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Tor Rendezvous Specification - Version 3
This document specifies how the hidden service version 3 protocol works. This
text used to be proposal 224-rend-spec-ng.txt.
Table of contents:
0. Hidden services: overview and preliminaries.
0.1. Improvements over previous versions.
0.2. Notation and vocabulary
0.3. Cryptographic building blocks
0.4. Protocol building blocks [BUILDING-BLOCKS]
0.5. Assigned relay cell types
0.6. Acknowledgments
1. Protocol overview
1.1. View from 10,000 feet
1.2. In more detail: naming hidden services [NAMING]
1.3. In more detail: Access control [IMD:AC]
1.4. In more detail: Distributing hidden service descriptors. [IMD:DIST]
1.5. In more detail: Scaling to multiple hosts
1.6. In more detail: Backward compatibility with older hidden service
1.7. In more detail: Keeping crypto keys offline
1.8. In more detail: Encryption Keys And Replay Resistance
1.9. In more detail: A menagerie of keys
1.9.1. In even more detail: Client authorization [CLIENT-AUTH]
2. Generating and publishing hidden service descriptors [HSDIR]
2.1. Deriving blinded keys and subcredentials [SUBCRED]
2.2. Locating, uploading, and downloading hidden service descriptors
2.2.1. Dividing time into periods [TIME-PERIODS]
2.2.2. When to publish a hidden service descriptor [WHEN-HSDESC]
2.2.3. Where to publish a hidden service descriptor [WHERE-HSDESC]
2.2.4. Using time periods and SRVs to fetch/upload HS descriptors
2.2.5. Expiring hidden service descriptors [EXPIRE-DESC]
2.2.6. URLs for anonymous uploading and downloading
2.3. Publishing shared random values [PUB-SHAREDRANDOM]
2.3.1. Client behavior in the absense of shared random values
2.3.2. Hidden services and changing shared random values
2.4. Hidden service descriptors: outer wrapper [DESC-OUTER]
2.5. Hidden service descriptors: encryption format [HS-DESC-ENC]
2.5.1. First layer of encryption [HS-DESC-FIRST-LAYER]
2.5.1.1. First layer encryption logic
2.5.1.2. First layer plaintext format
2.5.1.3. Client behavior
2.5.1.4. Obfuscating the number of authorized clients
2.5.2. Second layer of encryption [HS-DESC-SECOND-LAYER]
2.5.2.1. Second layer encryption keys
2.5.2.2. Second layer plaintext format
2.5.3. Deriving hidden service descriptor encryption keys [HS-DESC-ENCRYPTION-KEYS]
3. The introduction protocol [INTRO-PROTOCOL]
3.1. Registering an introduction point [REG_INTRO_POINT]
3.1.1. Extensible ESTABLISH_INTRO protocol. [EST_INTRO]
3.1.1.1. Denial-of-Server Defense Extension. [EST_INTRO_DOS_EXT]
3.1.2. Registering an introduction point on a legacy Tor node [LEGACY_EST_INTRO]
3.1.3. Acknowledging establishment of introduction point [INTRO_ESTABLISHED]
3.2. Sending an INTRODUCE1 cell to the introduction point. [SEND_INTRO1]
3.2.1. INTRODUCE1 cell format [FMT_INTRO1]
3.2.2. INTRODUCE_ACK cell format. [INTRO_ACK]
3.3. Processing an INTRODUCE2 cell at the hidden service. [PROCESS_INTRO2]
3.3.1. Introduction handshake encryption requirements [INTRO-HANDSHAKE-REQS]
3.3.2. Example encryption handshake: ntor with extra data [NTOR-WITH-EXTRA-DATA]
3.4. Authentication during the introduction phase. [INTRO-AUTH]
3.4.1. Ed25519-based authentication.
4. The rendezvous protocol
4.1. Establishing a rendezvous point [EST_REND_POINT]
4.2. Joining to a rendezvous point [JOIN_REND]
4.2.1. Key expansion
4.3. Using legacy hosts as rendezvous points
5. Encrypting data between client and host
6. Encoding onion addresses [ONIONADDRESS]
7. Open Questions:
-1. Draft notes
This document describes a proposed design and specification for
hidden services in Tor version 0.2.5.x or later. It's a replacement
for the current rend-spec.txt, rewritten for clarity and for improved
design.
Look for the string "TODO" below: it describes gaps or uncertainties
in the design.
Change history:
2013-11-29: Proposal first numbered. Some TODO and XXX items remain.
2014-01-04: Clarify some unclear sections.
2014-01-21: Fix a typo.
2014-02-20: Move more things to the revised certificate format in the
new updated proposal 220.
2015-05-26: Fix two typos.
0. Hidden services: overview and preliminaries.
Hidden services aim to provide responder anonymity for bidirectional
stream-based communication on the Tor network. Unlike regular Tor
connections, where the connection initiator receives anonymity but
the responder does not, hidden services attempt to provide
bidirectional anonymity.
Participants:
Operator -- A person running a hidden service
Host, "Server" -- The Tor software run by the operator to provide
a hidden service.
User -- A person contacting a hidden service.
Client -- The Tor software running on the User's computer
Hidden Service Directory (HSDir) -- A Tor node that hosts signed
statements from hidden service hosts so that users can make
contact with them.
Introduction Point -- A Tor node that accepts connection requests
for hidden services and anonymously relays those requests to the
hidden service.
Rendezvous Point -- A Tor node to which clients and servers
connect and which relays traffic between them.
0.1. Improvements over previous versions.
Here is a list of improvements of this proposal over the legacy hidden
services:
a) Better crypto (replaced SHA1/DH/RSA1024 with SHA3/ed25519/curve25519)
b) Improved directory protocol leaking less to directory servers.
c) Improved directory protocol with smaller surface for targeted attacks.
d) Better onion address security against impersonation.
e) More extensible introduction/rendezvous protocol.
f) Offline keys for onion services
g) Advanced client authorization
0.2. Notation and vocabulary
Unless specified otherwise, all multi-octet integers are big-endian.
We write sequences of bytes in two ways:
1. A sequence of two-digit hexadecimal values in square brackets,
as in [AB AD 1D EA].
2. A string of characters enclosed in quotes, as in "Hello". The
characters in these strings are encoded in their ascii
representations; strings are NOT nul-terminated unless
explicitly described as NUL terminated.
We use the words "byte" and "octet" interchangeably.
We use the vertical bar | to denote concatenation.
We use INT_N(val) to denote the network (big-endian) encoding of the
unsigned integer "val" in N bytes. For example, INT_4(1337) is [00 00
05 39]. Values are truncated like so: val % (2 ^ (N * 8)). For example,
INT_4(42) is 42 % 4294967296 (32 bit).
0.3. Cryptographic building blocks
This specification uses the following cryptographic building blocks:
* A pseudorandom number generator backed by a strong entropy source.
The output of the PRNG should always be hashed before being posted on
the network to avoid leaking raw PRNG bytes to the network
(see [PRNG-REFS]).
* A stream cipher STREAM(iv, k) where iv is a nonce of length
S_IV_LEN bytes and k is a key of length S_KEY_LEN bytes.
* A public key signature system SIGN_KEYGEN()->seckey, pubkey;
SIGN_SIGN(seckey,msg)->sig; and SIGN_CHECK(pubkey, sig, msg) ->
{ "OK", "BAD" }; where secret keys are of length SIGN_SECKEY_LEN
bytes, public keys are of length SIGN_PUBKEY_LEN bytes, and
signatures are of length SIGN_SIG_LEN bytes.
This signature system must also support key blinding operations
as discussed in appendix [KEYBLIND] and in section [SUBCRED]:
SIGN_BLIND_SECKEY(seckey, blind)->seckey2 and
SIGN_BLIND_PUBKEY(pubkey, blind)->pubkey2 .
* A public key agreement system "PK", providing
PK_KEYGEN()->seckey, pubkey; PK_VALID(pubkey) -> {"OK", "BAD"};
and PK_HANDSHAKE(seckey, pubkey)->output; where secret keys are
of length PK_SECKEY_LEN bytes, public keys are of length
PK_PUBKEY_LEN bytes, and the handshake produces outputs of
length PK_OUTPUT_LEN bytes.
* A cryptographic hash function H(d), which should be preimage and
collision resistant. It produces hashes of length HASH_LEN
bytes.
* A cryptographic message authentication code MAC(key,msg) that
produces outputs of length MAC_LEN bytes.
* A key derivation function KDF(message, n) that outputs n bytes.
As a first pass, I suggest:
* Instantiate STREAM with AES256-CTR.
* Instantiate SIGN with Ed25519 and the blinding protocol in
[KEYBLIND].
* Instantiate PK with Curve25519.
* Instantiate H with SHA3-256.
* Instantiate KDF with SHAKE-256.
* Instantiate MAC(key=k, message=m) with H(k_len | k | m),
where k_len is htonll(len(k)).
For legacy purposes, we specify compatibility with older versions of
the Tor introduction point and rendezvous point protocols. These used
RSA1024, DH1024, AES128, and SHA1, as discussed in
rend-spec.txt.
As in [proposal 220], all signatures are generated not over strings
themselves, but over those strings prefixed with a distinguishing
value.
0.4. Protocol building blocks [BUILDING-BLOCKS]
In sections below, we need to transmit the locations and identities
of Tor nodes. We do so in the link identification format used by
EXTEND2 cells in the Tor protocol.
NSPEC (Number of link specifiers) [1 byte]
NSPEC times:
LSTYPE (Link specifier type) [1 byte]
LSLEN (Link specifier length) [1 byte]
LSPEC (Link specifier) [LSLEN bytes]
Link specifier types are as described in tor-spec.txt. Every set of
link specifiers MUST include at minimum specifiers of type [00]
(TLS-over-TCP, IPv4), [02] (legacy node identity) and [03] (ed25519
identity key).
As of 0.4.1.1-alpha, Tor includes both IPv4 and IPv6 link specifiers
in v3 onion service protocol link specifier lists. All available
addresses SHOULD be included as link specifiers, regardless of the
address that Tor actually used to connect/extend to the remote relay.
We also incorporate Tor's circuit extension handshakes, as used in
the CREATE2 and CREATED2 cells described in tor-spec.txt. In these
handshakes, a client who knows a public key for a server sends a
message and receives a message from that server. Once the exchange is
done, the two parties have a shared set of forward-secure key
material, and the client knows that nobody else shares that key
material unless they control the secret key corresponding to the
server's public key.
0.5. Assigned relay cell types
These relay cell types are reserved for use in the hidden service
protocol.
32 -- RELAY_COMMAND_ESTABLISH_INTRO
Sent from hidden service host to introduction point;
establishes introduction point. Discussed in
[REG_INTRO_POINT].
33 -- RELAY_COMMAND_ESTABLISH_RENDEZVOUS
Sent from client to rendezvous point; creates rendezvous
point. Discussed in [EST_REND_POINT].
34 -- RELAY_COMMAND_INTRODUCE1
Sent from client to introduction point; requests
introduction. Discussed in [SEND_INTRO1]
35 -- RELAY_COMMAND_INTRODUCE2
Sent from introduction point to hidden service host; requests
introduction. Same format as INTRODUCE1. Discussed in
[FMT_INTRO1] and [PROCESS_INTRO2]
36 -- RELAY_COMMAND_RENDEZVOUS1
Sent from hidden service host to rendezvous point;
attempts to join host's circuit to
client's circuit. Discussed in [JOIN_REND]
37 -- RELAY_COMMAND_RENDEZVOUS2
Sent from rendezvous point to client;
reports join of host's circuit to
client's circuit. Discussed in [JOIN_REND]
38 -- RELAY_COMMAND_INTRO_ESTABLISHED
Sent from introduction point to hidden service host;
reports status of attempt to establish introduction
point. Discussed in [INTRO_ESTABLISHED]
39 -- RELAY_COMMAND_RENDEZVOUS_ESTABLISHED
Sent from rendezvous point to client; acknowledges
receipt of ESTABLISH_RENDEZVOUS cell. Discussed in
[EST_REND_POINT]
40 -- RELAY_COMMAND_INTRODUCE_ACK
Sent from introduction point to client; acknowledges
receipt of INTRODUCE1 cell and reports success/failure.
Discussed in [INTRO_ACK]
0.6. Acknowledgments
This design includes ideas from many people, including
Christopher Baines,
Daniel J. Bernstein,
Matthew Finkel,
Ian Goldberg,
George Kadianakis,
Aniket Kate,
Tanja Lange,
Robert Ransom,
Roger Dingledine,
Aaron Johnson,
Tim Wilson-Brown ("teor"),
special (John Brooks),
s7r
It's based on Tor's original hidden service design by Roger
Dingledine, Nick Mathewson, and Paul Syverson, and on improvements to
that design over the years by people including
Tobias Kamm,
Thomas Lauterbach,
Karsten Loesing,
Alessandro Preite Martinez,
Robert Ransom,
Ferdinand Rieger,
Christoph Weingarten,
Christian Wilms,
We wouldn't be able to do any of this work without good attack
designs from researchers including
Alex Biryukov,
Lasse Øverlier,
Ivan Pustogarov,
Paul Syverson
Ralf-Philipp Weinmann,
See [ATTACK-REFS] for their papers.
Several of these ideas have come from conversations with
Christian Grothoff,
Brian Warner,
Zooko Wilcox-O'Hearn,
And if this document makes any sense at all, it's thanks to
editing help from
Matthew Finkel
George Kadianakis,
Peter Palfrader,
Tim Wilson-Brown ("teor"),
[XXX Acknowledge the huge bunch of people working on 8106.]
[XXX Acknowledge the huge bunch of people working on 8244.]
Please forgive me if I've missed you; please forgive me if I've
misunderstood your best ideas here too.
1. Protocol overview
In this section, we outline the hidden service protocol. This section
omits some details in the name of simplicity; those are given more
fully below, when we specify the protocol in more detail.
1.1. View from 10,000 feet
A hidden service host prepares to offer a hidden service by choosing
several Tor nodes to serve as its introduction points. It builds
circuits to those nodes, and tells them to forward introduction
requests to it using those circuits.
Once introduction points have been picked, the host builds a set of
documents called "hidden service descriptors" (or just "descriptors"
for short) and uploads them to a set of HSDir nodes. These documents
list the hidden service's current introduction points and describe
how to make contact with the hidden service.
When a client wants to connect to a hidden service, it first chooses
a Tor node at random to be its "rendezvous point" and builds a
circuit to that rendezvous point. If the client does not have an
up-to-date descriptor for the service, it contacts an appropriate
HSDir and requests such a descriptor.
The client then builds an anonymous circuit to one of the hidden
service's introduction points listed in its descriptor, and gives the
introduction point an introduction request to pass to the hidden
service. This introduction request includes the target rendezvous
point and the first part of a cryptographic handshake.
Upon receiving the introduction request, the hidden service host
makes an anonymous circuit to the rendezvous point and completes the
cryptographic handshake. The rendezvous point connects the two
circuits, and the cryptographic handshake gives the two parties a
shared key and proves to the client that it is indeed talking to the
hidden service.
Once the two circuits are joined, the client can send Tor RELAY cells
to the server. RELAY_BEGIN cells open streams to an external process
or processes configured by the server; RELAY_DATA cells are used to
communicate data on those streams, and so forth.
1.2. In more detail: naming hidden services [NAMING]
A hidden service's name is its long term master identity key. This is
encoded as a hostname by encoding the entire key in Base 32, including a
version byte and a checksum, and then appending the string ".onion" at the
end. The result is a 56-character domain name.
(This is a change from older versions of the hidden service protocol,
where we used an 80-bit truncated SHA1 hash of a 1024 bit RSA key.)
The names in this format are distinct from earlier names because of
their length. An older name might look like:
unlikelynamefora.onion
yyhws9optuwiwsns.onion
And a new name following this specification might look like:
l5satjgud6gucryazcyvyvhuxhr74u6ygigiuyixe3a6ysis67ororad.onion
Please see section [ONIONADDRESS] for the encoding specification.
1.3. In more detail: Access control [IMD:AC]
Access control for a hidden service is imposed at multiple points through
the process above. Furthermore, there is also the option to impose
additional client authorization access control using pre-shared secrets
exchanged out-of-band between the hidden service and its clients.
The first stage of access control happens when downloading HS descriptors.
Specifically, in order to download a descriptor, clients must know which
blinded signing key was used to sign it. (See the next section for more info
on key blinding.)
To learn the introduction points, clients must decrypt the body of the
hidden service descriptor. To do so, clients must know the _unblinded_
public key of the service, which makes the descriptor unuseable by entities
without that knowledge (e.g. HSDirs that don't know the onion address).
Also, if optional client authorization is enabled, hidden service
descriptors are superencrypted using each authorized user's identity x25519
key, to further ensure that unauthorized entities cannot decrypt it.
In order to make the introduction point send a rendezvous request to the
service, the client needs to use the per-introduction-point authentication
key found in the hidden service descriptor.
The final level of access control happens at the server itself, which may
decide to respond or not respond to the client's request depending on the
contents of the request. The protocol is extensible at this point: at a
minimum, the server requires that the client demonstrate knowledge of the
contents of the encrypted portion of the hidden service descriptor. If
optional client authorization is enabled, the service may additionally
require the client to prove knowledge of a pre-shared private key.
1.4. In more detail: Distributing hidden service descriptors. [IMD:DIST]
Periodically, hidden service descriptors become stored at different
locations to prevent a single directory or small set of directories
from becoming a good DoS target for removing a hidden service.
For each period, the Tor directory authorities agree upon a
collaboratively generated random value. (See section 2.3 for a
description of how to incorporate this value into the voting
practice; generating the value is described in other proposals,
including [SHAREDRANDOM-REFS].) That value, combined with hidden service
directories' public identity keys, determines each HSDir's position
in the hash ring for descriptors made in that period.
Each hidden service's descriptors are placed into the ring in
positions based on the key that was used to sign them. Note that
hidden service descriptors are not signed with the services' public
keys directly. Instead, we use a key-blinding system [KEYBLIND] to
create a new key-of-the-day for each hidden service. Any client that
knows the hidden service's credential can derive these blinded
signing keys for a given period. It should be impossible to derive
the blinded signing key lacking that credential.
The body of each descriptor is also encrypted with a key derived from
the credential.
To avoid a "thundering herd" problem where every service generates
and uploads a new descriptor at the start of each period, each
descriptor comes online at a time during the period that depends on
its blinded signing key. The keys for the last period remain valid
until the new keys come online.
1.5. In more detail: Scaling to multiple hosts
This design is compatible with our current approaches for scaling hidden
services. Specifically, hidden service operators can use onionbalance to
achieve high availability between multiple nodes on the HSDir
layer. Furthermore, operators can use proposal 255 to load balance their
hidden services on the introduction layer. See [SCALING-REFS] for further
discussions on this topic and alternative designs.
1.6. In more detail: Backward compatibility with older hidden service
protocols
This design is incompatible with the clients, server, and hsdir node
protocols from older versions of the hidden service protocol as
described in rend-spec.txt. On the other hand, it is designed to
enable the use of older Tor nodes as rendezvous points and
introduction points.
1.7. In more detail: Keeping crypto keys offline
In this design, a hidden service's secret identity key may be
stored offline. It's used only to generate blinded signing keys,
which are used to sign descriptor signing keys.
In order to operate a hidden service, the operator can generate in
advance a number of blinded signing keys and descriptor signing
keys (and their credentials; see [DESC-OUTER] and [HS-DESC-ENC]
below), and their corresponding descriptor encryption keys, and
export those to the hidden service hosts.
As a result, in the scenario where the Hidden Service gets
compromised, the adversary can only impersonate it for a limited
period of time (depending on how many signing keys were generated
in advance).
It's important to not send the private part of the blinded signing
key to the Hidden Service since an attacker can derive from it the
secret master identity key. The secret blinded signing key should
only be used to create credentials for the descriptor signing keys.
(NOTE: although the protocol allows them, offline keys are not
implemented as of 0.3.2.1-alpha.)
1.8. In more detail: Encryption Keys And Replay Resistance
To avoid replays of an introduction request by an introduction point,
a hidden service host must never accept the same request
twice. Earlier versions of the hidden service design used an
authenticated timestamp here, but including a view of the current
time can create a problematic fingerprint. (See proposal 222 for more
discussion.)
1.9. In more detail: A menagerie of keys
[In the text below, an "encryption keypair" is roughly "a keypair you
can do Diffie-Hellman with" and a "signing keypair" is roughly "a
keypair you can do ECDSA with."]
Public/private keypairs defined in this document:
Master (hidden service) identity key -- A master signing keypair
used as the identity for a hidden service. This key is long
term and not used on its own to sign anything; it is only used
to generate blinded signing keys as described in [KEYBLIND]
and [SUBCRED]. The public key is encoded in the ".onion"
address according to [NAMING].
Blinded signing key -- A keypair derived from the identity key,
used to sign descriptor signing keys. It changes periodically for
each service. Clients who know a 'credential' consisting of the
service's public identity key and an optional secret can derive
the public blinded identity key for a service. This key is used
as an index in the DHT-like structure of the directory system
(see [SUBCRED]).
Descriptor signing key -- A key used to sign hidden service
descriptors. This is signed by blinded signing keys. Unlike
blinded signing keys and master identity keys, the secret part
of this key must be stored online by hidden service hosts. The
public part of this key is included in the unencrypted section
of HS descriptors (see [DESC-OUTER]).
Introduction point authentication key -- A short-term signing
keypair used to identify a hidden service to a given
introduction point. A fresh keypair is made for each
introduction point; these are used to sign the request that a
hidden service host makes when establishing an introduction
point, so that clients who know the public component of this key
can get their introduction requests sent to the right
service. No keypair is ever used with more than one introduction
point. (previously called a "service key" in rend-spec.txt)
Introduction point encryption key -- A short-term encryption
keypair used when establishing connections via an introduction
point. Plays a role analogous to Tor nodes' onion keys. A fresh
keypair is made for each introduction point.
Symmetric keys defined in this document:
Descriptor encryption keys -- A symmetric encryption key used to
encrypt the body of hidden service descriptors. Derived from the
current period and the hidden service credential.
Public/private keypairs defined elsewhere:
Onion key -- Short-term encryption keypair
(Node) identity key
Symmetric key-like things defined elsewhere:
KH from circuit handshake -- An unpredictable value derived as
part of the Tor circuit extension handshake, used to tie a request
to a particular circuit.
1.9.1. In even more detail: Client authorization keys [CLIENT-AUTH]
When client authorization is enabled, each authorized client of a hidden
service has two more assymetric keypairs which are shared with the hidden
service. An entity without those keys is not able to use the hidden
service. Throughout this document, we assume that these pre-shared keys are
exchanged between the hidden service and its clients in a secure out-of-band
fashion.
Specifically, each authorized client possesses:
- An x25519 keypair used to compute decryption keys that allow the client to
decrypt the hidden service descriptor. See [HS-DESC-ENC].
- An ed25519 keypair which allows the client to compute signatures which
prove to the hidden service that the client is authorized. These
signatures are inserted into the INTRODUCE1 cell, and without them the
introduction to the hidden service cannot be completed. See [INTRO-AUTH].
The right way to exchange these keys is to have the client generate keys and
send the corresponding public keys to the hidden service out-of-band. An
easier but less secure way of doing this exchange would be to have the
hidden service generate the keypairs and pass the corresponding private keys
to its clients. See section [CLIENT-AUTH-MGMT] for more details on how these
keys should be managed.
[TODO: Also specify stealth client authorization.]
(NOTE: client authorization is implemented as of 0.3.5.1-alpha.)
2. Generating and publishing hidden service descriptors [HSDIR]
Hidden service descriptors follow the same metaformat as other Tor
directory objects. They are published anonymously to Tor servers with the
HSDir flag, HSDir=2 protocol version and tor version >= 0.3.0.8 (because a
bug was fixed in this version).
2.1. Deriving blinded keys and subcredentials [SUBCRED]
In each time period (see [TIME-PERIODS] for a definition of time
periods), a hidden service host uses a different blinded private key
to sign its directory information, and clients use a different
blinded public key as the index for fetching that information.
For a candidate for a key derivation method, see Appendix [KEYBLIND].
Additionally, clients and hosts derive a subcredential for each
period. Knowledge of the subcredential is needed to decrypt hidden
service descriptors for each period and to authenticate with the
hidden service host in the introduction process. Unlike the
credential, it changes each period. Knowing the subcredential, even
in combination with the blinded private key, does not enable the
hidden service host to derive the main credential--therefore, it is
safe to put the subcredential on the hidden service host while
leaving the hidden service's private key offline.
The subcredential for a period is derived as:
subcredential = H("subcredential" | credential | blinded-public-key).
In the above formula, credential corresponds to:
credential = H("credential" | public-identity-key)
where public-identity-key is the public identity master key of the hidden
service.
2.2. Locating, uploading, and downloading hidden service descriptors
[HASHRING]
To avoid attacks where a hidden service's descriptor is easily
targeted for censorship, we store them at different directories over
time, and use shared random values to prevent those directories from
being predictable far in advance.
Which Tor servers hosts a hidden service depends on:
* the current time period,
* the daily subcredential,
* the hidden service directories' public keys,
* a shared random value that changes in each time period,
* a set of network-wide networkstatus consensus parameters.
(Consensus parameters are integer values voted on by authorities
and published in the consensus documents, described in
dir-spec.txt, section 3.3.)
Below we explain in more detail.
2.2.1. Dividing time into periods [TIME-PERIODS]
To prevent a single set of hidden service directory from becoming a
target by adversaries looking to permanently censor a hidden service,
hidden service descriptors are uploaded to different locations that
change over time.
The length of a "time period" is controlled by the consensus
parameter 'hsdir-interval', and is a number of minutes between 30 and
14400 (10 days). The default time period length is 1440 (one day).
Time periods start at the Unix epoch (Jan 1, 1970), and are computed by
taking the number of minutes since the epoch and dividing by the time
period. However, we want our time periods to start at 12:00UTC every day, so
we subtract a "rotation time offset" of 12*60 minutes from the number of
minutes since the epoch, before dividing by the time period (effectively
making "our" epoch start at Jan 1, 1970 12:00UTC).
Example: If the current time is 2016-04-13 11:15:01 UTC, making the seconds
since the epoch 1460546101, and the number of minutes since the epoch
24342435. We then subtract the "rotation time offset" of 12*60 minutes from
the minutes since the epoch, to get 24341715. If the current time period
length is 1440 minutes, by doing the division we see that we are currently
in time period number 16903.
Specifically, time period #16903 began 16903*1440*60 + (12*60*60) seconds
after the epoch, at 2016-04-12 12:00 UTC, and ended at 16904*1440*60 +
(12*60*60) seconds after the epoch, at 2016-04-13 12:00 UTC.
2.2.2. When to publish a hidden service descriptor [WHEN-HSDESC]
Hidden services periodically publish their descriptor to the responsible
HSDirs. The set of responsible HSDirs is determined as specified in
[WHERE-HSDESC].
Specifically, everytime a hidden service publishes its descriptor, it also
sets up a timer for a random time between 60 minutes and 120 minutes in the
future. When the timer triggers, the hidden service needs to publish its
descriptor again to the responsible HSDirs for that time period.
[TODO: Control republish period using a consensus parameter?]
2.2.2.1. Overlapping descriptors
Hidden services need to upload multiple descriptors so that they can be
reachable to clients with older or newer consensuses than them. Services
need to upload their descriptors to the HSDirs _before_ the beginning of
each upcoming time period, so that they are readily available for clients to
fetch them. Furthermore, services should keep uploading their old descriptor
even after the end of a time period, so that they can be reachable by
clients that still have consensuses from the previous time period.
Hence, services maintain two active descriptors at every point. Clients on
the other hand, don't have a notion of overlapping descriptors, and instead
always download the descriptor for the current time period and shared random
value. It's the job of the service to ensure that descriptors will be
available for all clients. See section [FETCHUPLOADDESC] for how this is
achieved.
[TODO: What to do when we run multiple hidden services in a single host?]
2.2.3. Where to publish a hidden service descriptor [WHERE-HSDESC]
This section specifies how the HSDir hash ring is formed at any given
time. Whenever a time value is needed (e.g. to get the current time period
number), we assume that clients and services use the valid-after time from
their latest live consensus.
The following consensus parameters control where a hidden service
descriptor is stored;
hsdir_n_replicas = an integer in range [1,16] with default value 2.
hsdir_spread_fetch = an integer in range [1,128] with default value 3.
hsdir_spread_store = an integer in range [1,128] with default value 4.
(Until 0.3.2.8-rc, the default was 3.)
To determine where a given hidden service descriptor will be stored
in a given period, after the blinded public key for that period is
derived, the uploading or downloading party calculates:
for replicanum in 1...hsdir_n_replicas:
hs_index(replicanum) = H("store-at-idx" |
blinded_public_key |
INT_8(replicanum) |
INT_8(period_length) |
INT_8(period_num) )
where blinded_public_key is specified in section [KEYBLIND], period_length
is the length of the time period in minutes, and period_num is calculated
using the current consensus "valid-after" as specified in section
[TIME-PERIODS].
Then, for each node listed in the current consensus with the HSDir flag,
we compute a directory index for that node as:
hsdir_index(node) = H("node-idx" | node_identity |
shared_random_value |
INT_8(period_num) |
INT_8(period_length) )
where shared_random_value is the shared value generated by the authorities
in section [PUB-SHAREDRANDOM], and node_identity is the ed25519 identity
key of the node.
Finally, for replicanum in 1...hsdir_n_replicas, the hidden service
host uploads descriptors to the first hsdir_spread_store nodes whose
indices immediately follow hs_index(replicanum). If any of those
nodes have already been selected for a lower-numbered replica of the
service, any nodes already chosen are disregarded (i.e. skipped over)
when choosing a replica's hsdir_spread_store nodes.
When choosing an HSDir to download from, clients choose randomly from
among the first hsdir_spread_fetch nodes after the indices. (Note
that, in order to make the system better tolerate disappearing
HSDirs, hsdir_spread_fetch may be less than hsdir_spread_store.)
Again, nodes from lower-numbered replicas are disregarded when
choosing the spread for a replica.
2.2.4. Using time periods and SRVs to fetch/upload HS descriptors [FETCHUPLOADDESC]
Hidden services and clients need to make correct use of time periods (TP)
and shared random values (SRVs) to successfuly fetch and upload
descriptors. Furthermore, to avoid problems with skewed clocks, both clients
and services use the 'valid-after' time of a live consensus as a way to take
decisions with regards to uploading and fetching descriptors. By using the
consensus times as the ground truth here, we minimize the desynchronization
of clients and services due to system clock. Whenever time-based decisions
are taken in this section, assume that they are consensus times and not
system times.
As [PUB-SHAREDRANDOM] specifies, consensuses contain two shared random
values (the current one and the previous one). Hidden services and clients
are asked to match these shared random values with descriptor time periods
and use the right SRV when fetching/uploading descriptors. This section
attempts to precisely specify how this works.
Let's start with an illustration of the system:
+------------------------------------------------------------------+
| |
| 00:00 12:00 00:00 12:00 00:00 12:00 |
| SRV#1 TP#1 SRV#2 TP#2 SRV#3 TP#3 |
| |
| $==========|-----------$===========|-----------$===========| |
| |
| |
+------------------------------------------------------------------+
Legend: [TP#1 = Time Period #1]
[SRV#1 = Shared Random Value #1]
["$" = descriptor rotation moment]
2.2.4.1. Client behavior for fetching descriptors [CLIENTFETCH]
And here is how clients use TPs and SRVs to fetch descriptors:
Clients always aim to synchronize their TP with SRV, so they always want to
use TP#N with SRV#N: To achieve this wrt time periods, clients always use
the current time period when fetching descriptors. Now wrt SRVs, if a client
is in the time segment between a new time period and a new SRV (i.e. the
segments drawn with "-") it uses the current SRV, else if the client is in a
time segment between a new SRV and a new time period (i.e. the segments
drawn with "="), it uses the previous SRV.
Example:
+------------------------------------------------------------------+
| |
| 00:00 12:00 00:00 12:00 00:00 12:00 |
| SRV#1 TP#1 SRV#2 TP#2 SRV#3 TP#3 |
| |
| $==========|-----------$===========|-----------$===========| |
| ^ ^ |
| C1 C2 |
+------------------------------------------------------------------+
If a client (C1) is at 13:00 right after TP#1, then it will use TP#1 and
SRV#1 for fetching descriptors. Also, if a client (C2) is at 01:00 right
after SRV#2, it will still use TP#1 and SRV#1.
2.2.4.2. Service behavior for uploading descriptors [SERVICEUPLOAD]
As discussed above, services maintain two active descriptors at any time. We
call these the "first" and "second" service descriptors. Services rotate
their descriptor everytime they receive a consensus with a valid_after time
past the next SRV calculation time. They rotate their descriptors by
discarding their first descriptor, pushing the second descriptor to the
first, and rebuilding their second descriptor with the latest data.
Services like clients also employ a different logic for picking SRV and TP
values based on their position in the graph above. Here is the logic:
2.2.4.2.1. First descriptor upload logic [FIRSTDESCUPLOAD]
Here is the service logic for uploading its first descriptor:
When a service is in the time segment between a new time period a new SRV
(i.e. the segments drawn with "-"), it uses the previous time period and
previous SRV for uploading its first descriptor: that's meant to cover
for clients that have a consensus that is still in the previous time period.
Example: Consider in the above illustration that the service is at 13:00
right after TP#1. It will upload its first descriptor using TP#0 and SRV#0.
So if a client still has a 11:00 consensus it will be able to access it
based on the client logic above.
Now if a service is in the time segment between a new SRV and a new time
period (i.e. the segments drawn with "=") it uses the current time period
and the previous SRV for its first descriptor: that's meant to cover clients
with an up-to-date consensus in the same time period as the service.
Example:
+------------------------------------------------------------------+
| |
| 00:00 12:00 00:00 12:00 00:00 12:00 |
| SRV#1 TP#1 SRV#2 TP#2 SRV#3 TP#3 |
| |
| $==========|-----------$===========|-----------$===========| |
| ^ |
| S |
+------------------------------------------------------------------+
Consider that the service is at 01:00 right after SRV#2: it will upload its
first descriptor using TP#1 and SRV#1.
2.2.4.2.2. Second descriptor upload logic [SECONDDESCUPLOAD]
Here is the service logic for uploading its second descriptor:
When a service is in the time segment between a new time period a new SRV
(i.e. the segments drawn with "-"), it uses the current time period and
current SRV for uploading its second descriptor: that's meant to cover for
clients that have an up-to-date consensus on the same TP as the service.
Example: Consider in the above illustration that the service is at 13:00
right after TP#1: it will upload its second descriptor using TP#1 and SRV#1.
Now if a service is in the time segment between a new SRV and a new time
period (i.e. the segments drawn with "=") it uses the next time period and
the current SRV for its second descriptor: that's meant to cover clients
with a newer consensus than the service (in the next time period).
Example:
+------------------------------------------------------------------+
| |
| 00:00 12:00 00:00 12:00 00:00 12:00 |
| SRV#1 TP#1 SRV#2 TP#2 SRV#3 TP#3 |
| |
| $==========|-----------$===========|-----------$===========| |
| ^ |
| S |
+------------------------------------------------------------------+
Consider that the service is at 01:00 right after SRV#2: it will upload its
second descriptor using TP#2 and SRV#2.
2.2.5. Expiring hidden service descriptors [EXPIRE-DESC]
Hidden services set their descriptor's "descriptor-lifetime" field to 180
minutes (3 hours). Hidden services ensure that their descriptor will remain
valid in the HSDir caches, by republishing their descriptors periodically as
specified in [WHEN-HSDESC].
Hidden services MUST also keep their introduction circuits alive for as long
as descriptors including those intro points are valid (even if that's after
the time period has changed).
2.2.6. URLs for anonymous uploading and downloading
Hidden service descriptors conforming to this specification are uploaded
with an HTTP POST request to the URL /tor/hs/<version>/publish relative to
the hidden service directory's root, and downloaded with an HTTP GET
request for the URL /tor/hs/<version>/<z> where <z> is a base64 encoding of
the hidden service's blinded public key and <version> is the protocol
version which is "3" in this case.
These requests must be made anonymously, on circuits not used for
anything else.
2.2.7. Client-side validation of onion addresses
When a Tor client receives a prop224 onion address from the user, it
MUST first validate the onion address before attempting to connect or
fetch its descriptor. If the validation fails, the client MUST
refuse to connect.
As part of the address validation, Tor clients should check that the
underlying ed25519 key does not have a torsion component. If Tor accepted
ed25519 keys with torsion components, attackers could create multiple
equivalent onion addresses for a single ed25519 key, which would map to the
same service. We want to avoid that because it could lead to phishing
attacks and surprising behaviors (e.g. imagine a browser plugin that blocks
onion addresses, but could be bypassed using an equivalent onion address
with a torsion component).
The right way for clients to detect such fraudulent addresses (which should
only occur malevolently and never natutally) is to extract the ed25519
public key from the onion address and multiply it by the ed25519 group order
and ensure that the result is the ed25519 identity element. For more
details, please see [TORSION-REFS].
2.3. Publishing shared random values [PUB-SHAREDRANDOM]
Our design for limiting the predictability of HSDir upload locations
relies on a shared random value (SRV) that isn't predictable in advance or
too influenceable by an attacker. The authorities must run a protocol
to generate such a value at least once per hsdir period. Here we
describe how they publish these values; the procedure they use to
generate them can change independently of the rest of this
specification. For more information see [SHAREDRANDOM-REFS].
According to proposal 250, we add two new lines in consensuses:
"shared-rand-previous-value" SP NUM_REVEALS SP VALUE NL
"shared-rand-current-value" SP NUM_REVEALS SP VALUE NL
2.3.1. Client behavior in the absense of shared random values
If the previous or current shared random value cannot be found in a
consensus, then Tor clients and services need to generate their own random
value for use when choosing HSDirs.
To do so, Tor clients and services use:
SRV = H("shared-random-disaster" | INT_8(period_length) | INT_8(period_num))
where period_length is the length of a time period in minutes, period_num is
calculated as specified in [TIME-PERIODS] for the wanted shared random value
that could not be found originally.
2.3.2. Hidden services and changing shared random values
It's theoretically possible that the consensus shared random values will
change or disappear in the middle of a time period because of directory
authorities dropping offline or misbehaving.
To avoid client reachability issues in this rare event, hidden services
should use the new shared random values to find the new responsible HSDirs
and upload their descriptors there.
XXX How long should they upload descriptors there for?
2.4. Hidden service descriptors: outer wrapper [DESC-OUTER]
The format for a hidden service descriptor is as follows, using the
meta-format from dir-spec.txt.
"hs-descriptor" SP version-number NL
[At start, exactly once.]
The version-number is a 32 bit unsigned integer indicating the version
of the descriptor. Current version is "3".
"descriptor-lifetime" SP LifetimeMinutes NL
[Exactly once]
The lifetime of a descriptor in minutes. An HSDir SHOULD expire the
hidden service descriptor at least LifetimeMinutes after it was
uploaded.
The LifetimeMinutes field can take values between 30 and 720 (12
hours).
"descriptor-signing-key-cert" NL certificate NL
[Exactly once.]
The 'certificate' field contains a certificate in the format from
proposal 220, wrapped with "-----BEGIN ED25519 CERT-----". The
certificate cross-certifies the short-term descriptor signing key with
the blinded public key. The certificate type must be [08], and the
blinded public key must be present as the signing-key extension.
"revision-counter" SP Integer NL
[Exactly once.]
The revision number of the descriptor. If an HSDir receives a
second descriptor for a key that it already has a descriptor for,
it should retain and serve the descriptor with the higher
revision-counter.
(Checking for monotonically increasing revision-counter values
prevents an attacker from replacing a newer descriptor signed by
a given key with a copy of an older version.)
"superencrypted" NL encrypted-string
[Exactly once.]
An encrypted blob, whose format is discussed in [HS-DESC-ENC] below. The
blob is base64 encoded and enclosed in -----BEGIN MESSAGE---- and
----END MESSAGE---- wrappers. (The resulting document does not end with
a newline character.)
"signature" SP signature NL
[exactly once, at end.]
A signature of all previous fields, using the signing key in the
descriptor-signing-key-cert line, prefixed by the string "Tor onion
service descriptor sig v3". We use a separate key for signing, so that
the hidden service host does not need to have its private blinded key
online.
HSDirs accept hidden service descriptors of up to 50k bytes (a consensus
parameter should also be introduced to control this value).
2.5. Hidden service descriptors: encryption format [HS-DESC-ENC]
Hidden service descriptors are protected by two layers of encryption.
Clients need to decrypt both layers to connect to the hidden service.
The first layer of encryption provides confidentiality against entities who
don't know the public key of the hidden service (e.g. HSDirs), while the
second layer of encryption is only useful when client authorization is enabled
and protects against entities that do not possess valid client credentials.
2.5.1. First layer of encryption [HS-DESC-FIRST-LAYER]
The first layer of HS descriptor encryption is designed to protect
descriptor confidentiality against entities who don't know the public
identity key of the hidden service.
2.5.1.1. First layer encryption logic
The encryption keys and format for the first layer of encryption are
generated as specified in [HS-DESC-ENCRYPTION-KEYS] with customization
parameters:
SECRET_DATA = blinded-public-key
STRING_CONSTANT = "hsdir-superencrypted-data"
The encryption scheme in [HS-DESC-ENCRYPTION-KEYS] uses the service
credential which is derived from the public identity key (see [SUBCRED]) to
ensure that only entities who know the public identity key can decrypt the
first descriptor layer.
The ciphertext is placed on the "superencrypted" field of the descriptor.
Before encryption the plaintext is padded with NUL bytes to the nearest
multiple of 10k bytes.
2.5.1.2. First layer plaintext format
After clients decrypt the first layer of encryption, they need to parse the
plaintext to get to the second layer ciphertext which is contained in the
"encrypted" field.
If client auth is enabled, the hidden service generates a fresh
descriptor_cookie key (32 random bytes) and encrypts it using each
authorized client's identity x25519 key. Authorized clients can use the
descriptor cookie to decrypt the second layer of encryption. Our encryption
scheme requires the hidden service to also generate an ephemeral x25519
keypair for each new descriptor.
If client auth is disabled, fake data is placed in each of the fields below
to obfuscate whether client authorization is enabled.
Here are all the supported fields:
"desc-auth-type" SP type NL
[Exactly once]
This field contains the type of authorization used to protect the
descriptor. The only recognized type is "x25519" and specifies the
encryption scheme described in this section.
If client authorization is disabled, the value here should be "x25519".
"desc-auth-ephemeral-key" SP key NL
[Exactly once]
This field contains an ephemeral x25519 public key generated by the
hidden service and encoded in base64. The key is used by the encryption
scheme below.
If client authorization is disabled, the value here should be a fresh
x25519 pubkey that will remain unused.
"auth-client" SP client-id SP iv SP encrypted-cookie
[At least once]
When client authorization is enabled, the hidden service inserts an
"auth-client" line for each of its authorized clients. If client
authorization is disabled, the fields here can be populated with random
data of the right size (that's 8 bytes for 'client-id', 16 bytes for 'iv'
and 16 bytes for 'encrypted-cookie' all encoded with base64).
When client authorization is enabled, each "auth-client" line contains
the descriptor cookie encrypted to each individual client. We assume that
each authorized client possesses a pre-shared x25519 keypair which is
used to decrypt the descriptor cookie.
We now describe the descriptor cookie encryption scheme. Here are the
relevant keys:
client_x = private x25519 key of authorized client
client_X = public x25519 key of authorized client
hs_y = private key of ephemeral x25519 keypair of hidden service
hs_Y = public key of ephemeral x25519 keypair of hidden service
descriptor_cookie = descriptor cookie used to encrypt the descriptor
And here is what the hidden service computes:
SECRET_SEED = x25519(hs_y, client_X)
KEYS = KDF(subcredential | SECRET_SEED, 40)
CLIENT-ID = fist 8 bytes of KEYS
COOKIE-KEY = last 32 bytes of KEYS
Here is a description of the fields in the "auth-client" line:
- The "client-id" field is CLIENT-ID from above encoded in base64.
- The "iv" field is 16 random bytes encoded in base64.
- The "encrypted-cookie" field contains the descriptor cookie ciphertext
as follows and is encoded in base64:
encrypted-cookie = STREAM(iv, COOKIE-KEY) XOR descriptor_cookie
See section [FIRST-LAYER-CLIENT-BEHAVIOR] for the client-side logic of
how to decrypt the descriptor cookie.
"encrypted" NL encrypted-string
[Exactly once]
An encrypted blob containing the second layer ciphertext, whose format is
discussed in [HS-DESC-SECOND-LAYER] below. The blob is base64 encoded
and enclosed in -----BEGIN MESSAGE---- and ----END MESSAGE---- wrappers.
2.5.1.3. Client behavior [FIRST-LAYER-CLIENT-BEHAVIOR]
The goal of clients at this stage is to decrypt the "encrypted" field as
described in [HS-DESC-SECOND-LAYER].
If client authorization is enabled, authorized clients need to extract the
descriptor cookie to proceed with decryption of the second layer as
follows:
An authorized client parsing the first layer of an encrypted descriptor,
extracts the ephemeral key from "desc-auth-ephemeral-key" and calculates
CLIENT-ID and COOKIE-KEY as described in the section above using their
x25519 private key. The client then uses CLIENT-ID to find the right
"auth-client" field which contains the ciphertext of the descriptor
cookie. The client then uses COOKIE-KEY and the iv to decrypt the
descriptor_cookie, which is used to decrypt the second layer of descriptor
encryption as described in [HS-DESC-SECOND-LAYER].
2.5.1.4. Hiding client authorization data
Hidden services should avoid leaking whether client authorization is
enabled or how many authorized clients there are.
Hence even when client authorization is disabled, the hidden service adds
fake "desc-auth-type", "desc-auth-ephemeral-key" and "auth-client" lines to
the descriptor, as described in [HS-DESC-FIRST-LAYER].
The hidden service also avoids leaking the number of authorized clients by
adding fake "auth-client" entries to its descriptor. Specifically,
descriptors always contain a number of authorized clients that is a
multiple of 16 by adding fake "auth-client" entries if needed.
[XXX consider randomization of the value 16]
Clients MUST accept descriptors with any number of "auth-client" lines as
long as the total descriptor size is within the max limit of 50k (also
controlled with a consensus parameter).
2.5.2. Second layer of encryption [HS-DESC-SECOND-LAYER]
The second layer of descriptor encryption is designed to protect descriptor
confidentiality against unauthorized clients. If client authorization is
enabled, it's encrypted using the descriptor_cookie, and contains needed
information for connecting to the hidden service, like the list of its
introduction points.
If client authorization is disabled, then the second layer of HS encryption
does not offer any additional security, but is still used.
2.5.2.1. Second layer encryption keys
The encryption keys and format for the second layer of encryption are
generated as specified in [HS-DESC-ENCRYPTION-KEYS] with customization
parameters as follows:
SECRET_DATA = blinded-public-key | descriptor_cookie
STRING_CONSTANT = "hsdir-encrypted-data"
If client authorization is disabled the 'descriptor_cookie' field is left blank.
The ciphertext is placed on the "encrypted" field of the descriptor.
2.5.2.2. Second layer plaintext format
After decrypting the second layer ciphertext, clients can finally learn the
list of intro points etc. The plaintext has the following format:
"create2-formats" SP formats NL
[Exactly once]
A space-separated list of integers denoting CREATE2 cell format numbers
that the server recognizes. Must include at least ntor as described in
tor-spec.txt. See tor-spec section 5.1 for a list of recognized
handshake types.
"intro-auth-required" SP types NL
[At most once]
A space-separated list of introduction-layer authentication types; see
section [INTRO-AUTH] for more info. A client that does not support at
least one of these authentication types will not be able to contact the
host. Recognized types are: 'password' and 'ed25519'.
"single-onion-service"
[None or at most once]
If present, this line indicates that the service is a Single Onion
Service (see prop260 for more details about that type of service). This
field has been introduced in 0.3.0 meaning 0.2.9 service don't include
this.
Followed by zero or more introduction points as follows (see section
[NUM_INTRO_POINT] below for accepted values):
"introduction-point" SP link-specifiers NL
[Exactly once per introduction point at start of introduction
point section]
The link-specifiers is a base64 encoding of a link specifier
block in the format described in BUILDING-BLOCKS.
As of 0.4.1.1-alpha, services include both IPv4 and IPv6 link
specifiers in descriptors. All available addresses SHOULD be
included in the descriptor, regardless of the address that the
onion service actually used to connect/extend to the intro
point.
The client SHOULD NOT reject any LSTYPE fields which it doesn't
recognize; instead, it should use them verbatim in its EXTEND
request to the introduction point.
The client MAY perform basic validity checks on the link
specifiers in the descriptor. These checks SHOULD NOT leak
detailed information about the client's version, configuration,
or consensus. (See 3.3 for service link specifier handling.)
"onion-key" SP "ntor" SP key NL
[Exactly once per introduction point]
The key is a base64 encoded curve25519 public key which is the onion
key of the introduction point Tor node used for the ntor handshake
when a client extends to it.
"auth-key" NL certificate NL
[Exactly once per introduction point]
The certificate is a proposal 220 certificate wrapped in "-----BEGIN
ED25519 CERT-----" cross-certifying the introduction point
authentication key using the descriptor signing key. The introduction
point authentication key is included in the mandatory signing-key
extension. The certificate type must be [09].
"enc-key" SP "ntor" SP key NL
[Exactly once per introduction point]
The key is a base64 encoded curve25519 public key used to encrypt
the introduction request to service.
"enc-key-cert" NL certificate NL
[Exactly once per introduction point]
Cross-certification of the encryption key using the descriptor
signing key.
For "ntor" keys, certificate is a proposal 220 certificate wrapped
in "-----BEGIN ED25519 CERT-----" armor, cross-certifying the
descriptor signing key with the ed25519 equivalent of a curve25519
public encryption key derived using the process in proposal 228
appendix A. The certificate type must be [0B], and the signing-key
extension is mandatory.
"legacy-key" NL key NL
[None or at most once per introduction point]
The key is an ASN.1 encoded RSA public key in PEM format used for a
legacy introduction point as described in [LEGACY_EST_INTRO].
This field is only present if the introduction point only supports
legacy protocol (v2) that is <= 0.2.9 or the protocol version value
"HSIntro 3".
"legacy-key-cert" NL certificate NL
[None or at most once per introduction point]
MUST be present if "legacy-key" is present.
The certificate is a proposal 220 RSA->Ed cross-certificate wrapped
in "-----BEGIN CROSSCERT-----" armor, cross-certifying the the RSA
public key found in "legacy-key" using the descriptor signing key.
To remain compatible with future revisions to the descriptor format,
clients should ignore unrecognized lines in the descriptor.
Other encryption and authentication key formats are allowed; clients
should ignore ones they do not recognize.
Clients who manage to extract the introduction points of the hidden service
can prroceed with the introduction protocol as specified in [INTRO-PROTOCOL].
2.5.3. Deriving hidden service descriptor encryption keys [HS-DESC-ENCRYPTION-KEYS]
In this section we present the generic encryption format for hidden service
descriptors. We use the same encryption format in both encryption layers,
hence we introduce two customization parameters SECRET_DATA and
STRING_CONSTANT which vary between the layers.
The SECRET_DATA parameter specifies the secret data that are used during
encryption key generation, while STRING_CONSTANT is merely a string constant
that is used as part of the KDF.
Here is the key generation logic:
SALT = 16 bytes from H(random), changes each time we rebuld the
descriptor even if the content of the descriptor hasn't changed.
(So that we don't leak whether the intro point list etc. changed)
secret_input = SECRET_DATA | subcredential | INT_8(revision_counter)
keys = KDF(secret_input | salt | STRING_CONSTANT, S_KEY_LEN + S_IV_LEN + MAC_KEY_LEN)
SECRET_KEY = first S_KEY_LEN bytes of keys
SECRET_IV = next S_IV_LEN bytes of keys
MAC_KEY = last MAC_KEY_LEN bytes of keys
The encrypted data has the format:
SALT hashed random bytes from above [16 bytes]
ENCRYPTED The ciphertext [variable]
MAC D_MAC of both above fields [32 bytes]
The final encryption format is ENCRYPTED = STREAM(SECRET_IV,SECRET_KEY) XOR Plaintext .
Where D_MAC = H(mac_key_len | MAC_KEY | salt_len | SALT | ENCRYPTED)
and
mac_key_len = htonll(len(MAC_KEY))
and
salt_len = htonll(len(SALT)).
2.5.4. Number of introduction points [NUM_INTRO_POINT]
This section defines how many introduction points an hidden service
descriptor can have at minimum, by default and the maximum:
Minimum: 0 - Default: 3 - Maximum: 20
A value of 0 would means that the service is still alive but doesn't want
to be reached by any client at the moment. Note that the descriptor size
increases considerably as more introduction points are added.
The reason for a maximum value of 20 is to give enough scalability to tools
like OnionBalance to be able to load balance up to 120 servers (20 x 6
HSDirs) but also in order for the descriptor size to not overwhelmed hidden
service directories with user defined values that could be gigantic.
3. The introduction protocol [INTRO-PROTOCOL]
The introduction protocol proceeds in three steps.
First, a hidden service host builds an anonymous circuit to a Tor
node and registers that circuit as an introduction point.
Single Onion Services attempt to build a non-anonymous single-hop circuit,
but use an anonymous 3-hop circuit if:
* the intro point is on an address that is configured as unreachable via
a direct connection, or
* the initial attempt to connect to the intro point over a single-hop
circuit fails, and they are retrying the intro point connection.
[After 'First' and before 'Second', the hidden service publishes its
introduction points and associated keys, and the client fetches
them as described in section [HSDIR] above.]
Second, a client builds an anonymous circuit to the introduction
point, and sends an introduction request.
Third, the introduction point relays the introduction request along
the introduction circuit to the hidden service host, and acknowledges
the introduction request to the client.
3.1. Registering an introduction point [REG_INTRO_POINT]
3.1.1. Extensible ESTABLISH_INTRO protocol. [EST_INTRO]
When a hidden service is establishing a new introduction point, it
sends an ESTABLISH_INTRO cell with the following contents:
AUTH_KEY_TYPE [1 byte]
AUTH_KEY_LEN [2 bytes]
AUTH_KEY [AUTH_KEY_LEN bytes]
N_EXTENSIONS [1 byte]
N_EXTENSIONS times:
EXT_FIELD_TYPE [1 byte]
EXT_FIELD_LEN [1 byte]
EXT_FIELD [EXT_FIELD_LEN bytes]
HANDSHAKE_AUTH [MAC_LEN bytes]
SIG_LEN [2 bytes]
SIG [SIG_LEN bytes]
The AUTH_KEY_TYPE field indicates the type of the introduction point
authentication key and the type of the MAC to use in
HANDSHAKE_AUTH. Recognized types are:
[00, 01] -- Reserved for legacy introduction cells; see
[LEGACY_EST_INTRO below]
[02] -- Ed25519; SHA3-256.
The AUTH_KEY_LEN field determines the length of the AUTH_KEY
field. The AUTH_KEY field contains the public introduction point
authentication key.
The EXT_FIELD_TYPE, EXT_FIELD_LEN, EXT_FIELD entries are reserved for
future extensions to the introduction protocol. Extensions with
unrecognized EXT_FIELD_TYPE values must be ignored.
The HANDSHAKE_AUTH field contains the MAC of all earlier fields in
the cell using as its key the shared per-circuit material ("KH")
generated during the circuit extension protocol; see tor-spec.txt
section 5.2, "Setting circuit keys". It prevents replays of
ESTABLISH_INTRO cells.
SIG_LEN is the length of the signature.
SIG is a signature, using AUTH_KEY, of all contents of the cell, up
to but not including SIG. These contents are prefixed with the string
"Tor establish-intro cell v1".
Upon receiving an ESTABLISH_INTRO cell, a Tor node first decodes the
key and the signature, and checks the signature. The node must reject
the ESTABLISH_INTRO cell and destroy the circuit in these cases:
* If the key type is unrecognized
* If the key is ill-formatted
* If the signature is incorrect
* If the HANDSHAKE_AUTH value is incorrect
* If the circuit is already a rendezvous circuit.
* If the circuit is already an introduction circuit.
[TODO: some scalability designs fail there.]
* If the key is already in use by another circuit.
Otherwise, the node must associate the key with the circuit, for use
later in INTRODUCE1 cells.
3.1.1.1. Denial-of-Service Defense Extension. [EST_INTRO_DOS_EXT]
This extension can be used to send Denial-of-Service (DoS) parameters to
the introduction point in order for it to apply them for the introduction
circuit.
If used, it needs to be encoded within the N_EXTENSIONS field of the
ESTABLISH_INTRO cell defined in the previous section. The content is
defined as follow:
EXT_FIELD_TYPE:
[01] -- Denial-of-Service Parameters.
If this flag is set, the extension should be used by the introduction
point to learn what values the denial of service subsystem should be
using.
EXT_FIELD content format is:
N_PARAMS [1 byte]
N_PARAMS times:
PARAM_TYPE [1 byte]
PARAM_VALUE [8 byte]
The PARAM_TYPE possible values are:
[01] -- DOS_INTRODUCE2_RATE_PER_SEC
The rate per second of INTRODUCE2 cell relayed to the
service.
[02] -- DOS_INTRODUCE2_BURST_PER_SEC
The burst per second of INTRODUCE2 cell relayed to the
service.
The PARAM_VALUE size is 8 bytes in order to accomodate 64bit values.
It MUST match the specified limit for the following PARAM_TYPE:
[01] -- Min: 0, Max: 2147483647
[02] -- Min: 0, Max: 2147483647
A value of 0 means the defense is disabled. If the rate per second is
set to 0 (param 0x01) then the burst value should be ignored. And
vice-versa, if the burst value is 0 (param 0x02), then the rate value
should be ignored. In other words, setting one single parameter to 0
disables the defense.
The burst can NOT be smaller than the rate. If so, the parameters
should be ignored by the introduction point.
Any valid value does have precedence over the network wide consensus
parameter.
Using this extension extends the payload of the ESTABLISH_INTRO cell by 19
bytes bringing it from 134 bytes to 155 bytes.
This extension can only be used with relays supporting the protocol version
"HSIntro=5".
Introduced in tor-0.4.2.1-alpha.
3.1.2. Registering an introduction point on a legacy Tor node
[LEGACY_EST_INTRO]
Tor nodes should also support an older version of the ESTABLISH_INTRO
cell, first documented in rend-spec.txt. New hidden service hosts
must use this format when establishing introduction points at older
Tor nodes that do not support the format above in [EST_INTRO].
In this older protocol, an ESTABLISH_INTRO cell contains:
KEY_LEN [2 bytes]
KEY [KEY_LEN bytes]
HANDSHAKE_AUTH [20 bytes]
SIG [variable, up to end of relay payload]
The KEY_LEN variable determines the length of the KEY field.
The KEY field is the ASN1-encoded legacy RSA public key that was also
included in the hidden service descriptor.
The HANDSHAKE_AUTH field contains the SHA1 digest of (KH | "INTRODUCE").
The SIG field contains an RSA signature, using PKCS1 padding, of all
earlier fields.
Older versions of Tor always use a 1024-bit RSA key for these introduction
authentication keys.
3.1.3. Acknowledging establishment of introduction point [INTRO_ESTABLISHED]
After setting up an introduction circuit, the introduction point reports its
status back to the hidden service host with an INTRO_ESTABLISHED cell.
The INTRO_ESTABLISHED cell has the following contents:
N_EXTENSIONS [1 byte]
N_EXTENSIONS times:
EXT_FIELD_TYPE [1 byte]
EXT_FIELD_LEN [1 byte]
EXT_FIELD [EXT_FIELD_LEN bytes]
Older versions of Tor send back an empty INTRO_ESTABLISHED cell instead.
Services must accept an empty INTRO_ESTABLISHED cell from a legacy relay.
3.2. Sending an INTRODUCE1 cell to the introduction point. [SEND_INTRO1]
In order to participate in the introduction protocol, a client must
know the following:
* An introduction point for a service.
* The introduction authentication key for that introduction point.
* The introduction encryption key for that introduction point.
The client sends an INTRODUCE1 cell to the introduction point,
containing an identifier for the service, an identifier for the
encryption key that the client intends to use, and an opaque blob to
be relayed to the hidden service host.
In reply, the introduction point sends an INTRODUCE_ACK cell back to
the client, either informing it that its request has been delivered,
or that its request will not succeed.
[TODO: specify what tor should do when receiving a malformed cell. Drop it?
Kill circuit? This goes for all possible cells.]
3.2.1. INTRODUCE1 cell format [FMT_INTRO1]
When a client is connecting to an introduction point, INTRODUCE1 cells
should be of the form:
LEGACY_KEY_ID [20 bytes]
AUTH_KEY_TYPE [1 byte]
AUTH_KEY_LEN [2 bytes]
AUTH_KEY [AUTH_KEY_LEN bytes]
N_EXTENSIONS [1 byte]
N_EXTENSIONS times:
EXT_FIELD_TYPE [1 byte]
EXT_FIELD_LEN [1 byte]
EXT_FIELD [EXT_FIELD_LEN bytes]
ENCRYPTED [Up to end of relay payload]
AUTH_KEY_TYPE is defined as in [EST_INTRO]. Currently, the only value of
AUTH_KEY_TYPE for this cell is an Ed25519 public key [02].
The LEGACY_KEY_ID field is used to distinguish between legacy and new style
INTRODUCE1 cells. In new style INTRODUCE1 cells, LEGACY_KEY_ID is 20 zero
bytes. Upon receiving an INTRODUCE1 cell, the introduction point checks the
LEGACY_KEY_ID field. If LEGACY_KEY_ID is non-zero, the INTRODUCE1 cell
should be handled as a legacy INTRODUCE1 cell by the intro point.
Upon receiving a INTRODUCE1 cell, the introduction point checks
whether AUTH_KEY matches the introduction point authentication key for an
active introduction circuit. If so, the introduction point sends an
INTRODUCE2 cell with exactly the same contents to the service, and sends an
INTRODUCE_ACK response to the client.
3.2.2. INTRODUCE_ACK cell format. [INTRO_ACK]
An INTRODUCE_ACK cell has the following fields:
STATUS [2 bytes]
N_EXTENSIONS [1 bytes]
N_EXTENSIONS times:
EXT_FIELD_TYPE [1 byte]
EXT_FIELD_LEN [1 byte]
EXT_FIELD [EXT_FIELD_LEN bytes]
Recognized status values are:
[00 00] -- Success: cell relayed to hidden service host.
[00 01] -- Failure: service ID not recognized
[00 02] -- Bad message format
[00 03] -- Can't relay cell to service
3.3. Processing an INTRODUCE2 cell at the hidden service. [PROCESS_INTRO2]
Upon receiving an INTRODUCE2 cell, the hidden service host checks whether
the AUTH_KEY or LEGACY_KEY_ID field matches the keys for this
introduction circuit.
The service host then checks whether it has received a cell with these
contents or rendezvous cookie before. If it has, it silently drops it as a
replay. (It must maintain a replay cache for as long as it accepts cells
with the same encryption key. Note that the encryption format below should
be non-malleable.)
If the cell is not a replay, it decrypts the ENCRYPTED field,
establishes a shared key with the client, and authenticates the whole
contents of the cell as having been unmodified since they left the
client. There may be multiple ways of decrypting the ENCRYPTED field,
depending on the chosen type of the encryption key. Requirements for
an introduction handshake protocol are described in
[INTRO-HANDSHAKE-REQS]. We specify one below in section
[NTOR-WITH-EXTRA-DATA].
The decrypted plaintext must have the form:
RENDEZVOUS_COOKIE [20 bytes]
N_EXTENSIONS [1 byte]
N_EXTENSIONS times:
EXT_FIELD_TYPE [1 byte]
EXT_FIELD_LEN [1 byte]
EXT_FIELD [EXT_FIELD_LEN bytes]
ONION_KEY_TYPE [1 bytes]
ONION_KEY_LEN [2 bytes]
ONION_KEY [ONION_KEY_LEN bytes]
NSPEC (Number of link specifiers) [1 byte]
NSPEC times:
LSTYPE (Link specifier type) [1 byte]
LSLEN (Link specifier length) [1 byte]
LSPEC (Link specifier) [LSLEN bytes]
PAD (optional padding) [up to end of plaintext]
Upon processing this plaintext, the hidden service makes sure that
any required authentication is present in the extension fields, and
then extends a rendezvous circuit to the node described in the LSPEC
fields, using the ONION_KEY to complete the extension. As mentioned
in [BUILDING-BLOCKS], the "TLS-over-TCP, IPv4" and "Legacy node
identity" specifiers must be present.
As of 0.4.1.1-alpha, clients include both IPv4 and IPv6 link specifiers
in INTRODUCE1 cells. All available addresses SHOULD be included in the
cell, regardless of the address that the client actually used to extend
to the rendezvous point.
The hidden service should handle invalid or unrecognised link specifiers
the same way as clients do in section 2.5.2.2. In particular, services
MAY perform basic validity checks on link specifiers, and SHOULD NOT
reject unrecognised link specifiers, to avoid information leaks.
The ONION_KEY_TYPE field is:
[01] NTOR: ONION_KEY is 32 bytes long.
The ONION_KEY field describes the onion key that must be used when
extending to the rendezvous point. It must be of a type listed as
supported in the hidden service descriptor.
When using a legacy introduction point, the INTRODUCE cells must be padded
to a certain length using the PAD field in the encrypted portion.
Upon receiving a well-formed INTRODUCE2 cell, the hidden service host
will have:
* The information needed to connect to the client's chosen
rendezvous point.
* The second half of a handshake to authenticate and establish a
shared key with the hidden service client.
* A set of shared keys to use for end-to-end encryption.
3.3.1. Introduction handshake encryption requirements [INTRO-HANDSHAKE-REQS]
When decoding the encrypted information in an INTRODUCE2 cell, a
hidden service host must be able to:
* Decrypt additional information included in the INTRODUCE2 cell,
to include the rendezvous token and the information needed to
extend to the rendezvous point.
* Establish a set of shared keys for use with the client.
* Authenticate that the cell has not been modified since the client
generated it.
Note that the old TAP-derived protocol of the previous hidden service
design achieved the first two requirements, but not the third.
3.3.2. Example encryption handshake: ntor with extra data
[NTOR-WITH-EXTRA-DATA]
[TODO: relocate this]
This is a variant of the ntor handshake (see tor-spec.txt, section
5.1.4; see proposal 216; and see "Anonymity and one-way
authentication in key-exchange protocols" by Goldberg, Stebila, and
Ustaoglu).
It behaves the same as the ntor handshake, except that, in addition
to negotiating forward secure keys, it also provides a means for
encrypting non-forward-secure data to the server (in this case, to
the hidden service host) as part of the handshake.
Notation here is as in section 5.1.4 of tor-spec.txt, which defines
the ntor handshake.
The PROTOID for this variant is "tor-hs-ntor-curve25519-sha3-256-1".
We also use the following tweak values:
t_hsenc = PROTOID | ":hs_key_extract"
t_hsverify = PROTOID | ":hs_verify"
t_hsmac = PROTOID | ":hs_mac"
m_hsexpand = PROTOID | ":hs_key_expand"
To make an INTRODUCE1 cell, the client must know a public encryption
key B for the hidden service on this introduction circuit. The client
generates a single-use keypair:
x,X = KEYGEN()
and computes:
intro_secret_hs_input = EXP(B,x) | AUTH_KEY | X | B | PROTOID
info = m_hsexpand | subcredential
hs_keys = KDF(intro_secret_hs_input | t_hsenc | info, S_KEY_LEN+MAC_LEN)
ENC_KEY = hs_keys[0:S_KEY_LEN]
MAC_KEY = hs_keys[S_KEY_LEN:S_KEY_LEN+MAC_KEY_LEN]
and sends, as the ENCRYPTED part of the INTRODUCE1 cell:
CLIENT_PK [PK_PUBKEY_LEN bytes]
ENCRYPTED_DATA [Padded to length of plaintext]
MAC [MAC_LEN bytes]
Substituting those fields into the INTRODUCE1 cell body format
described in [FMT_INTRO1] above, we have
LEGACY_KEY_ID [20 bytes]
AUTH_KEY_TYPE [1 byte]
AUTH_KEY_LEN [2 bytes]
AUTH_KEY [AUTH_KEY_LEN bytes]
N_EXTENSIONS [1 bytes]
N_EXTENSIONS times:
EXT_FIELD_TYPE [1 byte]
EXT_FIELD_LEN [1 byte]
EXT_FIELD [EXT_FIELD_LEN bytes]
ENCRYPTED:
CLIENT_PK [PK_PUBKEY_LEN bytes]
ENCRYPTED_DATA [Padded to length of plaintext]
MAC [MAC_LEN bytes]
(This format is as documented in [FMT_INTRO1] above, except that here
we describe how to build the ENCRYPTED portion.)
Here, the encryption key plays the role of B in the regular ntor
handshake, and the AUTH_KEY field plays the role of the node ID.
The CLIENT_PK field is the public key X. The ENCRYPTED_DATA field is
the message plaintext, encrypted with the symmetric key ENC_KEY. The
MAC field is a MAC of all of the cell from the AUTH_KEY through the
end of ENCRYPTED_DATA, using the MAC_KEY value as its key.
To process this format, the hidden service checks PK_VALID(CLIENT_PK)
as necessary, and then computes ENC_KEY and MAC_KEY as the client did
above, except using EXP(CLIENT_PK,b) in the calculation of
intro_secret_hs_input. The service host then checks whether the MAC is
correct. If it is invalid, it drops the cell. Otherwise, it computes
the plaintext by decrypting ENCRYPTED_DATA.
The hidden service host now completes the service side of the
extended ntor handshake, as described in tor-spec.txt section 5.1.4,
with the modified PROTOID as given above. To be explicit, the hidden
service host generates a keypair of y,Y = KEYGEN(), and uses its
introduction point encryption key 'b' to compute:
intro_secret_hs_input = EXP(X,b) | AUTH_KEY | X | B | PROTOID
info = m_hsexpand | subcredential
hs_keys = KDF(intro_secret_hs_input | t_hsenc | info, S_KEY_LEN+MAC_LEN)
HS_DEC_KEY = hs_keys[0:S_KEY_LEN]
HS_MAC_KEY = hs_keys[S_KEY_LEN:S_KEY_LEN+MAC_KEY_LEN]
(The above are used to check the MAC and then decrypt the
encrypted data.)
rend_secret_hs_input = EXP(X,y) | EXP(X,b) | AUTH_KEY | B | X | Y | PROTOID
NTOR_KEY_SEED = MAC(rend_secret_hs_input, t_hsenc)
verify = MAC(rend_secret_hs_input, t_hsverify)
auth_input = verify | AUTH_KEY | B | Y | X | PROTOID | "Server"
AUTH_INPUT_MAC = MAC(auth_input, t_hsmac)
(The above are used to finish the ntor handshake.)
The server's handshake reply is:
SERVER_PK Y [PK_PUBKEY_LEN bytes]
AUTH AUTH_INPUT_MAC [MAC_LEN bytes]
These fields will be sent to the client in a RENDEZVOUS1 cell using the
HANDSHAKE_INFO element (see [JOIN_REND]).
The hidden service host now also knows the keys generated by the
handshake, which it will use to encrypt and authenticate data
end-to-end between the client and the server. These keys are as
computed in tor-spec.txt section 5.1.4.
3.4. Authentication during the introduction phase. [INTRO-AUTH]
Hidden services may restrict access only to authorized users.
One mechanism to do so is the credential mechanism, where only users who
know the credential for a hidden service may connect at all.
3.4.1. Ed25519-based authentication.
To authenticate with an Ed25519 private key, the user must include an
extension field in the encrypted part of the INTRODUCE1 cell with an
EXT_FIELD_TYPE type of [02] and the contents:
Nonce [16 bytes]
Pubkey [32 bytes]
Signature [64 bytes]
Nonce is a random value. Pubkey is the public key that will be used
to authenticate. [TODO: should this be an identifier for the public
key instead?] Signature is the signature, using Ed25519, of:
"hidserv-userauth-ed25519"
Nonce (same as above)
Pubkey (same as above)
AUTH_KEY (As in the INTRODUCE1 cell)
The hidden service host checks this by seeing whether it recognizes
and would accept a signature from the provided public key. If it
would, then it checks whether the signature is correct. If it is,
then the correct user has authenticated.
Replay prevention on the whole cell is sufficient to prevent replays
on the authentication.
Users SHOULD NOT use the same public key with multiple hidden
services.
4. The rendezvous protocol
Before connecting to a hidden service, the client first builds a
circuit to an arbitrarily chosen Tor node (known as the rendezvous
point), and sends an ESTABLISH_RENDEZVOUS cell. The hidden service
later connects to the same node and sends a RENDEZVOUS cell. Once
this has occurred, the relay forwards the contents of the RENDEZVOUS
cell to the client, and joins the two circuits together.
Single Onion Services attempt to build a non-anonymous single-hop circuit,
but use an anonymous 3-hop circuit if:
* the rend point is on an address that is configured as unreachable via
a direct connection, or
* the initial attempt to connect to the rend point over a single-hop
circuit fails, and they are retrying the rend point connection.
4.1. Establishing a rendezvous point [EST_REND_POINT]
The client sends the rendezvous point a RELAY_COMMAND_ESTABLISH_RENDEZVOUS
cell containing a 20-byte value.
RENDEZVOUS_COOKIE [20 bytes]
Rendezvous points MUST ignore any extra bytes in an
ESTABLISH_RENDEZVOUS cell. (Older versions of Tor did not.)
The rendezvous cookie is an arbitrary 20-byte value, chosen randomly
by the client. The client SHOULD choose a new rendezvous cookie for
each new connection attempt. If the rendezvous cookie is already in
use on an existing circuit, the rendezvous point should reject it and
destroy the circuit.
Upon receiving an ESTABLISH_RENDEZVOUS cell, the rendezvous point associates
the cookie with the circuit on which it was sent. It replies to the client
with an empty RENDEZVOUS_ESTABLISHED cell to indicate success. Clients MUST
ignore any extra bytes in a RENDEZVOUS_ESTABLISHED cell.
The client MUST NOT use the circuit which sent the cell for any
purpose other than rendezvous with the given location-hidden service.
The client should establish a rendezvous point BEFORE trying to
connect to a hidden service.
4.2. Joining to a rendezvous point [JOIN_REND]
To complete a rendezvous, the hidden service host builds a circuit to
the rendezvous point and sends a RENDEZVOUS1 cell containing:
RENDEZVOUS_COOKIE [20 bytes]
HANDSHAKE_INFO [variable; depends on handshake type
used.]
where RENDEZVOUS_COOKIE is the cookie suggested by the client during the
introduction (see [PROCESS_INTRO2]) and HANDSHAKE_INFO is defined in
[NTOR-WITH-EXTRA-DATA].
If the cookie matches the rendezvous cookie set on any
not-yet-connected circuit on the rendezvous point, the rendezvous
point connects the two circuits, and sends a RENDEZVOUS2 cell to the
client containing the HANDSHAKE_INFO field of the RENDEZVOUS1 cell.
Upon receiving the RENDEZVOUS2 cell, the client verifies that HANDSHAKE_INFO
correctly completes a handshake. To do so, the client parses SERVER_PK from
HANDSHAKE_INFO and reverses the final operations of section
[NTOR-WITH-EXTRA-DATA] as shown here:
rend_secret_hs_input = EXP(Y,x) | EXP(B,x) | AUTH_KEY | B | X | Y | PROTOID
NTOR_KEY_SEED = MAC(ntor_secret_input, t_hsenc)
verify = MAC(ntor_secret_input, t_hsverify)
auth_input = verify | AUTH_KEY | B | Y | X | PROTOID | "Server"
AUTH_INPUT_MAC = MAC(auth_input, t_hsmac)
Finally the client verifies that the received AUTH field of HANDSHAKE_INFO
is equal to the computed AUTH_INPUT_MAC.
Now both parties use the handshake output to derive shared keys for use on
the circuit as specified in the section below:
4.2.1. Key expansion
The hidden service and its client need to derive crypto keys from the
NTOR_KEY_SEED part of the handshake output. To do so, they use the KDF
construction as follows:
K = KDF(NTOR_KEY_SEED | m_hsexpand, HASH_LEN * 2 + S_KEY_LEN * 2)
The first HASH_LEN bytes of K form the forward digest Df; the next HASH_LEN
bytes form the backward digest Db; the next S_KEY_LEN bytes form Kf, and the
final S_KEY_LEN bytes form Kb. Excess bytes from K are discarded.
Subsequently, the rendezvous point passes relay cells, unchanged, from each
of the two circuits to the other. When Alice's OP sends RELAY cells along
the circuit, it authenticates with Df, and encrypts them with the Kf, then
with all of the keys for the ORs in Alice's side of the circuit; and when
Alice's OP receives RELAY cells from the circuit, it decrypts them with the
keys for the ORs in Alice's side of the circuit, then decrypts them with Kb,
and checks integrity with Db. Bob's OP does the same, with Kf and Kb
interchanged.
[TODO: Should we encrypt HANDSHAKE_INFO as we did INTRODUCE2
contents? It's not necessary, but it could be wise. Similarly, we
should make it extensible.]
4.3. Using legacy hosts as rendezvous points
The behavior of ESTABLISH_RENDEZVOUS is unchanged from older versions
of this protocol, except that relays should now ignore unexpected
bytes at the end.
Old versions of Tor required that RENDEZVOUS cell payloads be exactly
168 bytes long. All shorter rendezvous payloads should be padded to
this length with random bytes, to make them difficult to distinguish from
older protocols at the rendezvous point.
Relays older than 0.2.9.1 should not be used for rendezvous points by next
generation onion services because they enforce too-strict length checks to
rendezvous cells. Hence the "HSRend" protocol from proposal#264 should be
used to select relays for rendezvous points.
5. Encrypting data between client and host
A successfully completed handshake, as embedded in the
INTRODUCE/RENDEZVOUS cells, gives the client and hidden service host
a shared set of keys Kf, Kb, Df, Db, which they use for sending
end-to-end traffic encryption and authentication as in the regular
Tor relay encryption protocol, applying encryption with these keys
before other encryption, and decrypting with these keys before other
decryption. The client encrypts with Kf and decrypts with Kb; the
service host does the opposite.
6. Encoding onion addresses [ONIONADDRESS]
The onion address of a hidden service includes its identity public key, a
version field and a basic checksum. All this information is then base32
encoded as shown below:
onion_address = base32(PUBKEY | CHECKSUM | VERSION) + ".onion"
CHECKSUM = H(".onion checksum" | PUBKEY | VERSION)[:2]
where:
- PUBKEY is the 32 bytes ed25519 master pubkey of the hidden service.
- VERSION is an one byte version field (default value '\x03')
- ".onion checksum" is a constant string
- CHECKSUM is truncated to two bytes before inserting it in onion_address
Here are a few example addresses:
pg6mmjiyjmcrsslvykfwnntlaru7p5svn6y2ymmju6nubxndf4pscryd.onion
sp3k262uwy4r2k3ycr5awluarykdpag6a7y33jxop4cs2lu5uz5sseqd.onion
xa4r2iadxm55fbnqgwwi5mymqdcofiu3w6rpbtqn7b2dyn7mgwj64jyd.onion
For more information about this encoding, please see our discussion thread
at [ONIONADDRESS-REFS].
7. Open Questions:
Scaling hidden services is hard. There are on-going discussions that
you might be able to help with. See [SCALING-REFS].
How can we improve the HSDir unpredictability design proposed in
[SHAREDRANDOM]? See [SHAREDRANDOM-REFS] for discussion.
How can hidden service addresses become memorable while retaining
their self-authenticating and decentralized nature? See
[HUMANE-HSADDRESSES-REFS] for some proposals; many more are possible.
Hidden Services are pretty slow. Both because of the lengthy setup
procedure and because the final circuit has 6 hops. How can we make
the Hidden Service protocol faster? See [PERFORMANCE-REFS] for some
suggestions.
References:
[KEYBLIND-REFS]:
https://trac.torproject.org/projects/tor/ticket/8106
https://lists.torproject.org/pipermail/tor-dev/2012-September/004026.html
[KEYBLIND-PROOF]:
https://lists.torproject.org/pipermail/tor-dev/2013-December/005943.html
[SHAREDRANDOM-REFS]:
https://gitweb.torproject.org/torspec.git/tree/proposals/250-commit-reveal-consensus.txt
https://trac.torproject.org/projects/tor/ticket/8244
[SCALING-REFS]:
https://lists.torproject.org/pipermail/tor-dev/2013-October/005556.html
[HUMANE-HSADDRESSES-REFS]:
https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/ideas/xxx-onion-nyms.txt
http://archives.seul.org/or/dev/Dec-2011/msg00034.html
[PERFORMANCE-REFS]:
"Improving Efficiency and Simplicity of Tor circuit
establishment and hidden services" by Overlier, L., and
P. Syverson
[TODO: Need more here! Do we have any? :( ]
[ATTACK-REFS]:
"Trawling for Tor Hidden Services: Detection, Measurement,
Deanonymization" by Alex Biryukov, Ivan Pustogarov,
Ralf-Philipp Weinmann
"Locating Hidden Servers" by Lasse Øverlier and Paul
Syverson
[ED25519-REFS]:
"High-speed high-security signatures" by Daniel
J. Bernstein, Niels Duif, Tanja Lange, Peter Schwabe, and
Bo-Yin Yang. http://cr.yp.to/papers.html#ed25519
[ED25519-B-REF]:
https://tools.ietf.org/html/draft-josefsson-eddsa-ed25519-03#section-5:
[PRNG-REFS]:
http://projectbullrun.org/dual-ec/ext-rand.html
https://lists.torproject.org/pipermail/tor-dev/2015-November/009954.html
[SRV-TP-REFS]:
https://lists.torproject.org/pipermail/tor-dev/2016-April/010759.html
[VANITY-REFS]:
https://github.com/Yawning/horse25519
[ONIONADDRESS-REFS]:
https://lists.torproject.org/pipermail/tor-dev/2017-January/011816.html
[TORSION-REFS]:
https://lists.torproject.org/pipermail/tor-dev/2017-April/012164.html
https://getmonero.org/2017/05/17/disclosure-of-a-major-bug-in-cryptonote-based-currencies.html
Appendix A. Signature scheme with key blinding [KEYBLIND]
A.1. Key derivation overview
As described in [IMD:DIST] and [SUBCRED] above, we require a "key
blinding" system that works (roughly) as follows:
There is a master keypair (sk, pk).
Given the keypair and a nonce n, there is a derivation function
that gives a new blinded keypair (sk_n, pk_n). This keypair can
be used for signing.
Given only the public key and the nonce, there is a function
that gives pk_n.
Without knowing pk, it is not possible to derive pk_n; without
knowing sk, it is not possible to derive sk_n.
It's possible to check that a signature was made with sk_n while
knowing only pk_n.
Someone who sees a large number of blinded public keys and
signatures made using those public keys can't tell which
signatures and which blinded keys were derived from the same
master keypair.
You can't forge signatures.
[TODO: Insert a more rigorous definition and better references.]
A.2. Tor's key derivation scheme
We propose the following scheme for key blinding, based on Ed25519.
(This is an ECC group, so remember that scalar multiplication is the
trapdoor function, and it's defined in terms of iterated point
addition. See the Ed25519 paper [Reference ED25519-REFS] for a fairly
clear writeup.)
Let B be the ed25519 basepoint as found in section 5 of [ED25519-B-REF]:
B = (15112221349535400772501151409588531511454012693041857206046113283949847762202,
46316835694926478169428394003475163141307993866256225615783033603165251855960)
Assume B has prime order l, so lB=0. Let a master keypair be written as
(a,A), where a is the private key and A is the public key (A=aB).
To derive the key for a nonce N and an optional secret s, compute the
blinding factor like this:
h = H(BLIND_STRING | A | s | B | N)
BLIND_STRING = "Derive temporary signing key" | INT_1(0)
N = "key-blind" | INT_8(period-number) | INT_8(period_length)
B = "(1511[...]2202, 4631[...]5960)"
then clamp the blinding factor 'h' according to the ed25519 spec:
h[0] &= 248;
h[31] &= 63;
h[31] |= 64;
and do the key derivation as follows:
private key for the period:
a' = h a mod l
RH' = SHA-512(RH_BLIND_STRING | RH)[:32]
RH_BLIND_STRING = "Derive temporary signing key hash input"
public key for the period:
A' = h A = (ha)B
Generating a signature of M: given a deterministic random-looking r
(see EdDSA paper), take R=rB, S=r+hash(R,A',M)ah mod l. Send signature
(R,S) and public key A'.
Verifying the signature: Check whether SB = R+hash(R,A',M)A'.
(If the signature is valid,
SB = (r + hash(R,A',M)ah)B
= rB + (hash(R,A',M)ah)B
= R + hash(R,A',M)A' )
This boils down to regular Ed25519 with key pair (a', A').
See [KEYBLIND-REFS] for an extensive discussion on this scheme and
possible alternatives. Also, see [KEYBLIND-PROOF] for a security
proof of this scheme.
Appendix B. Selecting nodes [PICKNODES]
Picking introduction points
Picking rendezvous points
Building paths
Reusing circuits
(TODO: This needs a writeup)
Appendix C. Recommendations for searching for vanity .onions [VANITY]
EDITORIAL NOTE: The author thinks that it's silly to brute-force the
keyspace for a key that, when base-32 encoded, spells out the name of
your website. It also feels a bit dangerous to me. If you train your
users to connect to
llamanymityx4fi3l6x2gyzmtmgxjyqyorj9qsb5r543izcwymle.onion
I worry that you're making it easier for somebody to trick them into
connecting to
llamanymityb4sqi0ta0tsw6uovyhwlezkcrmczeuzdvfauuemle.onion
Nevertheless, people are probably going to try to do this, so here's a
decent algorithm to use.
To search for a public key with some criterion X:
Generate a random (sk,pk) pair.
While pk does not satisfy X:
Add the number 8 to sk
Add the point 8*B to pk
Return sk, pk.
We add 8 and 8*B, rather than 1 and B, so that sk is always a valid
Curve25519 private key, with the lowest 3 bits equal to 0.
This algorithm is safe [source: djb, personal communication] [TODO:
Make sure I understood correctly!] so long as only the final (sk,pk)
pair is used, and all previous values are discarded.
To parallelize this algorithm, start with an independent (sk,pk) pair
generated for each independent thread, and let each search proceed
independently.
See [VANITY-REFS] for a reference implementation of this vanity .onion
search scheme.
Appendix D. Numeric values reserved in this document
[TODO: collect all the lists of commands and values mentioned above]
Appendix E. Reserved numbers
We reserve these certificate type values for Ed25519 certificates:
[08] short-term descriptor signing key, signed with blinded
public key. (Section 2.4)
[09] intro point authentication key, cross-certifying the descriptor
signing key. (Section 2.5)
[0B] ed25519 key derived from the curve25519 intro point encryption key,
cross-certifying the descriptor signing key. (Section 2.5)
Note: The value "0A" is skipped because it's reserved for the onion key
cross-certifying ntor identity key from proposal 228.
Appendix F. Hidden service directory format [HIDSERVDIR-FORMAT]
This appendix section specifies the contents of the HiddenServiceDir directory:
- "hostname" [FILE]
This file contains the onion address of the onion service.
- "private_key_ed25519" [FILE]
This file contains the private master ed25519 key of the onion service.
[TODO: Offline keys]
- "./authorized_clients/" [DIRECTORY]
"./authorized_clients/alice.auth" [FILE]
"./authorized_clients/bob.auth" [FILE]
"./authorized_clients/charlie.auth" [FILE]
If client authorization is enabled, this directory MUST contain a ".auth"
file for each authorized client. Each such file contains the public key of
the respective client. The files are transmitted to the service operator by
the client.
See section [CLIENT-AUTH-MGMT] for more details and the format of the client file.
(NOTE: client authorization is implemented as of 0.3.5.1-alpha.)
Appendix G. Managing authorized client data [CLIENT-AUTH-MGMT]
Hidden services and clients can configure their authorized client data either
using the torrc, or using the control port. This section presents a suggested
scheme for configuring client authorization. Please see appendix
[HIDSERVDIR-FORMAT] for more information about relevant hidden service files.
(NOTE: client authorization is implemented as of 0.3.5.1-alpha.)
G.1. Configuring client authorization using torrc
G.1.1. Hidden Service side configuration
A hidden service that wants to enable client authorization, needs to
populate the "authorized_clients/" directory of its HiddenServiceDir
directory with the ".auth" files of its authorized clients.
When Tor starts up with a configured onion service, Tor checks its
<HiddenServiceDir>/authorized_clients/ directory for ".auth" files, and if
any recognized and parseable such files are found, then client
authorization becomes activated for that service.
G.1.2. Service-side bookkeeping
This section contains more details on how onion services should be keeping
track of their client ".auth" files.
For the "descriptor" authentication type, the ".auth" file MUST contain
the x25519 public key of that client. Here is a suggested file format:
<auth-type>:<key-type>:<base32-encoded-public-key>
Here is an an example:
descriptor:x25519:OM7TGIVRYMY6PFX6GAC6ATRTA5U6WW6U7A4ZNHQDI6OVL52XVV2Q
Tor SHOULD ignore lines it does not recognize.
Tor SHOULD ignore files that don't use the ".auth" suffix.
G.1.3. Client side configuration
A client who wants to register client authorization data for onion
services needs to add the following line to their torrc to indicate the
directory which hosts ".auth_private" files containing client-side
credentials for onion services:
ClientOnionAuthDir <DIR>
The <DIR> contains a file with the suffix ".auth_private" for each onion
service the client is authorized with. Tor should scan the directory for
".auth_private" files to find which onion services require client
authorization from this client.
For the "descriptor" auth-type, a ".auth_private" file contains the
private x25519 key:
<onion-address>:descriptor:x25519:<base32-encoded-privkey>
The keypair used for client authorization is created by a third party tool
for which the public key needs to be transferred to the service operator
in a secure out-of-band way. The third party tool SHOULD add appropriate
headers to the private key file to ensure that users won't accidentally
give out their private key.
G.2. Configuring client authorization using the control port
G.2.1. Service side
A hidden service also has the option to configure authorized clients
using the control port. The idea is that hidden service operators can use
controller utilities that manage their access control instead of using
the filesystem to register client keys.
Specifically, we require a new control port command ADD_ONION_CLIENT_AUTH
which is able to register x25519/ed25519 public keys tied to a specific
authorized client.
[XXX figure out control port command format]
Hidden services who use the control port interface for client auth need
to perform their own key management.
G.2.2. Client side
There should also be a control port interface for clients to register
authorization data for hidden services without having to use the
torrc. It should allow both generation of client authorization private
keys, and also to import client authorization data provided by a hidden
service
This way, Tor Browser can present "Generate client auth keys" and "Import
client auth keys" dialogs to users when they try to visit a hidden service
that is protected by client authorization.
Specifically, we require two new control port commands:
IMPORT_ONION_CLIENT_AUTH_DATA
GENERATE_ONION_CLIENT_AUTH_DATA
which import and generate client authorization data respectively.
[XXX how does key management work here?]
[XXX what happens when people use both the control port interface and the
filesystem interface?]
Appendix F. Two methods for managing revision counters.
Implementations MAY generate revision counters in any way they please,
so long as they are monotonically increasing over the lifetime of each
blinded public key. But to avoid fingerprinting, implementors SHOULD
choose a strategy also used by other Tor implementations. Here we
describe two, and additionally list some strategies that implementors
should NOT use.
F.1. Increment-on-generation
This is the simplest strategy, and the one used by Tor through at
least version 0.3.4.0-alpha.
Whenever using a new blinded key, the service records the
highest revision counter it has used with that key. When generating
a descriptor, the service uses the smallest non-negative number
higher than any number it has already used.
In other words, the revision counters under this system start fresh
with each blinded key as 0, 1, 2, 3, and so on.
F.2. Encrypted time in period
This scheme is what we recommend for situations when multiple
service instances need to coordinate their revision counters,
without an actual coordination mechanism.
Let T be the number of seconds that have elapsed since the descriptor
became valid, plus 1. (T must be at least 1.) Implementations can use the
number of seconds since the start time of the shared random protocol run
that corresponds to this descriptor.
Let S be a secret that all the service providers share. For
example, it could be the private signing key corresponding to the
current blinded key.
Let K be an AES-256 key, generated as
K = H("rev-counter-generation" | S)
Use K, and AES in counter mode with IV=0, to generate a stream of T
* 2 bytes. Consider these bytes as a sequence of T 16-bit
little-endian words. Add these words.
Let the sum of these words be the revision counter.
Cryptowiki attributes roughly this scheme to G. Bebek in:
G. Bebek. Anti-tamper database research: Inference control
techniques. Technical Report EECS 433 Final Report, Case
Western Reserve University, November 2002.
Although we believe it is suitable for use in this application, it
is not a perfect order-preserving encryption algorithm (and all
order-preserving encryption has weaknesses). Please think twice
before using it for anything else.
(This scheme can be optimized pretty easily by caching the encryption of
X*1, X*2, X*3, etc for some well chosen X.)
For a slow reference implementation, see src/test/ope_ref.py in the
Tor source repository. [XXXX for now, see the same file in Nick's
"ope_hax" branch -- it isn't merged yet.]
This scheme is not currently implemented in Tor.
F.X. Some revision-counter strategies to avoid
Though it might be tempting, implementations SHOULD NOT use the
current time or the current time within the period directly as their
revision counter -- doing so leaks their view of the current time,
which can be used to link the onion service to other services run on
the same host.
Similarly, implementations SHOULD NOT let the revision counter
increase forever without resetting it -- doing so links the service
across changes in the blinded public key.