send this message in response to a ClientHello message when
it was able to find an acceptable set of algorithms and the client's
"key_share" extension was acceptable. If it is not able to find an acceptable
set of parameters, the server will respond with a "handshake_failure" fatal alert.
Structure of this message:
%%% Key Exchange Messages
struct {
ProtocolVersion version;
Random random;
CipherSuite cipher_suite;
Extension extensions<0..2^16-1>;
} ServerHello;
version
: This field contains the version of TLS negotiated for this session. Servers
MUST select the lower of the highest supported server version and the version
offered by the client in the ClientHello. In particular, servers MUST accept
ClientHello messages with versions higher than those supported and negotiate
the highest mutually supported version. For this version of the
specification, the version is 0x0304. (See {{backward-compatibility}} for
details about backward compatibility.)
random
: This structure is generated by the server and MUST be
generated independently of the ClientHello.random.
cipher_suite
: The single cipher suite selected by the server from the list in
ClientHello.cipher_suites. A client which receives a cipher suite
that was not offered MUST abort the handshake.
extensions
: A list of extensions. Note that only extensions offered by the
client can appear in the server's list. In TLS 1.3, as opposed to
previous versions of TLS, the server's extensions are split between
the ServerHello and the EncryptedExtensions {{encrypted-extensions}}
message. The ServerHello MUST only include extensions which are
required to establish the cryptographic context. Currently the only
such extensions are "key_share", "pre_shared_key", and "signature_algorithms".
Clients MUST check the ServerHello for the presence of any forbidden
extensions and if any are found MUST abort the handshake with a
"illegal_parameter" alert. In prior versions of TLS, the extensions
field could be omitted entirely if not needed, similar to
ClientHello. As of TLS 1.3, all clients and servers will send at
least one extension (at least "key_share" or "pre_shared_key").
{:br }
TLS 1.3 has a downgrade protection mechanism embedded in the server's
random value. TLS 1.3 server implementations which respond to a
ClientHello indicating only support for TLS 1.2 or below
MUST set the last eight bytes of their Random value
to the bytes:
44 4F 57 4E 47 52 44 01
TLS 1.3 server implementations which respond to a
ClientHello indicating only support for TLS 1.1 or below
SHOULD set the last eight
bytes of their Random value to the bytes:
44 4F 57 4E 47 52 44 00
TLS 1.3 clients receiving a TLS 1.2 or below ServerHello MUST check
that the last eight octets are not equal to either of these values. TLS
1.2 clients SHOULD also perform this check if the ServerHello
indicates TLS 1.1 or below. If a match is found, the client MUST abort
the handshake with an "illegal_parameter" alert. This mechanism
provides limited protection against downgrade attacks over and above
that provided by the Finished exchange: because the ServerKeyExchange
includes a signature over both random values, it is not possible for
an active attacker to modify the randoms without detection as long as
ephemeral ciphers are used. It does not provide downgrade protection
when static RSA is used.
Note: This is an update to TLS 1.2 so in practice many TLS 1.2 clients
and servers will not behave as specified above.
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH
Implementations of draft versions (see {{draft-version-indicator}}) of this
specification SHOULD NOT implement this mechanism on either client and server.
A pre-RFC client connecting to RFC servers, or vice versa, will appear to
downgrade to TLS 1.2. With the mechanism enabled, this will cause an
interoperability failure.
### Hello Retry Request
When this message will be sent:
> Servers send this message in response to a ClientHello
message if they were able to find an acceptable set of algorithms and
groups that are mutually supported, but
the client's ClientHello did not contain sufficient information to
proceed with the handshake.
If a server cannot successfully select algorithms, it MUST abort
the handshake with a "handshake_failure" alert.
Structure of this message:
%%% Key Exchange Messages
struct {
ProtocolVersion server_version;
Extension extensions<2..2^16-1>;
} HelloRetryRequest;
{:br }
The version and extensions fields have the
same meanings as their corresponding values in the ServerHello.
The server SHOULD send only the extensions necessary for the client to
generate a correct ClientHello pair (currently no such extensions
exist). As with ServerHello, a
HelloRetryRequest MUST NOT contain any extensions that were not first
offered by the client in its ClientHello, with the exception of optionally the
"cookie" (see {{cookie}}) extension.
Upon receipt of a HelloRetryRequest, the client MUST verify that the
extensions block is not empty and otherwise MUST abort the handshake
with a "decode_error" alert. Clients MUST abort the handshake with
an "illegal_parameter" alert if the HelloRetryRequest would not result in
any change in the ClientHello. If a client receives a second
HelloRetryRequest in the same connection (i.e., where
the ClientHello was itself in response to a HelloRetryRequest), it
MUST abort the handshake with an "unexpected_message" alert.
Otherwise, the client MUST process all extensions in the HelloRetryRequest and
send a second updated ClientHello. The HelloRetryRequest extensions defined in
this specification are:
- cookie (see {{cookie}})
- key_share (see {{key-share}})
## Hello Extensions
The extension format is:
%%% Key Exchange Messages
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
enum {
supported_groups(10),
signature_algorithms(13),
key_share(40),
pre_shared_key(41),
early_data(42),
supported_versions(43),
cookie(44),
psk_key_exchange_modes(45),
ticket_early_data_info(46),
(65535)
} ExtensionType;
Here:
- "extension_type" identifies the particular extension type.
- "extension_data" contains information specific to the particular
extension type.
The initial set of extensions is defined in {{RFC6066}}.
The list of extension types is maintained by IANA as described in
{{iana-considerations}}.
An extension type MUST NOT appear in the ServerHello or HelloRetryRequest
unless the same extension type appeared in the corresponding ClientHello.
If a client receives an extension type in ServerHello or HelloRetryRequest
that it did not request in the associated ClientHello, it MUST abort the
handshake with an "unsupported_extension" fatal alert.
Nonetheless, "server-oriented" extensions may be provided within
this framework. Such an extension (say, of type x) would require the client to
first send an extension of type x in a ClientHello with empty extension_data to
indicate that it supports the extension type. In this case, the client is
offering the capability to understand the extension type, and the server is
taking the client up on its offer.
When multiple extensions of different types are present in the ClientHello or
ServerHello messages, the extensions MAY appear in any order. There MUST NOT be
more than one extension of the same type.
Finally, note that extensions can be sent both when starting a new session and
when in resumption-PSK mode. A client that requests session
resumption does not in general know whether the server will accept this
request, and therefore it SHOULD send the same extensions as it would send
normally.
In general, the specification of each extension type needs to describe the
effect of the extension both during full handshake and session resumption. Most
current TLS extensions are relevant only when a session is initiated: when an
older session is resumed, the server does not process these extensions in
ClientHello, and does not include them in ServerHello. However, some
extensions may specify different behavior during session resumption.
[[TODO: update this and the previous paragraph to cover PSK-based resumption.]]
There are subtle (and not so subtle) interactions that may occur in this
protocol between new features and existing features which may result in a
significant reduction in overall security. The following considerations should
be taken into account when designing new extensions:
- Some cases where a server does not agree to an extension are error
conditions, and some are simply refusals to support particular features. In
general, error alerts should be used for the former, and a field in the
server extension response for the latter.
- Extensions should, as far as possible, be designed to prevent any attack that
forces use (or non-use) of a particular feature by manipulation of handshake
messages. This principle should be followed regardless of whether the feature
is believed to cause a security problem.
Often the fact that the extension fields are included in the inputs to the
Finished message hashes will be sufficient, but extreme care is needed when
the extension changes the meaning of messages sent in the handshake phase.
Designers and implementors should be aware of the fact that until the
handshake has been authenticated, active attackers can modify messages and
insert, remove, or replace extensions.
### Supported Versions
%%% Version Extension
struct {
ProtocolVersion versions<2..254>;
} SupportedVersions;
The "supported_versions" extension is used by the client to indicate
which versions of TLS it supports. The extension contains a list of
supported versions in preference order, with the most preferred
version first. Implementations of this specification MUST send this
extension containing all versions of TLS which they are
prepared to negotiate (for this specification, that means minimally
0x0304, but if previous versions of TLS are supported, they MUST
be present as well).
Servers which are compliant with this specification MUST use only the
"supported_versions" extension, if present, to determine client
preferences and MUST only select a version of TLS present in that
extension. They MUST ignore any unknown versions. If the extension is
not present, they MUST negotiate TLS 1.2 or prior as specified in
{{RFC5246}}, even if ClientHello.legacy_version is 0x0304 or later.
The server MUST NOT send the "supported_versions" extension. The
server's selected version is contained in the ServerHello.version field as
in previous versions of TLS.
#### Draft Version Indicator
RFC EDITOR: PLEASE REMOVE THIS SECTION
While the eventual version indicator for the RFC version of TLS 1.3 will
be 0x0304, implementations of draft versions of this specification SHOULD
instead advertise 0x7f00 | draft_version
in ServerHello.version, and HelloRetryRequest.server_version.
For instance, draft-17 would be encoded as the 0x7f11.
This allows pre-RFC implementations to safely negotiate with each other,
even if they would otherwise be incompatible.
### Cookie
%%% Cookie Extension
struct {
opaque cookie<1..2^16-1>;
} Cookie;
Cookies serve two primary purposes:
- Allowing the server to force the client to demonstrate reachability
at their apparent network address (thus providing a measure of DoS
protection). This is primarily useful for non-connection-oriented
transports (see {{?RFC6347}} for an example of this).
- Allowing the server to offload state to the client, thus allowing it to send
a HelloRetryRequest without storing any state. The server does this by
pickling that post-ClientHello hash state into the cookie (protected
with some suitable integrity algorithm).
When sending a HelloRetryRequest, the server MAY provide a "cookie" extension to the
client (this is an exception to the usual rule that the only extensions that
may be sent are those that appear in the ClientHello). When sending the
new ClientHello, the client MUST echo the value of the extension.
Clients MUST NOT use cookies in subsequent connections.
### Signature Algorithms
The client uses the "signature_algorithms" extension to indicate to the server
which signature algorithms may be used in digital signatures. Clients which
desire the server to authenticate via a certificate MUST send this extension.
If a server
is authenticating via a certificate and the client has not sent a
"signature_algorithms" extension then the server MUST
abort the handshake with a "missing_extension" alert
(see {{mti-extensions}}).
Servers which are authenticating via a certificate MUST indicate so
by sending the client an empty "signature_algorithms" extension.
The "extension_data" field of this extension in a ClientHello contains a
SignatureSchemeList value:
%%% Signature Algorithm Extension
enum {
/* RSASSA-PKCS1-v1_5 algorithms */
rsa_pkcs1_sha1 (0x0201),
rsa_pkcs1_sha256 (0x0401),
rsa_pkcs1_sha384 (0x0501),
rsa_pkcs1_sha512 (0x0601),
/* ECDSA algorithms */
ecdsa_secp256r1_sha256 (0x0403),
ecdsa_secp384r1_sha384 (0x0503),
ecdsa_secp521r1_sha512 (0x0603),
/* RSASSA-PSS algorithms */
rsa_pss_sha256 (0x0804),
rsa_pss_sha384 (0x0805),
rsa_pss_sha512 (0x0806),
/* EdDSA algorithms */
ed25519 (0x0807),
ed448 (0x0808),
/* Reserved Code Points */
dsa_sha1_RESERVED (0x0202),
dsa_sha256_RESERVED (0x0402),
dsa_sha384_RESERVED (0x0502),
dsa_sha512_RESERVED (0x0602),
ecdsa_sha1_RESERVED (0x0203),
obsolete_RESERVED (0x0000..0x0200),
obsolete_RESERVED (0x0204..0x0400),
obsolete_RESERVED (0x0404..0x0500),
obsolete_RESERVED (0x0504..0x0600),
obsolete_RESERVED (0x0604..0x06FF),
private_use (0xFE00..0xFFFF),
(0xFFFF)
} SignatureScheme;
struct {
SignatureScheme supported_signature_algorithms<2..2^16-2>;
} SignatureSchemeList;
Note: This enum is named "SignatureScheme" because there is already
a "SignatureAlgorithm" type in TLS 1.2, which this replaces.
We use the term "signature algorithm" throughout the text.
Each SignatureScheme value lists a single signature algorithm that the
client is willing to verify. The values are indicated in descending order
of preference. Note that a signature algorithm takes as input an
arbitrary-length message, rather than a digest. Algorithms which
traditionally act on a digest should be defined in TLS to first
hash the input with a specified hash algorithm and then proceed as usual.
The code point groups listed above have the following meanings:
RSASSA-PKCS1-v1_5 algorithms
: Indicates a signature algorithm using RSASSA-PKCS1-v1_5 {{RFC3447}}
with the corresponding hash algorithm as defined in {{SHS}}. These values
refer solely to signatures which appear in certificates (see
{{server-certificate-selection}}) and are not defined for use in signed
TLS handshake messages.
ECDSA algorithms
: Indicates a signature algorithm using ECDSA {{ECDSA}}, the corresponding
curve as defined in ANSI X9.62 {{X962}} and FIPS 186-4 {{DSS}}, and the
corresponding hash algorithm as defined in {{SHS}}. The signature is
represented as a DER-encoded {{X690}} ECDSA-Sig-Value structure.
RSASSA-PSS algorithms
: Indicates a signature algorithm using RSASSA-PSS {{RFC3447}} with MGF1. The
digest used in the mask generation function and the digest being signed are
both the corresponding hash algorithm as defined in {{SHS}}. When used in
signed TLS handshake messages, the length of the salt MUST be equal to the
length of the digest output. This codepoint is defined for use with TLS 1.2
as well as TLS 1.3.
EdDSA algorithms
: Indicates a signature algorithm using EdDSA as defined in
{{I-D.irtf-cfrg-eddsa}} or its successors. Note that these correspond to the
"PureEdDSA" algorithms and not the "prehash" variants.
{:br }
rsa_pkcs1_sha1, dsa_sha1, and ecdsa_sha1 SHOULD NOT be offered. Clients
offering these values for backwards compatibility MUST list them as the lowest
priority (listed after all other algorithms in SignatureSchemeList).
TLS 1.3 servers MUST NOT offer a SHA-1
signed certificate unless no valid certificate chain can be produced without it
(see {{server-certificate-selection}}).
The signatures on certificates that are self-signed or certificates that are
trust anchors are not validated since they begin a certification path (see
{{RFC5280}}, Section 3.2). A certificate that begins a certification
path MAY use a signature algorithm that is not advertised as being supported
in the "signature_algorithms" extension.
Note that TLS 1.2 defines this extension differently. TLS 1.3 implementations
willing to negotiate TLS 1.2 MUST behave in accordance with the requirements of
{{RFC5246}} when negotiating that version. In particular:
* TLS 1.2 ClientHellos MAY omit this extension.
* In TLS 1.2, the extension contained hash/signature pairs. The pairs are
encoded in two octets, so SignatureScheme values have been allocated to
align with TLS 1.2's encoding. Some legacy pairs are left unallocated. These
algorithms are deprecated as of TLS 1.3. They MUST NOT be offered or
negotiated by any implementation. In particular, MD5 {{SLOTH}} and SHA-224
MUST NOT be used.
* ECDSA signature schemes align with TLS 1.2's ECDSA hash/signature pairs.
However, the old semantics did not constrain the signing curve. If TLS 1.2 is
negotiated, implementations MUST be prepared to accept a signature that uses
any curve that they advertised in the "supported_groups" extension.
* Implementations that advertise support for RSASSA-PSS (which is mandatory in
TLS 1.3), MUST be prepared to accept a signature using that scheme even when
TLS 1.2 is negotiated. In TLS 1.2, RSASSA-PSS is used with RSA cipher suites.
### Negotiated Groups
When sent by the client, the "supported_groups" extension indicates
the named groups which the client supports for key exchange, ordered
from most preferred to least preferred.
Note: In versions of TLS prior to TLS 1.3, this extension was named
"elliptic_curves" and only contained elliptic curve groups. See {{RFC4492}} and
{{RFC7919}}. This extension was also used to negotiate
ECDSA curves. Signature algorithms are now negotiated independently (see
{{signature-algorithms}}).
The "extension_data" field of this extension contains a
"NamedGroupList" value:
%%% Supported Groups Extension
enum {
/* Elliptic Curve Groups (ECDHE) */
obsolete_RESERVED (1..22),
secp256r1 (23), secp384r1 (24), secp521r1 (25),
obsolete_RESERVED (26..28),
x25519 (29), x448 (30),
/* Finite Field Groups (DHE) */
ffdhe2048 (256), ffdhe3072 (257), ffdhe4096 (258),
ffdhe6144 (259), ffdhe8192 (260),
/* Reserved Code Points */
ffdhe_private_use (0x01FC..0x01FF),
ecdhe_private_use (0xFE00..0xFEFF),
obsolete_RESERVED (0xFF01..0xFF02),
(0xFFFF)
} NamedGroup;
struct {
NamedGroup named_group_list<2..2^16-1>;
} NamedGroupList;
Elliptic Curve Groups (ECDHE)
: Indicates support of the corresponding named curve.
Note that some curves are also recommended in ANSI
X9.62 {{X962}} and FIPS 186-4 {{DSS}}. Others are recommended
in {{RFC7748}}.
Values 0xFE00 through 0xFEFF are reserved for private use.
Finite Field Groups (DHE)
: Indicates support of the corresponding finite field
group, defined in {{RFC7919}}.
Values 0x01FC through 0x01FF are reserved for private use.
{:br }
Items in named_group_list are ordered according to the client's
preferences (most preferred choice first).
As of TLS 1.3, servers are permitted to send the "supported_groups"
extension to the client. If the server has a group it prefers to the
ones in the "key_share" extension but is still willing to accept the
ClientHello, it SHOULD send "supported_groups" to update the client's
view of its preferences; this extension SHOULD contain all groups
the server supports, regardless of whether they are currently
supported by the client. Clients MUST NOT act upon any information
found in "supported_groups" prior to successful completion of the
handshake, but MAY use the information learned from a successfully
completed handshake to change what groups they offer to a server in
subsequent connections.
### Key Share
The "key_share" extension contains the endpoint's cryptographic parameters.
Clients MAY send an empty client_shares vector in order to request
group selection from the server at the cost of an additional round trip.
(see {{hello-retry-request}})
%%% Key Exchange Messages
struct {
NamedGroup group;
opaque key_exchange<1..2^16-1>;
} KeyShareEntry;
group
: The named group for the key being exchanged.
Finite Field Diffie-Hellman {{DH}} parameters are described in
{{ffdhe-param}}; Elliptic Curve Diffie-Hellman parameters are
described in {{ecdhe-param}}.
key_exchange
: Key exchange information. The contents of this field are
determined by the specified group and its corresponding
definition. Endpoints MUST NOT send empty or otherwise
invalid key_exchange values for any reason.
{:br }
The "extension_data" field of this extension contains a
"KeyShare" value:
%%% Key Exchange Messages
struct {
select (Handshake.msg_type) {
case client_hello:
KeyShareEntry client_shares<0..2^16-1>;
case hello_retry_request:
NamedGroup selected_group;
case server_hello:
KeyShareEntry server_share;
};
} KeyShare;
client_shares
: A list of offered KeyShareEntry values in descending order of client preference.
This vector MAY be empty if the client is requesting a HelloRetryRequest.
The ordering of values here SHOULD match that of the ordering of offered support
in the "supported_groups" extension.
selected_group
: The mutually supported group the server intends to negotiate and
is requesting a retried ClientHello/KeyShare for.
server_share
: A single KeyShareEntry value that is in the same group as one of the
client's shares.
{:br }
Clients offer an arbitrary number of KeyShareEntry values, each
representing a single set of key exchange parameters. For instance, a
client might offer shares for several elliptic curves or multiple
FFDHE groups. The key_exchange values for each KeyShareEntry MUST be
generated independently. Clients MUST NOT offer multiple
KeyShareEntry values for the same group. Clients MUST NOT offer any
KeyShareEntry values for groups not listed in the client's
"supported_groups" extension. Servers MAY check for violations of
these rules and and MAY abort the handshake with an
"illegal_parameter" alert if one is violated.
Upon receipt of this extension in a HelloRetryRequest, the client MUST first
verify that the selected_group field corresponds to a group which was provided
in the "supported_groups" extension in the original ClientHello. It MUST then
verify that the selected_group field does not correspond to a group which was
provided in the "key_share" extension in the original ClientHello. If either of
these checks fails, then the client MUST abort the handshake with an
"illegal_parameter" alert. Otherwise, when sending the new ClientHello, the
client MUST append a new KeyShareEntry for the group indicated in the
selected_group field to the groups in its original KeyShare. The remaining
KeyShareEntry values MUST be preserved.
Note that a HelloRetryRequest might not include the "key_share" extension if
other extensions are sent, such as if the server is only sending a cookie.
If using (EC)DHE key establishment, servers offer exactly one
KeyShareEntry in the ServerHello. This value MUST correspond to the KeyShareEntry value offered
by the client that the server has selected for the negotiated key exchange.
Servers MUST NOT send a KeyShareEntry for any group not
indicated in the "supported_groups" extension.
If a HelloRetryRequest was received, the client MUST verify that the
selected NamedGroup matches that supplied in the selected_group field and MUST
abort the connection with an "illegal_parameter" alert if it does not.
[[TODO: Recommendation about what the client offers.
Presumably which integer DH groups and which curves.]]
#### Diffie-Hellman Parameters {#ffdhe-param}
Diffie-Hellman {{DH}} parameters for both clients and servers are encoded in
the opaque key_exchange field of a KeyShareEntry in a KeyShare structure.
The opaque value contains the
Diffie-Hellman public value (Y = g^X mod p) for the specified group
(see {{RFC7919}} for group definitions)
encoded as a big-endian integer, padded with zeros to the size of p in
bytes.
Note: For a given Diffie-Hellman group, the padding results in all public keys
having the same length.
Peers SHOULD validate each other's public key Y by ensuring that 1 < Y
< p-1. This check ensures that the remote peer is properly behaved and
isn't forcing the local system into a small subgroup.
#### ECDHE Parameters {#ecdhe-param}
ECDHE parameters for both clients and servers are encoded in the
the opaque key_exchange field of a KeyShareEntry in a KeyShare structure.
For secp256r1, secp384r1 and secp521r1, the contents are the byte string
representation of an elliptic curve public value following the conversion
routine in Section 4.3.6 of ANSI X9.62 {{X962}}.
Although X9.62 supports multiple point formats, any given curve
MUST specify only a single point format. All curves currently
specified in this document MUST only be used with the uncompressed
point format (the format for all ECDH functions is considered
uncompressed).
For x25519 and x448, the contents are the byte string inputs and outputs of the
corresponding functions defined in {{RFC7748}}, 32 bytes for x25519 and 56
bytes for x448.
Note: Versions of TLS prior to 1.3 permitted point format negotiation;
TLS 1.3 removes this feature in favor of a single point format
for each curve.
### Pre-Shared Key Extension
The "pre_shared_key" extension is used to indicate the identity of the
pre-shared key to be used with a given handshake in association
with PSK key establishment (see {{RFC4279}} for background).
The "extension_data" field of this extension contains a
"PreSharedKeyExtension" value:
%%% Key Exchange Messages
struct {
opaque identity<0..2^16-1>;
uint32 obfuscated_ticket_age;
} PskIdentity;
opaque PskBinderEntry<32..255>;
struct {
select (Handshake.msg_type) {
case client_hello:
PskIdentity identities<6..2^16-1>;
PskBinderEntry binders<33..2^16-1>;
case server_hello:
uint16 selected_identity;
};
} PreSharedKeyExtension;
identities
: A list of the identities (labels for keys) that the client is willing
to negotiate with the server. If sent alongside the "early_data"
extension (see {{early-data-indication}}), the first identity is the
one used for 0-RTT data.
obfuscated_ticket_age
: For each ticket, the time since the client learned about the server
configuration that it is using, in milliseconds. This value is
added modulo 2^32 to with the "ticket_age_add" value that was
included with the ticket, see {{NewSessionTicket}}. This addition
prevents passive observers from correlating sessions unless tickets
are reused. Note: because ticket lifetimes are restricted to a
week, 32 bits is enough to represent any plausible age, even in
milliseconds. External tickets SHOULD use an obfuscated_ticket_age of
0; servers MUST ignore this value for external tickets.
binders
: A series of HMAC values, one for
each PSK offered in the "pre_shared_keys" extension and in the same
order, computed as described below.
selected_identity
: The server's chosen identity expressed as a (0-based) index into
the identities in the client's list.
{: br}
Each PSK is associated with a single Hash algorithm. For PSKs established
via the ticket mechanism ({{NewSessionTicket}}), this is the Hash used for
the KDF. For externally established PSKs, the Hash algorithm MUST be set when the
PSK is established.
Prior to accepting PSK key establishment, the server MUST validate the
corresponding binder value (see {{psk-binder}} below) If this
value is not present or does not validate the server MUST abort the
handshake. Servers SHOULD NOT attempt to validate multiple binders;
rather they SHOULD select a single PSK and validate solely the
binder that corresponds to that PSK.
In order to accept PSK key establishment, the server sends a
"pre_shared_key" extension indicating the selected identity.
Clients MUST verify that the server's selected_identity is within the
range supplied by the client, that the server selected the cipher
suite associated with the PSK,
and that the "key_share", and
"signature_algorithms" extensions are consistent with the indicated
ke_modes and auth_modes values. If these values are not consistent,
the client MUST abort the handshake with an "illegal_parameter" alert.
This extension MUST be the last extension in the ClientHello (this
facilitates implementation as described below). Servers MUST check
that it is the last extension and otherwise fail the handshake with an
"illegal_parameter" alert.
If the server supplies an "early_data" extension, the client MUST
verify that the server's selected_identity is 0. If any
other value is returned, the client MUST abort the handshake
with an "unknown_psk_identity" alert.
#### PSK Binder
Each entry in the binders list is computed as an HMAC over the portion
of the ClientHello up to and including the PreSharedKeyExtension.identities
field. That is, it includes all of the ClientHello but not the binder
list itself. The length fields for the message (including the overall
length, the length of the extensions block, and the length of the "pre_shared_key"
extension) are all set as if the binder were present.
The binding_value is computed in the same way as the Finished
message ({{finished}}) but with the BaseKey being the binder_key
(see {{key-schedule}}).
If the handshake includes a HelloRetryRequest, the initial ClientHello
and HelloRetryRequest are included in the transcript along with the
new ClientHello. For instance, if the client sends ClientHello1, its
binder will be computed over:
ClientHello1[truncated]
If the server responds with HelloRetryRequest, and the client then sends
ClientHello2, its binder will be computed over:
ClientHello1 + HelloRetryRequest + ClientHello2[truncated]
The full ClientHello is included in all other handshake hash computations.
### Pre-Shared Key Exchange Modes
In order to use PSKs, clients MUST also send a "psk_key_exchange_modes"
extension. The semantics of this extension are that the client only
supports the use of PSKs with these modes, which restricts both the
use of PSKs offered in this ClientHello and those which the server
might supply via NewSessionTicket.
A clients MUST provide a "psk_key_exchange_modes" extension if it offers
a "pre_shared_key" extension. If clients offer "pre_shared_key" without
a "psk_key_exchange_modes" extension, servers MUST abort the handshake.
Servers MUST NOT select a key exchange mode that is not listed by the
client. This extension also restricts the modes for use with PSK resumption;
servers SHOULD NOT send NewSessionTicket with tickets that are not
compatible with the advertised modes.
%%% Key Exchange Messages
enum { psk_ke(0), psk_dhe_ke(1), (255) } PskKeyExchangeMode;
struct {
PskKeyExchangeMode ke_modes<1..255>;
} PskKeyExchangeModes;
psk_ke
: PSK-only key establishment. In this mode, the server MUST not
supply a "key_share" value.
psk_dhe_ke
: PSK key establishment with (EC)DHE key establishment. In this mode,
the client and servers MUST supply "key_share" values as described
in {{key-share}}.
{:br}
### PSK Binder
%%% Key Exchange Messages
### Early Data Indication
When PSK resumption is used, the client can send application data
in its first flight of messages. If the client opts to do so, it MUST
supply an "early_data" extension as well as the "pre_shared_key"
extension.
The "extension_data" field of this extension contains an
"EarlyDataIndication" value:
%%% Key Exchange Messages
struct {
} EarlyDataIndication;
A server MUST validate that the ticket age for the selected PSK
identity (computed by un-masking PskIdentity.obfuscated_ticket_age)
is within a small tolerance of the time since the ticket was
issued (see {{replay-time}}). If it is not, the server SHOULD proceed
with the handshake but reject 0-RTT, and SHOULD NOT take any other action
that assumes that this ClientHello is fresh.
The parameters for the 0-RTT data (symmetric cipher suite,
ALPN, etc.) are the same as those which were negotiated in the connection
which established the PSK. The PSK used to encrypt the early data
MUST be the first PSK listed in the client's "pre_shared_key" extension.
0-RTT messages sent in the first flight have the same content types
as their corresponding messages sent in other flights (handshake,
application_data, and alert respectively) but are protected under
different keys. After all the 0-RTT application data messages (if
any) have been sent, an "end_of_early_data" alert of type
"warning" is sent to indicate the end of the flight.
0-RTT MUST always be followed by an "end_of_early_data" alert,
which will be encrypted with the 0-RTT traffic keys.
A server which receives an "early_data" extension
can behave in one of two ways:
- Ignore the extension and return no response. This indicates that the
server has ignored any early data and an ordinary 1-RTT handshake is
required.
- Return an empty extension, indicating that it intends to
process the early data. It is not possible for the server
to accept only a subset of the early data messages.
- Request that the client send another ClientHello by responding with a
HelloRetryRequest. A client MUST NOT include the "early_data" extension in
its followup ClientHello.
In order to accept early data, the server server MUST have accepted a
PSK cipher suite and selected the the first key offered in the
client's "pre_shared_key" extension. In addition, it MUST verify that
the following values are consistent with those negotiated in the
connection during which the ticket was established.
- The TLS version number, AEAD algorithm, and the hash for HKDF.
- The selected ALPN {{!RFC7443}} value, if any.
Future extensions MUST define their interaction with 0-RTT.
If any of these checks fail, the server MUST NOT respond
with the extension and must discard all the remaining first
flight data (thus falling back to 1-RTT). If the client attempts
a 0-RTT handshake but the server rejects it, it will generally
not have the 0-RTT record protection keys and must instead
trial decrypt each record with the 1-RTT handshake keys
until it finds one that decrypts properly, and then pick up
the handshake from that point.
If the server chooses to accept the "early_data" extension,
then it MUST comply with the same error handling requirements
specified for all records when processing early data records.
Specifically, if the server fails to decrypt any 0-RTT record following
an accepted "early_data" extension it MUST terminate the connection
with a "bad_record_mac" alert as per {{record-payload-protection}}.
If the server rejects the "early_data" extension, the client
application MAY opt to retransmit the data once the handshake has
been completed. TLS stacks SHOULD not do this automatically and
client applications MUST take care that the negotiated parameters
are consistent with those it expected. For example, if the
ALPN value has changed, it is likely unsafe to retransmit the
original application layer data.
#### Processing Order
Clients are permitted to "stream" 0-RTT data until they
receive the server's Finished, only then sending the "end_of_early_data"
alert. In order to avoid deadlock, when accepting "early_data",
servers MUST process the client's ClientHello and then immediately
send the ServerHello, rather than waiting for the client's
"end_of_early_data" alert.
#### Replay Properties {#replay-time}
As noted in {{zero-rtt-data}}, TLS provides a limited mechanism for
replay protection for data sent by the client in the first flight.
The "obfuscated_ticket_age" parameter in the client's
"pre_shared_key" extension SHOULD be used by
servers to limit the time over which the first flight might be
replayed. A server can store the time at which it sends a session
ticket to the client, or encode the time in the ticket. Then, each
time it receives an "pre_shared_key" extension, it can subtract the base value and
check to see if the value used by the client matches its expectations.
The ticket age (the value with "ticket_age_add" subtracted) provided by the
client will be shorter than the
actual time elapsed on the server by a single round trip time. This
difference is comprised of the delay in sending the NewSessionTicket
message to the client, plus the time taken to send the ClientHello to
the server. For this reason, a server SHOULD measure the round trip
time prior to sending the NewSessionTicket message and account for
that in the value it saves.
To properly validate the ticket age, a server needs to save at least two items:
- The time that the server generated the session ticket and the estimated round
trip time can be added together to form a baseline time.
- The "ticket_age_add" parameter from the NewSessionTicket is needed to recover
the ticket age from the "obfuscated_ticket_age" parameter.
There are several potential sources of error that make an exact
measurement of time difficult. Variations in client and server clocks
are likely to be minimal, outside of gross time corrections. Network
propagation delays are most likely causes of a mismatch in legitimate
values for elapsed time. Both the NewSessionTicket and ClientHello
messages might be retransmitted and therefore delayed, which might be
hidden by TCP.
A small allowance for errors in clocks and variations in measurements
is advisable. However, any allowance also increases the opportunity
for replay. In this case, it is better to reject early data and fall back
to a full 1-RTT handshake than to risk greater exposure to replay attacks.
In common network topologies for browser clients, small allowances on the
order of ten seconds are reasonable. Clock skew distributions are not
symmetric, so the optimal tradeoff may involve an asymmetric replay window.
## Server Parameters
The next two messages from the server, EncryptedExtensions and
CertificateRequest, contain encrypted information from the server
that determines the rest of the handshake.
### Encrypted Extensions
When this message will be sent:
> In all handshakes, the server MUST send the
EncryptedExtensions message immediately after the
ServerHello message. This is the first message that is encrypted
under keys derived from handshake_traffic_secret.
Meaning of this message:
> The EncryptedExtensions message contains any extensions
which should be protected, i.e., any which are not needed to
establish the cryptographic context.
The same extension types MUST NOT appear in both the ServerHello and
EncryptedExtensions. All server-sent extensions other than those explicitly
listed in {{server-hello}} or designated in the IANA registry MUST only
appear in EncryptedExtensions. Extensions which are designated to
appear in ServerHello MUST NOT appear in EncryptedExtensions. Clients
MUST check EncryptedExtensions for the presence of any forbidden
extensions and if any are found MUST abort the handshake with an
"illegal_parameter" alert.
Structure of this message:
%%% Server Parameters Messages
struct {
Extension extensions<0..2^16-1>;
} EncryptedExtensions;
extensions
: A list of extensions.
{:br }
### Certificate Request
When this message will be sent:
> A server which is authenticating with a certificate can optionally
request a certificate from the client. This message, if sent, will
follow EncryptedExtensions.
Structure of this message:
%%% Server Parameters Messages
opaque DistinguishedName<1..2^16-1>;
struct {
opaque certificate_extension_oid<1..2^8-1>;
opaque certificate_extension_values<0..2^16-1>;
} CertificateExtension;
struct {
opaque certificate_request_context<0..2^8-1>;
SignatureScheme
supported_signature_algorithms<2..2^16-2>;
DistinguishedName certificate_authorities<0..2^16-1>;
CertificateExtension certificate_extensions<0..2^16-1>;
} CertificateRequest;
certificate_request_context
: An opaque string which identifies the certificate request and
which will be echoed in the client's Certificate message. The
certificate_request_context MUST be unique within the scope
of this connection (thus preventing replay of client
CertificateVerify messages). This field SHALL be zero length
unless used for the post-handshake authentication exchanges
described in Section 4.5.2.
supported_signature_algorithms
: A list of the signature algorithms that the server is
able to verify, listed in descending order of preference. Any
certificates provided by the client MUST be signed using a
signature algorithm found in supported_signature_algorithms.
certificate_authorities
: A list of the distinguished names {{X501}} of acceptable
certificate_authorities, represented in DER-encoded {{X690}} format. These
distinguished names may specify a desired distinguished name for a
root CA or for a subordinate CA; thus, this message can be used to
describe known roots as well as a desired authorization space. If
the certificate_authorities list is empty, then the client MAY
send any certificate that meets the rest of the selection criteria
in the CertificateRequest, unless there is some external arrangement
to the contrary.
certificate_extensions
: A list of certificate extension OIDs {{RFC5280}} with their allowed
values, represented in DER-encoded {{X690}} format. Some certificate
extension OIDs allow multiple values (e.g. Extended Key Usage).
If the server has included a non-empty certificate_extensions list,
the client certificate MUST contain all of the specified extension
OIDs that the client recognizes. For each extension OID recognized
by the client, all of the specified values MUST be present in the
client certificate (but the certificate MAY have other values as
well). However, the client MUST ignore and skip any unrecognized
certificate extension OIDs. If the client has ignored some of the
required certificate extension OIDs, and supplied a certificate
that does not satisfy the request, the server MAY at its discretion
either continue the session without client authentication, or
abort the handshake with an "unsupported_certificate" alert.
PKIX RFCs define a variety of certificate extension OIDs and their
corresponding value types. Depending on the type, matching
certificate extension values are not necessarily bitwise-equal. It
is expected that TLS implementations will rely on their PKI
libraries to perform certificate selection using certificate
extension OIDs.
This document defines matching rules for two standard certificate
extensions defined in {{RFC5280}}:
- The Key Usage extension in a certificate matches the request when
all key usage bits asserted in the request are also asserted in the
Key Usage certificate extension.
- The Extended Key Usage extension in a certificate matches the
request when all key purpose OIDs present in the request are also
found in the Extended Key Usage certificate extension. The special
anyExtendedKeyUsage OID MUST NOT be used in the request.
Separate specifications may define matching rules for other certificate
extensions.
{:br }
Servers which are authenticating with a PSK MUST not send the CertificateRequest
message.
## Authentication Messages
As discussed in {{protocol-overview}}, TLS uses a common
set of messages for authentication, key confirmation, and handshake
integrity: Certificate, CertificateVerify, and Finished. These
messages are always sent as the last messages in their handshake
flight. The Certificate and CertificateVerify messages are only
sent under certain circumstances, as defined below. The Finished
message is always sent as part of the Authentication block.
The computations for the Authentication messages all uniformly
take the following inputs:
- The certificate and signing key to be used.
- A Handshake Context based on the hash of the handshake messages
- A base key to be used to compute a MAC key.
Based on these inputs, the messages then contain:
Certificate
: The certificate to be used for authentication and any
supporting certificates in the chain. Note that certificate-based
client authentication is not available in the 0-RTT case.
CertificateVerify
: A signature over the value Hash(Handshake Context + Certificate)
Finished
: A MAC over the value Hash(Handshake Context + Certificate + CertificateVerify)
using a MAC key derived from the base key.
{:br}
Because the CertificateVerify signs the Handshake Context +
Certificate and the Finished MACs the Handshake Context + Certificate
+ CertificateVerify, this is mostly equivalent to keeping a running hash
of the handshake messages (exactly so in the pure 1-RTT cases). Note,
however, that subsequent post-handshake authentications do not include
each other, just the messages through the end of the main handshake.
The following table defines the Handshake Context and MAC Base Key
for each scenario:
| Mode | Handshake Context | Base Key |
|------|-------------------|----------|
| 1-RTT (Server) | ClientHello ... later of EncryptedExtensions/CertificateRequest | [sender]_handshake_traffic_secret |
| 1-RTT (Client) | ClientHello ... ServerFinished | [sender]_handshake_traffic_secret |
| Post-Handshake | ClientHello ... ClientFinished + CertificateRequest | [sender]_traffic_secret_N |
The [sender] in this table denotes the sending side.
In all cases, the handshake context is formed by concatenating the
indicated handshake messages, including the handshake message type
and length fields.
### Certificate
When this message will be sent:
> The server MUST send a Certificate message whenever the agreed-upon
key exchange method uses certificates for authentication (this
includes all key exchange methods defined in this document except PSK).
> The client MUST send a Certificate message if and only if server has
requested client authentication via a CertificateRequest message
({{certificate-request}}). If the server requests client authentication
but no suitable certificate is available, the client
MUST send a Certificate message containing no certificates (i.e., with
the "certificate_list" field having length 0).
Meaning of this message:
> This message conveys the endpoint's certificate chain to the peer.
Structure of this message:
%%% Authentication Messages
opaque ASN1Cert<1..2^24-1>;
struct {
ASN1Cert cert_data;
Extension extensions<0..2^16-1>;
} CertificateEntry;
struct {
opaque certificate_request_context<0..2^8-1>;
CertificateEntry certificate_list<0..2^24-1>;
} Certificate;
certificate_request_context
: If this message is in response to a CertificateRequest, the
value of certificate_request_context in that message. Otherwise,
in the case of server authentication this field SHALL be zero length.
certificate_list
: This is a sequence (chain) of CertificateEntry structures, each
containing a single certificate and set of extensions. The sender's
certificate MUST come in the first CertificateEntry in the list.
Each following certificate SHOULD directly certify one preceding it.
Because certificate validation requires that trust anchors be distributed
independently, a certificate that specifies a
trust anchor MAY be omitted from the chain, provided that
supported peers are known to possess any omitted certificates.
extensions:
: A set of extension values for the CertificateEntry. The "Extension"
format is defined in {{hello-extensions}}. Valid extensions include
OCSP Status extensions ({{!RFC6066}} and {{!RFC6961}}) and
SignedCertificateTimestamps ({{!RFC6962}}). Any extension presented
in a Certificate message must only be presented if the corresponding
ClientHello extension was presented in the initial handshake.
If an extension applies the the entire chain, it SHOULD be included
in the first CertificateEntry.
{:br }
Note: Prior to TLS 1.3, "certificate_list" ordering required each certificate
to certify the one immediately preceding it,
however some implementations allowed some flexibility. Servers sometimes send
both a current and deprecated intermediate for transitional purposes, and others
are simply configured incorrectly, but these cases can nonetheless be validated
properly. For maximum compatibility, all implementations SHOULD be prepared to
handle potentially extraneous certificates and arbitrary orderings from any TLS
version, with the exception of the end-entity certificate which MUST be first.
The server's certificate list MUST always be non-empty. A client will
send an empty certificate list if it does not have an appropriate
certificate to send in response to the server's authentication
request.
#### OCSP Status and SCT Extensions
{{!RFC6066}} and {{!RFC6961}} provide extensions to negotiate the server
sending OCSP responses to the client. In TLS 1.2 and below, the
server sends an empty extension to indicate negotiation of this
extension and the OCSP information is carried in a CertificateStatus
message. In TLS 1.3, the server's OCSP information is carried in
an extension in the CertificateEntry containing the assoiciated
certificate. Specifically:
The body of the "status_request" or "status_request_v2" extension
from the server MUST be a CertificateStatus structure as defined
in {{RFC6066}} and {{RFC6961}} respectively.
Similarly, {{!RFC6962}} provides a mechanism for a server to send a
Signed Certificate Timestamp (SCT) as an extension in the ServerHello.
In TLS 1.3, the server's SCT information is carried in an extension in
CertificateEntry.
#### Server Certificate Selection
The following rules apply to the certificates sent by the server:
- The certificate type MUST be X.509v3 {{RFC5280}}, unless explicitly negotiated
otherwise (e.g., {{RFC5081}}).
- The server's end-entity certificate's public key (and associated
restrictions) MUST be compatible with the selected authentication
algorithm (currently RSA or ECDSA).
- The certificate MUST allow the key to be used for signing (i.e., the
digitalSignature bit MUST be set if the Key Usage extension is present) with
a signature scheme indicated in the client's "signature_algorithms" extension.
- The "server_name" and "trusted_ca_keys" extensions {{RFC6066}} are used to
guide certificate selection. As servers MAY require the presence of the "server_name"
extension, clients SHOULD send this extension, when applicable.
All certificates provided by the server MUST be signed by a
signature algorithm that appears in the "signature_algorithms"
extension provided by the client, if they are able to provide such
a chain (see {{signature-algorithms}}).
Certificates that are self-signed
or certificates that are expected to be trust anchors are not validated as
part of the chain and therefore MAY be signed with any algorithm.
If the server cannot produce a certificate chain that is signed only via the
indicated supported algorithms, then it SHOULD continue the handshake by sending
the client a certificate chain of its choice that may include algorithms
that are not known to be supported by the client. This fallback chain MAY
use the deprecated SHA-1 hash algorithm only if the "signature_algorithms"
extension provided by the client permits it.
If the client cannot construct an acceptable chain using the provided
certificates and decides to abort the handshake, then it MUST abort the
handshake with an "unsupported_certificate" alert.
If the server has multiple certificates, it chooses one of them based on the
above-mentioned criteria (in addition to other criteria, such as transport
layer endpoint, local configuration and preferences).
#### Client Certificate Selection
The following rules apply to certificates sent by the client:
In particular:
- The certificate type MUST be X.509v3 {{RFC5280}}, unless explicitly negotiated
otherwise (e.g., {{RFC5081}}).
- If the certificate_authorities list in the certificate request
message was non-empty, one of the certificates in the certificate
chain SHOULD be issued by one of the listed CAs.
- The certificates MUST be signed using an acceptable signature
algorithm, as described in {{certificate-request}}. Note that this
relaxes the constraints on certificate-signing algorithms found in
prior versions of TLS.
- If the certificate_extensions list in the certificate request message
was non-empty, the end-entity certificate MUST match the extension OIDs
recognized by the client, as described in {{certificate-request}}.
Note that, as with the server certificate, there are certificates that use
algorithm combinations that cannot be currently used with TLS.
#### Receiving a Certificate Message
In general, detailed certificate validation procedures are out of scope for
TLS (see {{RFC5280}}). This section provides TLS-specific requirements.
If the server supplies an empty Certificate message, the client MUST abort
the handshake with a "decode_error" alert.
If the client does not send any certificates,
the server MAY at its discretion either continue the handshake without client
authentication, or abort the handshake with a "certificate_required" alert. Also, if some
aspect of the certificate chain was unacceptable (e.g., it was not signed by a
known, trusted CA), the server MAY at its discretion either continue the
handshake (considering the client unauthenticated) or abort the handshake.
Any endpoint receiving any certificate signed using any signature algorithm
using an MD5 hash MUST abort the handshake with a "bad_certificate" alert.
SHA-1 is deprecated and it is RECOMMENDED that
any endpoint receiving any certificate signed using any signature algorithm
using a SHA-1 hash abort the handshake with a "bad_certificate" alert.
All endpoints are RECOMMENDED to transition to SHA-256 or better as soon
as possible to maintain interoperability with implementations
currently in the process of phasing out SHA-1 support.
Note that a certificate containing a key for one signature algorithm
MAY be signed using a different signature algorithm (for instance,
an RSA key signed with an ECDSA key).
### Certificate Verify
When this message will be sent:
> This message is used to provide explicit proof that an endpoint
possesses the private key corresponding to its certificate
and also provides integrity for the handshake up
to this point. Servers MUST send this message when
authenticating via a certificate.
Clients MUST send this
message whenever authenticating via a Certificate (i.e., when
the Certificate message is non-empty). When sent, this message MUST appear immediately
after the Certificate Message and immediately prior to the Finished
message.
Structure of this message:
%%% Authentication Messages
struct {
SignatureScheme algorithm;
opaque signature<0..2^16-1>;
} CertificateVerify;
The algorithm field specifies the signature algorithm used (see
{{signature-algorithms}} for the definition of this field). The
signature is a digital signature using that algorithm that covers the
hash output described in {{authentication-messages}} namely:
Hash(Handshake Context + Certificate)
In TLS 1.3, the digital signature process takes as input:
- A signing key
- A context string
- The actual content to be signed
The digital signature is then computed using the signing key over
the concatenation of:
- 64 bytes of octet 32
- The context string
- A single 0 byte which servers as the separator
- The content to be signed
This structure is intended to prevent an attack on previous versions
of previous versions of TLS in which the ServerKeyExchange format meant that
attackers could obtain a signature of a message with a chosen, 32-byte
prefix. The initial 64 byte pad clears that prefix.
The context string for a server signature is
"TLS 1.3, server CertificateVerify"
and for a client signature is "TLS 1.3, client
CertificateVerify".
For example, if Hash(Handshake Context + Certificate) was 32 bytes of
01 (this length would make sense for SHA-256, the input to the final
signing process for a server CertificateVerify would be:
2020202020202020202020202020202020202020202020202020202020202020
2020202020202020202020202020202020202020202020202020202020202020
544c5320312e332c207365727665722043657274696669636174655665726966
79
00
0101010101010101010101010101010101010101010101010101010101010101
If sent by a server, the signature algorithm MUST be one offered in the
client's "signature_algorithms" extension unless no valid certificate chain can be
produced without unsupported algorithms (see {{signature-algorithms}}).
If sent by a client, the signature algorithm used in the signature
MUST be one of those present in the supported_signature_algorithms
field of the CertificateRequest message.
In addition, the signature algorithm MUST be compatible with the key
in the sender's end-entity certificate. RSA signatures MUST use an
RSASSA-PSS algorithm, regardless of whether RSASSA-PKCS1-v1_5 algorithms
appear in "signature_algorithms". SHA-1 MUST NOT be used in any signatures in
CertificateVerify. All SHA-1 signature algorithms in this specification are
defined solely for use in legacy certificates, and are not valid for
CertificateVerify signatures.
Note: When used with non-certificate-based handshakes (e.g., PSK), the
client's signature does not cover the server's certificate directly,
although it does cover the server's Finished message, which
transitively includes the server's certificate when the PSK derives
from a certificate-authenticated handshake. {{PSK-FINISHED}}
describes a concrete attack on this mode if the Finished is omitted
from the signature. It is unsafe to use certificate-based client
authentication when the client might potentially share the same
PSK/key-id pair with two different endpoints. In order to ensure
this, implementations MUST NOT mix certificate-based client
authentication with PSK.
### Finished
When this message will be sent:
> The Finished message is the final message in the authentication
block. It is essential for providing authentication of the handshake
and of the computed keys.
Meaning of this message:
> Recipients of Finished messages MUST verify that the contents are
correct. Once a side has sent its Finished message and received and
validated the Finished message from its peer, it may begin to send and
receive application data over the connection.
The key used to compute the finished message is computed from the
Base key defined in {{authentication-messages}} using HKDF (see
{{key-schedule}}). Specifically:
finished_key = HKDF-Expand-Label(BaseKey, "finished", "", Hash.length)
Structure of this message:
%%% Authentication Messages
struct {
opaque verify_data[Hash.length];
} Finished;
The verify_data value is computed as follows:
verify_data =
HMAC(finished_key, Hash(
Handshake Context +
Certificate* +
CertificateVerify*
)
)
* Only included if present.
Where HMAC {{RFC2104}} uses the Hash algorithm for the handshake.
As noted above, the HMAC input can generally be implemented by a running
hash, i.e., just the handshake hash at this point.
In previous versions of TLS, the verify_data was always 12 octets long. In
the current version of TLS, it is the size of the HMAC output for the
Hash used for the handshake.
Note: Alerts and any other record types are not handshake messages
and are not included in the hash computations.
Any records following a 1-RTT Finished message MUST be encrypted under the
application traffic key. In particular, this includes any alerts sent by the
server in response to client Certificate and CertificateVerify messages.
## Post-Handshake Messages
TLS also allows other messages to be sent after the main handshake.
These messages use a handshake content type and are encrypted under the application
traffic key.
Handshake messages sent after the handshake MUST NOT be interleaved with other
record types. That is, if a message is split over two or more handshake
records, there MUST NOT be any other records between them.
### New Session Ticket Message {#NewSessionTicket}
At any time after the server has received the client Finished message, it MAY send
a NewSessionTicket message. This message creates a pre-shared key
(PSK) binding between the ticket value and the following two values derived
from the resumption master secret:
resumption_psk = HKDF-Expand-Label( resumption_secret, "resumption psk", "", Hash.Length)
The client MAY use this PSK for future handshakes by including the
ticket value in the "pre_shared_key" extension in its ClientHello
({{pre-shared-key-extension}}). Servers MAY send multiple tickets on a
single connection, either immediately after each other or
after specific events. For instance, the server might send
a new ticket after post-handshake
authentication in order to encapsulate the additional client
authentication state. Clients SHOULD attempt to use each
ticket no more than once, with more recent tickets being used
first.
Any ticket MUST only be resumed with a cipher suite that is identical
to that negotiated connection where the ticket was established.
Note: Although the resumption_psk depends on the client's second
flight, servers which do not request client authentication MAY compute
the remainder of the transcript independently and then send a
NewSessionTicket immediately upon sending its Finished rather than
waiting for the client Finished.
%%% Ticket Establishment
struct {
uint32 ticket_lifetime;
uint32 ticket_age_add;
opaque ticket<1..2^16-1>;
Extension extensions<0..2^16-2>;
} NewSessionTicket;
ticket_lifetime
: Indicates the lifetime in seconds as a 32-bit unsigned integer in
network byte order from the time of ticket issuance.
Servers MUST NOT use any value more than 604800 seconds (7 days).
The value of zero indicates that the ticket should be discarded
immediately. Clients MUST NOT cache session tickets for longer than
7 days, regardless of the ticket_lifetime. It MAY delete the ticket
earlier based on local policy. A server MAY treat a ticket as valid
for a shorter period of time than what is stated in the
ticket_lifetime.
ticket_age_add
: A randomly generated 32-bit value that is used to obscure the age of
the ticket that the client includes in the "early_data" extension.
The client-side ticket age is added to this value modulo 2^32 to
obtain the value that is transmitted by the client.
ticket
: The value of the ticket to be used as the PSK identifier.
The ticket itself is an opaque label. It MAY either be a database
lookup key or a self-encrypted and self-authenticated value. Section
4 of {{RFC5077}} describes a recommended ticket construction mechanism.
ticket_extensions
: A set of extension values for the ticket. Clients MUST ignore
unrecognized extensions.
{:br }
This document defines one ticket extension, "ticket_early_data_info"
%%% Ticket Establishment
struct {
uint32 max_early_data_size;
} TicketEarlyDataInfo;
This extension indicates that the ticket may be used to send 0-RTT data
({{early-data-indication}})). It contains the following value:
max_early_data_size
: The maximum amount of 0-RTT data that the client is allowed to send when using
this ticket, in bytes. Only Application Data payload is counted. A server
receiving more than max_early_data_size bytes of 0-RTT data
SHOULD terminate the connection with an "unexpected_message" alert.
{:br }
### Post-Handshake Authentication
The server is permitted to request client authentication at any time
after the handshake has completed by sending a CertificateRequest
message. The client SHOULD respond with the appropriate Authentication
messages. If the client chooses to authenticate, it MUST send
Certificate, CertificateVerify, and Finished. If it declines, it
MUST send a Certificate message containing no certificates followed by Finished.
Note: Because client authentication may require prompting the user,
servers MUST be prepared for some delay, including receiving an
arbitrary number of other messages between sending the
CertificateRequest and receiving a response. In addition, clients which receive multiple
CertificateRequests in close succession MAY respond to them in a
different order than they were received (the
certificate_request_context value allows the server to disambiguate
the responses).
### Key and IV Update {#key-update}
%%% Updating Keys
enum { update_not_requested(0), update_requested(1), (255)
} KeyUpdateRequest;
struct {
KeyUpdateRequest request_update;
} KeyUpdate;
request_update
: Indicates that the recipient of the KeyUpdate should respond with its
own KeyUpdate. If an implementation receives any other value, it MUST
terminate the connection with an "illegal_parameter" alert.
{:br }
The KeyUpdate handshake message is used to indicate that the sender is
updating its sending cryptographic keys. This message can be sent by
the server after sending its first flight and the client after sending
its second flight. Implementations that receive a KeyUpdate message
prior to receiving a Finished message as part of the 1-RTT handshake
MUST terminate the connection with an "unexpected_message" alert.
After sending a KeyUpdate message, the sender SHALL send all its traffic using the
next generation of keys, computed as described in
{{updating-traffic-keys}}. Upon receiving a KeyUpdate, the receiver
MUST update its receiving keys.
If the request_udate field is set to "update_requested" then the receiver MUST
send a KeyUpdate of its own with request_update set to "update_not_requested" prior
to sending its next application data record. This mechanism allows either side to force an update to the
entire connection, but causes an implementation which
receives multiple KeyUpdates while it is silent to respond with
a single update. Note that implementations may receive an arbitrary
number of messages between sending a KeyUpdate and receiving the
peer's KeyUpdate because those messages may already be in flight.
However, because send and receive keys are derived from independent
traffic secrets, retaining the receive traffic secret does not threaten
the forward secrecy of data sent before the sender changed keys.
If implementations independently send their own KeyUpdates with
request_update set to "update_requested", and they cross in flight, then each side
will also send a response, with the result that each side increments
by two generations.
Both sender and receiver MUST encrypt their KeyUpdate
messages with the old keys. Additionally, both sides MUST enforce that
a KeyUpdate with the old key is received before accepting any messages
encrypted with the new key. Failure to do so may allow message truncation
attacks.
## Handshake Layer and Key Changes
Handshake messages MUST NOT span key changes. Because
the ServerHello, Finished, and KeyUpdate messages signal a key change,
upon receiving these messages a receiver MUST verify that the end
of these messages aligns with a record boundary; if not, then it MUST
terminate the connection with an "unexpected_message" alert.
# Record Protocol
The TLS record protocol takes messages to be transmitted, fragments
the data into manageable blocks, protects the records, and transmits
the result. Received data is decrypted and verified, reassembled, and
then delivered to higher-level clients.
TLS records are typed, which allows multiple higher level protocols to
be multiplexed over the same record layer. This document specifies
three content types: handshake, application data, and alert.
Implementations MUST NOT send record types not defined in this
document unless negotiated by some extension. If a TLS implementation
receives an unexpected record type, it MUST terminate the connection
with an "unexpected_message" alert. New record content type values are
assigned by IANA in the TLS Content Type Registry as described in
{{iana-considerations}}.
Application Data messages are carried by the record layer and are
fragmented and encrypted as described below. The messages are treated
as transparent data to the record layer.
## Record Layer
The TLS record layer receives uninterpreted data from higher layers in
non-empty blocks of arbitrary size.
The record layer fragments information blocks into TLSPlaintext records
carrying data in chunks of 2^14 bytes or less. Message boundaries are
not preserved in the record layer (i.e., multiple messages of the same
ContentType MAY be coalesced into a single TLSPlaintext record, or a single
message MAY be fragmented across several records).
Alert messages ({{alert-protocol}}) MUST NOT be fragmented across records.
%%% Record Layer
enum {
invalid_RESERVED(0),
change_cipher_spec_RESERVED(20),
alert(21),
handshake(22),
application_data(23),
(255)
} ContentType;
struct {
ContentType type;
ProtocolVersion legacy_record_version = 0x0301; /* TLS v1.x */
uint16 length;
opaque fragment[TLSPlaintext.length];
} TLSPlaintext;
type
: The higher-level protocol used to process the enclosed fragment.
legacy_record_version
: This value MUST be set to 0x0301 for all records.
This field is deprecated and MUST be ignored for all purposes.
length
: The length (in bytes) of the following TLSPlaintext.fragment. The
length MUST NOT exceed 2^14. An endpoint that receives a record
that exceeds this length MUST terminate the connection with a
"record_overflow" alert.
fragment
: The data being transmitted. This value transparent and treated as an
independent block to be dealt with by the higher-level protocol
specified by the type field.
{:br }
This document describes TLS Version 1.3, which uses the version 0x0304.
This version value is historical, deriving from the use of 0x0301
for TLS 1.0 and 0x0300 for SSL 3.0. In order to maximize backwards
compatibility, the record layer version identifies as simply TLS 1.0.
Endpoints supporting other versions negotiate the version to use
by following the procedure and requirements in {{backward-compatibility}}.
Implementations MUST NOT send zero-length fragments of Handshake or
Alert types, even if those fragments contain padding. Zero-length
fragments of Application Data MAY be sent as they are potentially
useful as a traffic analysis countermeasure.
When record protection has not yet been engaged, TLSPlaintext
structures are written directly onto the wire. Once record protection
has started, TLSPlaintext records are protected and sent as
described in the following section.
## Record Payload Protection
The record protection functions translate a TLSPlaintext structure into a
TLSCiphertext. The deprotection functions reverse the process. In TLS 1.3
as opposed to previous versions of TLS, all ciphers are modeled as
"Authenticated Encryption with Additional Data" (AEAD) {{RFC5116}}.
AEAD functions provide a unified encryption and authentication
operation which turns plaintext into authenticated ciphertext and
back again. Each encrypted record consists of a plaintext header followed
by an encrypted body, which itself contains a type and optional padding.
%%% Record Layer
struct {
opaque content[TLSPlaintext.length];
ContentType type;
uint8 zeros[length_of_padding];
} TLSInnerPlaintext;
struct {
ContentType opaque_type = 23; /* application_data, see TLSInnerPlaintext.type */
ProtocolVersion legacy_record_version = 0x0301; /* TLS v1.x */
uint16 length;
opaque encrypted_record[length];
} TLSCiphertext;
content
: The cleartext of TLSPlaintext.fragment.
type
: The content type of the record.
zeros
: An arbitrary-length run of zero-valued bytes may
appear in the cleartext after the type field. This provides an
opportunity for senders to pad any TLS record by a chosen amount as
long as the total stays within record size limits. See
{{record-padding}} for more details.
opaque_type
: The outer opaque_type field of a TLSCiphertext record is always set to the
value 23 (application_data) for outward compatibility with
middleboxes accustomed to parsing previous versions of TLS. The
actual content type of the record is found in TLSInnerPlaintext.type after
decryption.
legacy_record_version
: The legacy_record_version field is identical to TLSPlaintext.legacy_record_version and is always 0x0301.
Note that the handshake protocol including the ClientHello and ServerHello messages authenticates
the protocol version, so this value is redundant.
length
: The length (in bytes) of the following TLSCiphertext.fragment, which
is the sum of the lengths of the content and the padding, plus one
for the inner content type. The length MUST NOT exceed 2^14 + 256.
An endpoint that receives a record that exceeds this length MUST
terminate the connection with a "record_overflow" alert.
encrypted_record
: The AEAD encrypted form of the serialized TLSInnerPlaintext structure.
{:br }
AEAD algorithms take as input a single key, a nonce, a plaintext, and "additional
data" to be included in the authentication check, as described in Section 2.1
of {{RFC5116}}. The key is either the client_write_key or the server_write_key,
the nonce is derived from the sequence number (see {{nonce}}) and the
client_write_iv or server_write_iv, and the additional data input is empty
(zero length). Derivation of traffic keys is defined in {{traffic-key-calculation}}.
The plaintext is the concatenation of TLSPlaintext.fragment,
TLSPlaintext.type, and any padding bytes (zeros).
The AEAD output consists of the ciphertext output by the AEAD
encryption operation. The length of the plaintext is greater than
TLSPlaintext.length due to the inclusion of TLSPlaintext.type and
however much padding is supplied by the sender. The length of the
AEAD output will generally be larger than the plaintext, but by an
amount that varies with the AEAD algorithm. Since the ciphers might
incorporate padding, the amount of overhead could vary with different
lengths of plaintext. Symbolically,
AEADEncrypted =
AEAD-Encrypt(write_key, nonce, plaintext of fragment)
In order to decrypt and verify, the cipher takes as input the key,
nonce, and the AEADEncrypted value. The output is either the plaintext
or an error indicating that the decryption failed. There is no
separate integrity check. That is:
plaintext of fragment =
AEAD-Decrypt(write_key, nonce, AEADEncrypted)
If the decryption fails, the receiver MUST terminate the connection
with a "bad_record_mac" alert.
An AEAD algorithm used in TLS 1.3 MUST NOT produce an expansion of greater than 255
bytes. An endpoint that receives a record from its peer with
TLSCipherText.length larger than 2^14 + 256 octets MUST terminate
the connection with a "record_overflow" alert. This limit is derived from the maximum
TLSPlaintext length of 2^14 octets + 1 octet for ContentType + the
maximum AEAD expansion of 255 octets.
## Per-Record Nonce {#nonce}
A 64-bit sequence number is maintained separately for reading and writing
records. Each sequence number is set to zero at the beginning of a connection
and whenever the key is changed.
The sequence number is incremented after reading or writing each record.
The first record transmitted under a particular set of traffic keys
record key MUST use sequence number 0.
Sequence numbers do not wrap. If a TLS implementation would need to
wrap a sequence number, it MUST either rekey ({{key-update}}) or
terminate the connection.
The length of the per-record nonce (iv_length) is set to max(8 bytes,
N_MIN) for the AEAD algorithm (see {{RFC5116}} Section 4). An AEAD
algorithm where N_MAX is less than 8 bytes MUST NOT be used with TLS.
The per-record nonce for the AEAD construction is formed as follows:
1. The 64-bit record sequence number is padded to the left with zeroes
to iv_length.
2. The padded sequence number is XORed with the static client_write_iv
or server_write_iv, depending on the role.
The resulting quantity (of length iv_length) is used as the per-record
nonce.
Note: This is a different construction from that in TLS 1.2, which
specified a partially explicit nonce.
## Record Padding
All encrypted TLS records can be padded to inflate the size of the
TLSCipherText. This allows the sender to hide the size of the
traffic from an observer.
When generating a TLSCiphertext record, implementations MAY choose to
pad. An unpadded record is just a record with a padding length of
zero. Padding is a string of zero-valued bytes appended
to the ContentType field before encryption. Implementations MUST set
the padding octets to all zeros before encrypting.
Application Data records may contain a zero-length TLSInnerPlaintext.content if
the sender desires. This permits generation of plausibly-sized cover
traffic in contexts where the presence or absence of activity may be
sensitive. Implementations MUST NOT send Handshake or Alert records
that have a zero-length TLSInnerPlaintext.content.
The padding sent is automatically verified by the record protection
mechanism: Upon successful decryption of a TLSCiphertext.fragment,
the receiving implementation scans the field from the end toward the
beginning until it finds a non-zero octet. This non-zero octet is the
content type of the message.
This padding scheme was selected because it allows padding of any encrypted
TLS record by an arbitrary size (from zero up to TLS record size
limits) without introducing new content types. The design also
enforces all-zero padding octets, which allows for quick detection of
padding errors.
Implementations MUST limit their scanning to the cleartext returned
from the AEAD decryption. If a receiving implementation does not find
a non-zero octet in the cleartext, it MUST terminate the
connection with an "unexpected_message" alert.
The presence of padding does not change the overall record size
limitations -- the full fragment plaintext may not exceed 2^14 octets.
Selecting a padding policy that suggests when and how much to pad is a
complex topic, and is beyond the scope of this specification. If the
application layer protocol atop TLS has its own padding, it may be
preferable to pad application_data TLS records within the application
layer. Padding for encrypted handshake and alert TLS records must
still be handled at the TLS layer, though. Later documents may define
padding selection algorithms, or define a padding policy request
mechanism through TLS extensions or some other means.
## Limits on Key Usage
There are cryptographic limits on the amount of plaintext which can be
safely encrypted under a given set of keys. {{AEAD-LIMITS}} provides
an analysis of these limits under the assumption that the underlying
primitive (AES or ChaCha20) has no weaknesses. Implementations SHOULD
do a key update {{key-update}} prior to reaching these limits.
For AES-GCM, up to 2^24.5 full-size records (about 24 million)
may be encrypted on a
given connection while keeping a safety margin of approximately
2^-57 for Authenticated Encryption (AE) security. For
ChaCha20/Poly1305, the record sequence number would wrap before the
safety limit is reached.
# Alert Protocol
One of the content types supported by the TLS record layer is the
alert type. Like other messages, alert messages are encrypted as
specified by the current connection state.
Alert messages convey the severity of the message (warning or fatal)
and a description of the alert. Warning-level messages are used to
indicate orderly closure of the connection (see {{closure-alerts}}).
Upon receiving a warning-level alert, the TLS implementation SHOULD
indicate end-of-data to the application and, if appropriate for
the alert type, send a closure alert in response.
Fatal-level messages are used to indicate abortive closure of the
connection (See {{error-alerts}}). Upon receiving a fatal-level alert,
the TLS implementation SHOULD indicate an error to the application and
MUST NOT allow any further data to be sent or received on the
connection. Servers and clients MUST forget keys and secrets
associated with a failed connection. Stateful implementations of
session tickets (as in many clients) SHOULD discard tickets associated
with failed connections.
All the alerts listed in {{error-alerts}} MUST be sent as fatal and
MUST be treated as fatal regardless of the AlertLevel in the
message. Unknown alert types MUST be treated as fatal.
%%% Alert Messages
enum { warning(1), fatal(2), (255) } AlertLevel;
enum {
close_notify(0),
end_of_early_data(1),
unexpected_message(10),
bad_record_mac(20),
decryption_failed_RESERVED(21),
record_overflow(22),
decompression_failure_RESERVED(30),
handshake_failure(40),
no_certificate_RESERVED(41),
bad_certificate(42),
unsupported_certificate(43),
certificate_revoked(44),
certificate_expired(45),
certificate_unknown(46),
illegal_parameter(47),
unknown_ca(48),
access_denied(49),
decode_error(50),
decrypt_error(51),
export_restriction_RESERVED(60),
protocol_version(70),
insufficient_security(71),
internal_error(80),
inappropriate_fallback(86),
user_canceled(90),
no_renegotiation_RESERVED(100),
missing_extension(109),
unsupported_extension(110),
certificate_unobtainable(111),
unrecognized_name(112),
bad_certificate_status_response(113),
bad_certificate_hash_value(114),
unknown_psk_identity(115),
certificate_required(116),
(255)
} AlertDescription;
struct {
AlertLevel level;
AlertDescription description;
} Alert;
## Closure Alerts
The client and the server must share knowledge that the connection is ending in
order to avoid a truncation attack. Failure to properly close a connection does
not prohibit a session from being resumed.
close_notify
: This alert notifies the recipient that the sender will not send
any more messages on this connection. Any data received after a
closure MUST be ignored.
end_of_early_data
: This alert is sent by the client to indicate that all 0-RTT
application_data messages have been transmitted (or none will
be sent at all) and that this is the end of the flight. This
alert MUST be at the warning level. Servers MUST NOT send this
alert and clients receiving it MUST terminate the connection
with an "unexpected_message" alert.
user_canceled
: This alert notifies the recipient that the sender is canceling the
handshake for some reason unrelated to a protocol failure. If a user
cancels an operation after the handshake is complete, just closing the
connection by sending a "close_notify" is more appropriate. This alert
SHOULD be followed by a "close_notify". This alert is generally a warning.
{:br }
Either party MAY initiate a close by sending a "close_notify" alert. Any data
received after a closure alert is ignored. If a transport-level close is
received prior to a "close_notify", the receiver cannot know that all the
data that was sent has been received.
Each party MUST send a "close_notify" alert before closing the write side
of the connection, unless some other fatal alert has been transmitted. The
other party MUST respond with a "close_notify" alert of its own and close down
the connection immediately, discarding any pending writes. The initiator of the
close need not wait for the responding "close_notify" alert before closing the
read side of the connection.
If the application protocol using TLS provides that any data may be carried
over the underlying transport after the TLS connection is closed, the TLS
implementation must receive the responding "close_notify" alert before indicating
to the application layer that the TLS connection has ended. If the application
protocol will not transfer any additional data, but will only close the
underlying transport connection, then the implementation MAY choose to close
the transport without waiting for the responding "close_notify". No part of this
standard should be taken to dictate the manner in which a usage profile for TLS
manages its data transport, including when connections are opened or closed.
Note: It is assumed that closing a connection reliably delivers pending data
before destroying the transport.
## Error Alerts
Error handling in the TLS Handshake Protocol is very simple. When an
error is detected, the detecting party sends a message to its
peer. Upon transmission or receipt of a fatal alert message, both
parties immediately close the connection.
Whenever an implementation encounters a fatal error condition, it
SHOULD send an appropriate fatal alert and MUST close the connection
without sending or receiving any additional data. In the rest of this
specification, the phrase "{terminate the connection, abort the
handshake}" is used without a specific alert means that the
implementation SHOULD send the alert indicated by the descriptions
below. The phrase "{terminate the connection, abort the handshake}
with a X alert" MUST send alert X if it sends any alert. All
alerts defined in this section below, as well as all unknown alerts
are universally considered fatal as of TLS 1.3 (see
{{alert-protocol}}).
The following error alerts are defined:
unexpected_message
: An inappropriate message (e.g., the wrong handshake message, premature
application data, etc.) was received. This alert should never be
observed in communication between proper implementations.
bad_record_mac
: This alert is returned if a record is received which cannot be
deprotected. Because AEAD algorithms combine decryption and
verification, this alert is used for all deprotection failures.
This alert should never be observed in communication between
proper implementations, except when messages were corrupted
in the network.
record_overflow
: A TLSCiphertext record was received that had a length more than
2^14 + 256 bytes, or a record decrypted to a TLSPlaintext record
with more than 2^14 bytes.
This alert should never be observed in communication between
proper implementations, except when messages were corrupted
in the network.
handshake_failure
: Reception of a "handshake_failure" alert message indicates that the
sender was unable to negotiate an acceptable set of security
parameters given the options available.
bad_certificate
: A certificate was corrupt, contained signatures that did not
verify correctly, etc.
unsupported_certificate
: A certificate was of an unsupported type.
certificate_revoked
: A certificate was revoked by its signer.
certificate_expired
: A certificate has expired or is not currently valid.
certificate_unknown
: Some other (unspecified) issue arose in processing the
certificate, rendering it unacceptable.
illegal_parameter
: A field in the handshake was incorrect or inconsistent with
other fields. This alert is used for errors which conform to
the formal protocol syntax but are otherwise incorrect.
unknown_ca
: A valid certificate chain or partial chain was received, but the
certificate was not accepted because the CA certificate could not
be located or couldn't be matched with a known, trusted CA.
access_denied
: A valid certificate or PSK was received, but when access control was
applied, the sender decided not to proceed with negotiation.
decode_error
: A message could not be decoded because some field was out of the
specified range or the length of the message was incorrect.
This alert is used for errors where the message does not conform
to the formal protocol syntax.
This alert should never be observed in communication between
proper implementations, except when messages were corrupted
in the network.
decrypt_error
: A handshake cryptographic operation failed, including being unable
to correctly verify a signature or validate a Finished message
or a PSK binder.
protocol_version
: The protocol version the peer has attempted to negotiate is
recognized but not supported. (see {{backward-compatibility}})
insufficient_security
: Returned instead of "handshake_failure" when a negotiation has
failed specifically because the server requires ciphers more
secure than those supported by the client.
internal_error
: An internal error unrelated to the peer or the correctness of the
protocol (such as a memory allocation failure) makes it impossible
to continue.
inappropriate_fallback
: Sent by a server in response to an invalid connection retry attempt
from a client. (see [RFC7507])
missing_extension
: Sent by endpoints that receive a hello message not containing an
extension that is mandatory to send for the offered TLS version
or other negotiated parameters.
[[TODO: IANA Considerations.]]
unsupported_extension
: Sent by endpoints receiving any hello message containing an extension
known to be prohibited for inclusion in the given hello message, including
any extensions in a ServerHello or Certificate not first offered in the
corresponding ClientHello.
certificate_unobtainable
: Sent by servers when unable to obtain a certificate from a URL
provided by the client via the "client_certificate_url" extension
[RFC6066].
unrecognized_name
: Sent by servers when no server exists identified by the name
provided by the client via the "server_name" extension
[RFC6066].
bad_certificate_status_response
: Sent by clients when an invalid or unacceptable OCSP response is
provided by the server via the "status_request" extension
[RFC6066].
bad_certificate_hash_value
: Sent by servers when a retrieved object does not have the correct hash
provided by the client via the "client_certificate_url" extension
[RFC6066].
unknown_psk_identity
: Sent by servers when PSK key establishment is desired but no
acceptable PSK identity is provided by the client. Sending this alert
is OPTIONAL; servers MAY instead choose to send a "decrypt_error"
alert to merely indicate an invalid PSK identity.
certificate_required
: Sent by servers when a client certificate is desired but none was provided by
the client.
{:br }
[[TODO: IANA Considerations for new alert values.]]
New Alert values are assigned by IANA as described in {{iana-considerations}}.
# Cryptographic Computations
In order to begin connection protection, the TLS Record Protocol
requires specification of a suite of algorithms, a master secret, and
the client and server random values.
## Key Schedule
The TLS handshake establishes one or more input secrets which
are combined to create the actual working keying material, as detailed
below.
The key derivation process makes use of the HKDF-Extract and HKDF-Expand
functions as defined for HKDF {{RFC5869}}, as well as the functions
defined below:
HKDF-Expand-Label(Secret, Label, HashValue, Length) =
HKDF-Expand(Secret, HkdfLabel, Length)
Where HkdfLabel is specified as:
struct {
uint16 length = Length;
opaque label<9..255> = "TLS 1.3, " + Label;
opaque hash_value<0..255> = HashValue;
} HkdfLabel;
Derive-Secret(Secret, Label, Messages) =
HKDF-Expand-Label(Secret, Label,
Hash(Messages), Hash.Length)
The Hash function and the HKDF hash are the cipher suite hash algorithm.
Hash.length is its output length.
Given a set of n InputSecrets, the final "master secret" is computed
by iteratively invoking HKDF-Extract with InputSecret_1, InputSecret_2,
etc. The initial secret is simply a string of zeroes as long as the size
of the Hash that is the basis for the HKDF. Concretely, for the
present version of TLS 1.3, secrets are added in the following order:
- PSK
- (EC)DHE shared secret
This produces a full key derivation schedule shown in the diagram below.
In this diagram, the following formatting conventions apply:
- HKDF-Extract is drawn as taking the Salt argument from the top and the IKM argument
from the left.
- Derive-Secret's Secret argument is indicated by the arrow coming in
from the left. For instance, the Early Secret is the Secret for
generating the client_early_traffic_secret.
0
|
v
PSK -> HKDF-Extract | v Early Secret | +---------> Derive-Secret(., "client early traffic secret", | ClientHello) | = client_early_traffic_secret | +---------> Derive-Secret(., | "external psk binder key" | | "resumption psk binder key", | "") | = binder_key | +--------> Derive-Secret(., "early exporter master secret", | ClientHello) | = early_exporter_secret v (EC)DHE -> HKDF-Extract | v Handshake Secret | +---------> Derive-Secret(., "client handshake traffic secret", | ClientHello...ServerHello) | = client_handshake_traffic_secret | +---------> Derive-Secret(., "server handshake traffic secret", | ClientHello...ServerHello) | = server_handshake_traffic_secret | v 0 -> HKDF-Extract | v Master Secret | +---------> Derive-Secret(., "client application traffic secret", | ClientHello...Server Finished) | = client_traffic_secret_0 | +---------> Derive-Secret(., "server application traffic secret", | ClientHello...Server Finished) | = server_traffic_secret_0 | +---------> Derive-Secret(., "exporter master secret", | ClientHello...Server Finished) | = exporter_secret | +---------> Derive-Secret(., "resumption master secret", ClientHello...Client Finished) = resumption_secret
The general pattern here is that the secrets shown down the left side
of the diagram are just raw entropy without context, whereas the
secrets down the right side include handshake context and therefore
can be used to derive working keys without additional context.
Note that the different
calls to Derive-Secret may take different Messages arguments,
even with the same secret. In a 0-RTT exchange, Derive-Secret is
called with four distinct transcripts; in a 1-RTT only exchange
with three distinct transcripts.
If a given secret is not available, then the 0-value consisting of
a string of Hash.length zeroes is used. Note that this does not mean skipping
rounds, so if PSK is not in use Early Secret will still be
HKDF-Extract(0, 0). For the computation of the binder_secret, the label is "external
psk binder key" for external PSKs and "resumption psk binder key" for
resumption PSKs. The different labels prevents the substitution of one
type of PSK for the other.
There are multiple potential Early Secret values depending on
which PSK the server ultimately selects. The client will need to compute
one for each potential PSK; if no PSK is selected, it will then need to
compute the early secret corresponding to the zero PSK.
## Updating Traffic Keys and IVs {#updating-traffic-keys}
Once the handshake is complete, it is possible for either side to
update its sending traffic keys using the KeyUpdate handshake message
defined in {{key-update}}. The next generation of traffic keys is computed by
generating client_/server_traffic_secret_N+1 from
client_/server_traffic_secret_N as described in
this section then re-deriving the traffic keys as described in
{{traffic-key-calculation}}.
The next-generation traffic_secret is computed as:
traffic_secret_N+1 = HKDF-Expand-Label(
traffic_secret_N,
"application traffic secret", "", Hash.length)
Once client/server_traffic_secret_N+1 and its associated traffic keys have been computed,
implementations SHOULD delete client_/server_traffic_secret_N and its associated traffic
keys.
## Traffic Key Calculation
The traffic keying material is generated from the following input values:
* A secret value
* A purpose value indicating the specific value being generated
* The length of the key
The keying material is computed using:
key = HKDF-Expand-Label(Secret, purpose, "", key_length_
The following table describes the inputs to the key calculation for
each class of traffic keys:
| Record Type | Secret |
|:------------|--------|
| 0-RTT Application | client_early_traffic_secret |
| Handshake | [sender]_handshake_traffic_secret |
| Application Data | [sender]_traffic_secret_N |
The [sender] in this table denotes the sending side. The
following table indicates the purpose values for each type of key:
| Key Type | Purpose |
|:-----------------|:-------------------|
| key | "key" |
| iv | "iv" |
All the traffic keying material is recomputed whenever the
underlying Secret changes (e.g., when changing from the handshake to
application data keys or upon a key update).
### Diffie-Hellman
A conventional Diffie-Hellman computation is performed. The negotiated key (Z)
is converted to byte string by encoding in big-endian, padded with zeros up to
the size of the prime. This byte string is used as the shared secret, and is
used in the key schedule as specified above.
Note that this construction differs from previous versions of TLS which remove
leading zeros.
### Elliptic Curve Diffie-Hellman
For secp256r1, secp384r1 and secp521r1, ECDH calculations (including parameter
and key generation as well as the shared secret calculation) are
performed according to {{IEEE1363}} using the ECKAS-DH1 scheme with the identity
map as key derivation function (KDF), so that the shared secret is the
x-coordinate of the ECDH shared secret elliptic curve point represented
as an octet string. Note that this octet string (Z in IEEE 1363 terminology)
as output by FE2OSP, the Field Element to Octet String Conversion
Primitive, has constant length for any given field; leading zeros
found in this octet string MUST NOT be truncated.
(Note that this use of the identity KDF is a technicality. The
complete picture is that ECDH is employed with a non-trivial KDF
because TLS does not directly use this secret for anything
other than for computing other secrets.)
ECDH functions are used as follows:
* The public key to put into the KeyShareEntry.key_exchange structure is the
result of applying the ECDH function to the secret key of appropriate length
(into scalar input) and the standard public basepoint (into u-coordinate point
input).
* The ECDH shared secret is the result of applying ECDH function to the secret
key (into scalar input) and the peer's public key (into u-coordinate point
input). The output is used raw, with no processing.
For X25519 and X448, see {{RFC7748}}.
### Exporters
{{!RFC5705}} defines keying material exporters for TLS in terms of
the TLS PRF. This document replaces the PRF with HKDF, thus requiring
a new construction. The exporter interface remains the same. If context is
provided, the value is computed as:
HKDF-Expand-Label(Secret, label, context_value, key_length)
Where Secret is either the early_exporter_secret or the exporter_secret.
Implementations MUST use the exporter_secret unless explicitly specified
by the application. When adding TLS 1.3 to TLS 1.2 stacks, the exporter_secret
MUST be for the existing exporter interface.
If no context is provided, the value is computed as:
HKDF-Expand-Label(Secret, label, "", key_length)
Note that providing no context computes the same value as providing an empty
context. As of this document's publication, no allocated exporter label is used
with both modes. Future specifications MUST NOT provide an empty context and no
context with the same label and SHOULD provide a context, possibly empty, in
all exporter computations.
# Compliance Requirements
## MTI Cipher Suites
In the absence of an application profile standard specifying otherwise, a
TLS-compliant application MUST implement the TLS_AES_128_GCM_SHA256
cipher suite and SHOULD implement the TLS_AES_256_GCM_SHA384 and
TLS_CHACHA20_POLY1305_SHA256 cipher suites.
A TLS-compliant application MUST support digital signatures with
rsa_pkcs1_sha256 (for certificates), rsa_pss_sha256 (for
CertificateVerify and certificates), and ecdsa_secp256r1_sha256. A
TLS-compliant application MUST support key exchange with secp256r1
(NIST P-256) and SHOULD support key exchange with X25519 {{RFC7748}}.
## MTI Extensions
In the absence of an application profile standard specifying otherwise, a
TLS-compliant application MUST implement the following TLS extensions:
* Supported Versions ("supported_versions"; {{supported-versions}})
* Signature Algorithms ("signature_algorithms"; {{signature-algorithms}})
* Negotiated Groups ("supported_groups"; {{negotiated-groups}})
* Key Share ("key_share"; {{key-share}})
* Pre-Shared Key ("pre_shared_key"; {{pre-shared-key-extension}})
* Cookie ("cookie"; {{cookie}})
* Server Name Indication ("server_name"; Section 3 of {{RFC6066}})
All implementations MUST send and use these extensions when offering
applicable features:
* "supported_versions" is REQUIRED for all ClientHello messages.
* "signature_algorithms" is REQUIRED for certificate authentication.
* "supported_groups" and "key_share" are REQUIRED for DHE or ECDHE key exchange.
* "pre_shared_key" is REQUIRED for PSK key agreement.
A client is considered to be attempting to negotiate using this
specification if the ClientHello contains a "supported_versions"
extension with a version indicating TLS 1.3. Such a ClientHello message
MUST meet the following requirements:
* If not containing a "pre_shared_key" extension, it MUST contain both
a "signature_algorithms" extension and a "supported_groups" extension.
* If containing a "supported_groups" extension, it MUST also contain a
"key_share" extension, and vice versa. (an empty KeyShare.client_shares
vector is permitted)
Servers receiving a ClientHello which does not conform to these
requirements MUST abort the handshake with a "missing_extension"
alert.
Additionally, all implementations MUST support use of the "server_name"
extension with applications capable of using it.
Servers MAY require clients to send a valid "server_name" extension.
Servers requiring this extension SHOULD respond to a ClientHello
lacking a "server_name" extension by terminating the connection with a
"missing_extension" alert.
# Security Considerations
Security issues are discussed throughout this memo, especially in Appendices B,
C, and D.
# IANA Considerations
This document uses several registries that were originally created in
{{RFC4346}}. IANA has updated these to reference this document. The registries
and their allocation policies are below:
- TLS Cipher Suite Registry: Values with the first byte in the range
0-254 (decimal) are assigned via Specification Required {{RFC2434}}.
Values with the first byte 255 (decimal) are reserved for Private
Use {{RFC2434}}.
IANA [SHALL add/has added] the cipher suites listed in {{cipher-suites}} to
the registry. The "Value" and "Description" columns are taken from the table.
The "DTLS-OK" and "Recommended" columns are both marked as "Yes" for each new
cipher suite. [[This assumes {{?I-D.sandj-tls-iana-registry-updates}} has been
applied.]]
- TLS ContentType Registry: Future values are allocated via
Standards Action {{RFC2434}}.
- TLS Alert Registry: Future values are allocated via Standards
Action {{RFC2434}}.
- TLS HandshakeType Registry: Future values are allocated via
Standards Action {{RFC2434}}. IANA [SHALL update/has updated] this registry
to rename item 4 from "NewSessionTicket" to "new_session_ticket".
This document also uses a registry originally created in {{RFC4366}}. IANA has
updated it to reference this document. The registry and its allocation policy
is listed below:
- TLS ExtensionType Registry: Values with the first byte in the range
0-254 (decimal) are assigned via Specification Required {{RFC2434}}.
Values with the first byte 255 (decimal) are reserved for Private
Use {{RFC2434}}. IANA [SHALL update/has updated]
this registry to include the "key_share", "pre_shared_key", and
"early_data" extensions as defined in this document.
IANA [shall update/has updated] this registry to add a
"Recommended" column. IANA [shall/has] initially populated this
column with the values in the table below. This table has been generated
by marking Standards Track RFCs as "Yes" and all others as
"No".
IANA [shall update/has updated] this registry to include a "TLS
1.3" column with the following six values: "Client", indicating
that the server shall not send them. "Clear", indicating
that they shall be in the ServerHello. "Encrypted", indicating that
they shall be in the EncryptedExtensions block, "Certificate" indicating that
they shall be in the Certificate block, "Ticket" indicating that they
can appear in the NewSessionTicket message (only) and "No" indicating
that they are not used in TLS 1.3. This column [shall be/has been]
initially populated with the values in this document.
IANA [shall update/has updated] this registry to include a
"HelloRetryRequest" column with the following two values: "Yes", indicating
it may be sent in HelloRetryRequest, and "No", indicating it may not be sent
in HelloRetryRequest. This column [shall be/has been] initially populated
with the values in this document.
| Extension | Recommended | TLS 1.3 | HelloRetryRequest |
|:-----------------------------------------|------------:|------------:|------------------:|
| server_name [RFC6066] | Yes | Encrypted | No |
| max_fragment_length [RFC6066] | Yes | Encrypted | No |
| client_certificate_url [RFC6066] | Yes | Encrypted | No |
| trusted_ca_keys [RFC6066] | Yes | Encrypted | No |
| truncated_hmac [RFC6066] | Yes | No | No |
| status_request [RFC6066] | Yes | Certificate | No |
| user_mapping [RFC4681] | Yes | Encrypted | No |
| client_authz [RFC5878] | No | No | No |
| server_authz [RFC5878] | No | No | No |
| cert_type [RFC6091] | Yes | Encrypted | No |
| supported_groups [RFC7919] | Yes | Encrypted | No |
| ec_point_formats [RFC4492] | Yes | No | No |
| srp [RFC5054] | No | No | No |
| signature_algorithms [RFC5246] | Yes | Clear | No |
| use_srtp [RFC5764] | Yes | Encrypted | No |
| heartbeat [RFC6520] | Yes | Encrypted | No |
| application_layer_protocol_negotiation [RFC7301] | Yes | Encrypted | No |
| status_request_v2 [RFC6961] | Yes | Certificate | No |
| signed_certificate_timestamp [RFC6962] | No | Certificate | No |
| client_certificate_type [RFC7250] | Yes | Encrypted | No |
| server_certificate_type [RFC7250] | Yes | Certificate | No |
| padding [RFC7685] | Yes | Client | No |
| encrypt_then_mac [RFC7366] | Yes | No | No |
| extended_master_secret [RFC7627] | Yes | No | No |
| SessionTicket TLS [RFC4507] | Yes | No | No |
| renegotiation_info [RFC5746] | Yes | No | No |
| key_share [[this document]] | Yes | Clear | Yes |
| pre_shared_key [[this document]] | Yes | Clear | No |
| psk_key_exchange_modes [[this document]] | Yes | Client | No |
| early_data [[this document]] | Yes | Encrypted | No |
| cookie [[this document]] | Yes | Client | Yes |
| supported_versions [[this document]] | Yes | Client | No |
| ticket_early_data_info [[this document]] | Yes | Ticket | No |
In addition, this document defines two new registries to be maintained
by IANA
- TLS SignatureScheme Registry: Values with the first byte in the range
0-254 (decimal) are assigned via Specification Required {{RFC2434}}.
Values with the first byte 255 (decimal) are reserved for Private
Use {{RFC2434}}. Values with the first byte in the range 0-6 or with the
second byte in the range 0-3 that are not currently allocated are reserved for
backwards compatibility.
This registry SHALL have a "Recommended" column.
The registry [shall be/ has been] initially populated with the values described in
{{signature-algorithms}}. The following values SHALL be marked as
"Recommended": ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384,
rsa_pss_sha256, rsa_pss_sha384, rsa_pss_sha512, ed25519.
Finally, this document obsoletes the TLS HashAlgorithm Registry and the TLS
SignatureAlgorithm Registry, both originally created in {{RFC5246}}. IANA
[SHALL update/has updated] the TLS HashAlgorithm Registry to list values 7-223 as
"Reserved" and the TLS SignatureAlgorithm Registry to list values 4-233 as
"Reserved".
--- back
# Protocol Data Structures and Constant Values
This section describes protocol types and constants. Values listed as
_RESERVED were used in previous versions of TLS and are listed here
for completeness. TLS 1.3 implementations MUST NOT send them but
might receive them from older TLS implementations.
%%## Record Layer
%%## Alert Messages
%%## Handshake Protocol
%%### Key Exchange Messages
%%#### Version Extension
%%#### Cookie Extension
%%#### Signature Algorithm Extension
%%#### Supported Groups Extension
Values within "obsolete_RESERVED" ranges were used in previous versions
of TLS and MUST NOT be offered or negotiated by TLS 1.3 implementations.
The obsolete curves have various known/theoretical weaknesses or have
had very little usage, in some cases only due to unintentional
server configuration issues. They are no longer considered appropriate
for general use and should be assumed to be potentially unsafe. The set
of curves specified here is sufficient for interoperability with all
currently deployed and properly configured TLS implementations.
#### Deprecated Extensions
The following extensions are no longer applicable to TLS 1.3, although
TLS 1.3 clients MAY send them if they are willing to negotiate them
with prior versions of TLS. TLS 1.3 servers MUST ignore these
extensions if they are negotiating TLS 1.3:
truncated_hmac {{RFC6066}},
srp {{RFC5054}},
encrypt_then_mac {{RFC7366}},
extended_master_secret {{RFC7627}},
SessionTicket {{RFC5077}},
and renegotiation_info {{RFC5746}}.
%%### Server Parameters Messages
%%### Authentication Messages
%%### Ticket Establishment
%%### Updating Keys
## Cipher Suites
A symmetric cipher suite defines the pair of the AEAD algorithm and hash
algorithm to be used with HKDF.
Cipher suite names follow the naming convention:
~~~
CipherSuite TLS_AEAD_HASH = VALUE;
~~~
| Component | Contents |
|:----------|:---------|
| TLS | The string "TLS" |
| AEAD | The AEAD algorithm used for record protection |
| HASH | The hash algorithm used with HKDF |
| VALUE | The two byte ID assigned for this cipher suite |
This specification defines the following cipher suites for use with TLS 1.3.
| Description | Value |
|:--------------------------------|:------------|
| TLS_AES_128_GCM_SHA256 | {0x13,0x01} |
| TLS_AES_256_GCM_SHA384 | {0x13,0x02} |
| TLS_CHACHA20_POLY1305_SHA256 | {0x13,0x03} |
| TLS_AES_128_CCM_SHA256 | {0x13,0x04} |
| TLS_AES_128_CCM_8_SHA256 | {0x13,0x05} |
The corresponding AEAD algorithms AEAD_AES_128_GCM, AEAD_AES_256_GCM, and
AEAD_AES_128_CCM are defined in {{RFC5116}}. AEAD_CHACHA20_POLY1305 is defined
in {{RFC7539}}. AEAD_AES_128_CCM_8 is defined in {{RFC6655}}. The corresponding
hash algorithms are defined in {{SHS}}.
Although TLS 1.3 uses the same cipher suite space as previous versions
of TLS, TLS 1.3 cipher suites are defined differently, only specifying
the symmetric ciphers, and cannot be used for TLS 1.2. Similarly,
TLS 1.2 and lower cipher suites cannot be used with TLS 1.3.
New cipher suite values are assigned by IANA as described in
{{iana-considerations}}.
# Implementation Notes
The TLS protocol cannot prevent many common security mistakes. This section
provides several recommendations to assist implementors.
## API considerations for 0-RTT
0-RTT data has very different security properties from data
transmitted after a completed handshake: it can be
replayed. Implementations SHOULD provide different functions for
reading and writing 0-RTT data and data transmitted after the
handshake, and SHOULD NOT automatically resend 0-RTT data if it is
rejected by the server.
## Random Number Generation and Seeding
TLS requires a cryptographically secure pseudorandom number generator (PRNG).
In most cases, the operating system provides an appropriate facility such
as /dev/urandom, which should be used absent other (performance) concerns.
It is generally preferable to use an existing PRNG implementation in
preference to crafting a new one, and many adequate cryptographic libraries
are already available under favorable license terms. Should those prove
unsatisfactory, {{RFC4086}} provides guidance on the generation of random values.
## Certificates and Authentication
Implementations are responsible for verifying the integrity of certificates and
should generally support certificate revocation messages. Certificates should
always be verified to ensure proper signing by a trusted Certificate Authority
(CA). The selection and addition of trusted CAs should be done very carefully.
Users should be able to view information about the certificate and root CA.
## Cipher Suite Support
TLS supports a range of key sizes and security levels, including some that
provide no or minimal security. A proper implementation will probably not
support many cipher suites. Applications SHOULD also enforce minimum and
maximum key sizes. For example, certification paths containing keys or
signatures weaker than 2048-bit RSA or 224-bit ECDSA are not appropriate
for secure applications.
See also {{backwards-compatibility-security-restrictions}}.
## Implementation Pitfalls
Implementation experience has shown that certain parts of earlier TLS
specifications are not easy to understand, and have been a source of
interoperability and security problems. Many of these areas have been clarified
in this document, but this appendix contains a short list of the most important
things that require special attention from implementors.
TLS protocol issues:
- Do you correctly handle handshake messages that are fragmented to
multiple TLS records (see {{record-layer}})? Including corner cases
like a ClientHello that is split to several small fragments? Do
you fragment handshake messages that exceed the maximum fragment
size? In particular, the certificate and certificate request
handshake messages can be large enough to require fragmentation.
- Do you ignore the TLS record layer version number in all TLS
records? (see {{backward-compatibility}})
- Have you ensured that all support for SSL, RC4, EXPORT ciphers, and
MD5 (via the "signature_algorithm" extension) is completely removed from
all possible configurations that support TLS 1.3 or later, and that
attempts to use these obsolete capabilities fail correctly?
(see {{backward-compatibility}})
- Do you handle TLS extensions in ClientHello correctly, including
unknown extensions.
- When the server has requested a client certificate, but no
suitable certificate is available, do you correctly send an empty
Certificate message, instead of omitting the whole message (see
{{client-certificate-selection}})?
- When processing the plaintext fragment produced by AEAD-Decrypt and
scanning from the end for the ContentType, do you avoid scanning
past the start of the cleartext in the event that the peer has sent
a malformed plaintext of all-zeros?
- Do you properly ignore unrecognized cipher suites ({{client-hello}}),
hello extensions ({{hello-extensions}}), named groups ({{negotiated-groups}}),
and signature algorithms ({{signature-algorithms}})?
Cryptographic details:
- What countermeasures do you use to prevent timing attacks {{TIMING}}?
- When verifying RSA signatures, do you accept both NULL and missing parameters?
Do you verify that the RSA padding
doesn't have additional data after the hash value? {{FI06}}
- When using Diffie-Hellman key exchange, do you correctly preserve
leading zero bytes in the negotiated key (see {{diffie-hellman}})?
- Does your TLS client check that the Diffie-Hellman parameters sent
by the server are acceptable, (see {{ffdhe-param}})?
- Do you use a strong and, most importantly, properly seeded random number
generator (see {{random-number-generation-and-seeding}}) when generating Diffie-Hellman
private values, the ECDSA "k" parameter, and other security-critical values?
It is RECOMMENDED that implementations implement "deterministic ECDSA"
as specified in {{!RFC6979}}.
- Do you zero-pad Diffie-Hellman public key values to the group size (see
{{ffdhe-param}})?
- Do you verify signatures after making them to protect against RSA-CRT
key leaks? {{FW15}}
## Client Tracking Prevention
Clients SHOULD NOT reuse a session ticket for multiple connections. Reuse
of a session ticket allows passive observers to correlate different connections.
Servers that issue session tickets SHOULD offer at least as many session tickets
as the number of connections that a client might use; for example, a web browser
using HTTP/1.1 {{RFC7230}} might open six connections to a server. Servers SHOULD
issue new session tickets with every connection. This ensures that clients are
always able to use a new session ticket when creating a new connection.
## Unauthenticated Operation
Previous versions of TLS offered explicitly unauthenticated cipher suites based
on anonymous Diffie-Hellman. These modes have been deprecated in TLS 1.3.
However, it is still possible to negotiate parameters that do not provide
verifiable server authentication by several methods, including:
- Raw public keys {{RFC7250}}.
- Using a public key contained in a certificate but without
validation of the certificate chain or any of its contents.
Either technique used alone is vulnerable to man-in-the-middle attacks
and therefore unsafe for general use. However, it is also possible to
bind such connections to an external authentication mechanism via
out-of-band validation of the server's public key, trust on first
use, or channel bindings {{RFC5929}}. [[NOTE: TLS 1.3 needs a new
channel binding definition that has not yet been defined.]]
If no such mechanism is used, then the connection has no protection
against active man-in-the-middle attack; applications MUST NOT use TLS
in such a way absent explicit configuration or a specific application
profile.
# Backward Compatibility
The TLS protocol provides a built-in mechanism for version negotiation between
endpoints potentially supporting different versions of TLS.
TLS 1.x and SSL 3.0 use compatible ClientHello messages. Servers can also handle
clients trying to use future versions of TLS as long as the ClientHello format
remains compatible and the client supports the highest protocol version available
in the server.
Prior versions of TLS used the record layer version number for various
purposes. (TLSPlaintext.legacy_record_version & TLSCiphertext.legacy_record_version)
As of TLS 1.3, this field is deprecated and its value MUST be ignored by all
implementations. Version negotiation is performed using only the handshake versions.
(ClientHello.legacy_version,
ClientHello "supported_versions" extension & ServerHello.version)
In order to maximize interoperability with older endpoints, implementations
that negotiate the use of TLS 1.0-1.2 SHOULD set the record layer
version number to the negotiated version for the ServerHello and all
records thereafter.
For maximum compatibility with previously non-standard behavior and misconfigured
deployments, all implementations SHOULD support validation of certification paths
based on the expectations in this document, even when handling prior TLS versions'
handshakes. (see {{server-certificate-selection}})
TLS 1.2 and prior supported an "Extended Master Secret" {{?RFC7627}} extension
which digested large parts of the handshake transcript into the master secret.
Because TLS 1.3 always hashes in the transcript up to the server CertificateVerify,
implementations which support both TLS 1.3 and earlier versions SHOULD
indicate the use of the Extended Master Secret extension in their APIs
whenever TLS 1.3 is used.
## Negotiating with an older server
A TLS 1.3 client who wishes to negotiate with such older servers will send a
normal TLS 1.3 ClientHello containing 0x0303 (TLS 1.2) in
ClientHello.legacy_version but with the correct version in the
"supported_versions" extension. If the server does not support TLS 1.3 it
will respond with a ServerHello containing an older version number. If the
client agrees to use this version, the negotiation will proceed as appropriate
for the negotiated protocol. A client resuming a session SHOULD initiate the
connection using the version that was previously negotiated.
Note that 0-RTT data is not compatible with older servers.
See {{zero-rtt-backwards-compatibility}}.
If the version chosen by the server is not supported by the client (or not
acceptable), the client MUST abort the handshake with a "protocol_version" alert.
If a TLS server receives a ClientHello containing a version number greater than
the highest version supported by the server, it MUST reply according to the
highest version supported by the server.
Some legacy server implementations are known to not implement the TLS
specification properly and might abort connections upon encountering
TLS extensions or versions which it is not aware of. Interoperability
with buggy servers is a complex topic beyond the scope of this document.
Multiple connection attempts may be required in order to negotiate
a backwards compatible connection, however this practice is vulnerable
to downgrade attacks and is NOT RECOMMENDED.
## Negotiating with an older client
A TLS server can also receive a ClientHello indicating a version number smaller
than its highest supported version. If the "supported_versions" extension
is present, the server MUST negotiate the highest server-supported version
found in that extension. If the "supported_versions" extension is not
present, the server MUST negotiate the minimum of ClientHello.legacy_version
and TLS 1.2.For example, if
the server supports TLS 1.0, 1.1, and 1.2, and legacy_version is TLS 1.0, the
server will proceed with a TLS 1.0 ServerHello. If the server only supports
versions greater than ClientHello.legacy_version, it MUST abort the handshake
with a "protocol_version" alert.
Note that earlier versions of TLS did not clearly specify the record layer
version number value in all cases (TLSPlaintext.legacy_record_version). Servers
will receive various TLS 1.x versions in this field, however its value
MUST always be ignored.
## Zero-RTT backwards compatibility
0-RTT data is not compatible with older servers. An older server will respond
to the ClientHello with an older ServerHello, but it will not correctly skip
the 0-RTT data and fail to complete the handshake. This can cause issues when
a client attempts to use 0-RTT, particularly against multi-server deployments. For
example, a deployment could deploy TLS 1.3 gradually with some servers
implementing TLS 1.3 and some implementing TLS 1.2, or a TLS 1.3 deployment
could be downgraded to TLS 1.2.
A client that attempts to send 0-RTT data MUST fail a connection if it receives
a ServerHello with TLS 1.2 or older. A client that attempts to repair this
error SHOULD NOT send a TLS 1.2 ClientHello, but instead send a TLS 1.3
ClientHello without 0-RTT data.
To avoid this error condition, multi-server deployments SHOULD ensure a uniform
and stable deployment of TLS 1.3 without 0-RTT prior to enabling 0-RTT.
## Backwards Compatibility Security Restrictions
If an implementation negotiates use of TLS 1.2, then negotiation of cipher
suites also supported by TLS 1.3 SHOULD be preferred, if available.
The security of RC4 cipher suites is considered insufficient for the reasons
cited in {{RFC7465}}. Implementations MUST NOT offer or negotiate RC4 cipher suites
for any version of TLS for any reason.
Old versions of TLS permitted the use of very low strength ciphers.
Ciphers with a strength less than 112 bits MUST NOT be offered or
negotiated for any version of TLS for any reason.
The security of SSL 2.0 {{SSL2}} is considered insufficient for the reasons enumerated
in {{RFC6176}}, and MUST NOT be negotiated for any reason.
Implementations MUST NOT send an SSL version 2.0 compatible CLIENT-HELLO.
Implementations MUST NOT negotiate TLS 1.3 or later using an SSL version 2.0 compatible
CLIENT-HELLO. Implementations are NOT RECOMMENDED to accept an SSL version 2.0 compatible
CLIENT-HELLO in order to negotiate older versions of TLS.
Implementations MUST NOT send or accept any records with a version less than 0x0300.
The security of SSL 3.0 {{SSL3}} is considered insufficient for the reasons enumerated
in {{RFC7568}}, and MUST NOT be negotiated for any reason.
Implementations MUST NOT send a ClientHello.legacy_version or ServerHello.version
set to 0x0300 or less. Any endpoint receiving a Hello message with
ClientHello.legacy_version or ServerHello.version set to 0x0300 MUST
abort the handshake with a "protocol_version" alert.
Implementations MUST NOT use the Truncated HMAC extension, defined in
Section 7 of [RFC6066], as it is not applicable to AEAD algorithms and has
been shown to be insecure in some scenarios.
# Overview of Security Properties {#security-analysis}
[[TODO: This section is still a WIP and needs a bunch more work.]]
A complete security analysis of TLS is outside the scope of this document.
In this section, we provide an informal description the desired properties
as well as references to more detailed work in the research literature
which provides more formal definitions.
We cover properties of the handshake separately from those of the record layer.
## Handshake {#security-handshake}
The TLS handshake is an Authenticated Key Exchange (AKE) protocol which
is intended to provide both one-way authenticated (server-only) and
mutually authenticated (client and server) functionality. At the completion
of the handshake, each side outputs its view on the following values:
- A "session key" (the master secret) from which can be derived a set of working keys.
- A set of cryptographic parameters (algorithms, etc.)
- The identities of the communicating parties.
We assume that the attacker has complete control of the network in
between the parties {{RFC3552}}. Even under these conditions, the
handshake should provide the properties listed below. Note that
these properties are not necessarily independent, but reflect
the protocol consumers' needs.
Establishing the same session key.
: The handshake needs to output the same session key on both sides of the
handshake, provided that it completes successfully on each endpoint
(See {{CK01}}; defn 1, part 1).
Secrecy of the session key.
: The shared session key should be known only to the communicating
parties, not to the attacker (See {{CK01}}; defn 1, part 2). Note that
in a unilaterally authenticated connection, the attacker can establish
its own session keys with the server, but those session keys are
distinct from those established by the client.
Peer Authentication.
: The client's view of the peer identity should reflect the server's
identity. If the client is authenticated, the server's view of the
peer identity should match the client's identity.
Uniqueness of the session key:
: Any two distinct handshakes should produce distinct, unrelated session
keys {::comment}TODO{:/comment}
Downgrade protection.
: The cryptographic parameters should be the same on both sides and
should be the same as if the peers had been communicating in the
absence of an attack (See {{BBFKZG16}}; defns 8 and 9}).
Forward secret
: If the long-term keying material (in this case the signature keys in certificate-based
authentication modes or the PSK in PSK-(EC)DHE modes) are compromised after
the handshake is complete, this does not compromise the security of the
session key (See {{DOW92}}).
Protection of endpoint identities.
: The server's identity (certificate) should be protected against passive
attackers. The client's identity should be protected against both passive
and active attackers.
{:br}
Informally, the signature-based modes of TLS 1.3 provide for the
establishment of a unique, secret, shared, key established by an
(EC)DHE key exchange and authenticated by the server's signature over
the handshake transcript, as well as tied to the server's identity by
a MAC. If the client is authenticated by a certificate, it also signs
over the handshake transcript and provides a MAC tied to both
identities. {{SIGMA}} describes the analysis of this type of key
exchange protocol. If fresh (EC)DHE keys are used for each connection,
then the output keys are forward secret.
The PSK and resumption-PSK modes bootstrap from a long-term shared
secret into a unique per-connection short-term session key. This
secret may have been established in a previous handshake. If
PSK-(EC)DHE modes are used, this session key will also be forward
secret. The resumption-PSK mode has been designed so that the
resumption master secret computed by connection N and needed to form
connection N+1 is separate from the traffic keys used by connection N,
thus providing forward secrecy between the connections.
The PSK binder value forms a binding between a PSK
and the current handshake, as well as between the session where the
PSK was established and the session where it was used. This binding
transitively includes the original handshake transcript, because that
transcript is digested into the values which produce the Resumption
Master Secret. This requires that both the KDF used to produce the RMS
and the MAC used to compute the binder be collision
resistant. These are properties of HKDF and HMAC respectively when
used with collision resistant hash functions and producing output of
at least 256 bits. Any future replacement of these functions MUST
also provide collision resistance.
Note: The binder does not cover the binder values from other
PSKs, though they are included in the Finished MAC.
If an exporter is used, then it produces values which are unique
and secret (because they are generated from a unique session key).
Exporters computed with different labels and contexts are computationally
independent, so it is not feasible to compute one from another or
the session secret from the exported value. Note: exporters can
produce arbitrary-length values. If exporters are to be
used as channel bindings, the exported value MUST be large
enough to provide collision resistance. The exporters provided in
TLS 1.3 are derived from the same handshake contexts as the
early traffic keys and the application traffic keys respectively,
and thus have similar security properties. Note that they do
not include the client's certificate; future applications
which wish to bind to the client's certificate may need
to define a new exporter that includes the full handshake
transcript.
For all handshake modes, the Finished MAC (and where present, the
signature), prevents downgrade attacks. In addition, the use of
certain bytes in the random nonces as described in {{server-hello}}
allows the detection of downgrade to previous TLS versions.
As soon as the client and the server have exchanged enough information
to establish shared keys, the remainder of the handshake is encrypted,
thus providing protection against passive attackers. Because the server
authenticates before the client, the client can ensure that it only
reveals its identity to an authenticated server. Note that implementations
must use the provided record padding mechanism during the handshake
to avoid leaking information about the identities due to length.
The 0-RTT mode of operation generally provides the same security
properties as 1-RTT data, with the two exceptions that the 0-RTT
encryption keys do not provide full forward secrecy and that the
the server is not able to guarantee full uniqueness of the handshake
(non-replayability) without keeping potentially undue amounts of
state. See {{early-data-indication}} for one mechanism to limit
the exposure to replay.
The reader should refer to the following references for analysis of the
TLS handshake {{CHSV16}} {{FGSW16}} {{LXZFH16}}.
## Record Layer {#security-record-layer}
The record layer depends on the handshake producing a strong session
key which can be used to derive bidirectional traffic keys and nonces.
Assuming that is true, and the keys are used for no more data than
indicated in {{limits-on-key-usage}} then the record layer should provide the following
guarantees:
Confidentiality.
: An attacker should not be able to determine the plaintext contents
of a given record.
{::comment}Cite IND-CPA?{:/comment}
Integrity.
: An attacker should not be able to craft a new record which is
different from an existing record which will be accepted by the receiver.
{::comment}Cite INT-CTXT?{:/comment}
Order protection/non-replayability
: An attacker should not be able to cause the receiver to accept a
record which it has already accepted or cause the receiver to accept
record N+1 without having first processed record N.
[[TODO: If we merge in DTLS to this document, we will need to update
this guarantee.]]
Length concealment.
: Given a record with a given external length, the attacker should not be able
to determine the amount of the record that is content versus padding.
Forward security after key change.
: If the traffic key update mechanism described in {{key-update}} has been
used and the previous generation key is deleted, an attacker who compromises
the endpoint should not be able to decrypt traffic encrypted with the old key.
{:br}
Informally, TLS 1.3 provides these properties by AEAD-protecting the
plaintext with a strong key. AEAD encryption {{RFC5116}} provides confidentiality
and integrity for the data. Non-replayability is provided by using
a separate nonce for each record, with the nonce being derived from
the record sequence number ({{nonce}}), with the sequence
number being maintained independently at both sides thus records which
are delivered out of order result in AEAD deprotection failures.
The plaintext protected by the AEAD function consists of content plus
variable-length padding. Because the padding is also encrypted, the
attacker cannot directly determine the length of the padding, but
may be able to measure it indirectly by the use of timing channels
exposed during record processing (i.e., seeing how long it takes to
process a record). In general, it is not known how to remove this
type of channel because even a constant time padding removal
function will then feed the content into data-dependent functions.
Generation N+1 keys are derived from generation N keys via a key
derivation function {{updating-traffic-keys}}. As long as this function is truly one way, it
is not possible to compute the previous keys after a key change
(forward secrecy). However, TLS does not provide security for
data which is sent after the traffic secret is compromised,
even after a key update (backward secrecy); systems which want backward secrecy must do
a fresh handshake and establish a new session key with an (EC)DHE
exchange.
The reader should refer to the following references for analysis of the
TLS record layer.
# Working Group Information
The discussion list for the IETF TLS working group is located at the e-mail
address <tls@ietf.org>. Information on the group and information on how to
subscribe to the list is at <https://www.ietf.org/mailman/listinfo/tls>
Archives of the list can be found at:
<https://www.ietf.org/mail-archive/web/tls/current/index.html>
# Contributors
* Martin Abadi \\
University of California, Santa Cruz \\
abadi@cs.ucsc.edu
* Christopher Allen (co-editor of TLS 1.0) \\
Alacrity Ventures \\
ChristopherA@AlacrityManagement.com
* Steven M. Bellovin \\
Columbia University \\
smb@cs.columbia.edu
* David Benjamin \\
Google \\
davidben@google.com
* Benjamin Beurdouche
* Karthikeyan Bhargavan (co-author of [RFC7627]) \\
INRIA \\
karthikeyan.bhargavan@inria.fr
* Simon Blake-Wilson (co-author of [RFC4492]) \\
BCI \\
sblakewilson@bcisse.com
* Nelson Bolyard (co-author of [RFC4492]) \\
Sun Microsystems, Inc. \\
nelson@bolyard.com
* Ran Canetti \\
IBM \\
canetti@watson.ibm.com
* Pete Chown \\
Skygate Technology Ltd \\
pc@skygate.co.uk
* Antoine Delignat-Lavaud (co-author of [RFC7627]) \\
INRIA \\
antoine.delignat-lavaud@inria.fr
* Tim Dierks (co-editor of TLS 1.0, 1.1, and 1.2) \\
Independent \\
tim@dierks.org
* Taher Elgamal \\
Securify \\
taher@securify.com
* Pasi Eronen \\
Nokia \\
pasi.eronen@nokia.com
* Cedric Fournet \\
Microsoft \\
fournet@microsoft.com
* Anil Gangolli \\
anil@busybuddha.org
* David M. Garrett \\
dave@nulldereference.com
* Vipul Gupta (co-author of [RFC4492]) \\
Sun Microsystems Laboratories \\
vipul.gupta@sun.com
* Chris Hawk (co-author of [RFC4492]) \\
Corriente Networks LLC \\
chris@corriente.net
* Kipp Hickman
* Alfred Hoenes
* David Hopwood \\
Independent Consultant \\
david.hopwood@blueyonder.co.uk
* Subodh Iyengar \\
Facebook \\
subodh@fb.com
* Daniel Kahn Gillmor \\
ACLU \\
dkg@fifthhorseman.net
* Hubert Kario \\
Red Hat Inc. \\
hkario@redhat.com
* Phil Karlton (co-author of SSL 3.0)
* Paul Kocher (co-author of SSL 3.0) \\
Cryptography Research \\
paul@cryptography.com
* Hugo Krawczyk \\
IBM \\
hugo@ee.technion.ac.il
* Adam Langley (co-author of [RFC7627]) \\
Google \\
agl@google.com
* Xiaoyin Liu \\
University of North Carolina at Chapel Hill \\
xiaoyin.l@outlook.com
* Ilari Liusvaara \\
Independent \\
ilariliusvaara@welho.com
* Jan Mikkelsen \\
Transactionware \\
janm@transactionware.com
* Bodo Moeller (co-author of [RFC4492]) \\
Google \\
bodo@openssl.org
* Erik Nygren \\
Akamai Technologies \\
erik+ietf@nygren.org
* Magnus Nystrom \\
Microsoft \\
mnystrom@microsoft.com
* Alfredo Pironti (co-author of [RFC7627]) \\
INRIA \\
alfredo.pironti@inria.fr
* Andrei Popov \\
Microsoft \\
andrei.popov@microsoft.com
* Marsh Ray (co-author of [RFC7627]) \\
Microsoft \\
maray@microsoft.com
* Robert Relyea \\
Netscape Communications \\
relyea@netscape.com
* Kyle Rose \\
Akamai Technologies \\
krose@krose.org
* Jim Roskind \\
Netscape Communications \\
jar@netscape.com
* Michael Sabin
* Dan Simon \\
Microsoft, Inc. \\
dansimon@microsoft.com
* Nick Sullivan \\
CloudFlare Inc. \\
nick@cloudflare.com
* Bjoern Tackmann \\
University of California, San Diego \\
btackmann@eng.ucsd.edu
* Martin Thomson \\
Mozilla \\
mt@mozilla.com
* Filippo Valsorda \\
CloudFlare Inc. \\
filippo@cloudflare.com
* Tom Weinstein
* Hoeteck Wee \\
Ecole Normale Superieure, Paris \\
hoeteck@alum.mit.edu
* Tim Wright \\
Vodafone \\
timothy.wright@vodafone.com
* Kazu Yamamoto \\
Internet Initiative Japan Inc. \\
kazu@iij.ad.jp