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RTCWEB E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Standards Track March 12, 2012
Expires: September 13, 2012
RTCWEB Generic Identity Provider Interface
draft-rescorla-rtcweb-generic-idp-01
Abstract
Security for RTCWEB communications requires that the communicating
endpoints be able to authenticate each other. While authentication
may be mediated by the calling service, there are settings in which
this is undesirable. This document describes a generic mechanism for
leveraging existing identity providers (IdPs) such as BrowserID or
OAuth to provide this authentication service.
Legal
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This Internet-Draft will expire on September 13, 2012.
Copyright Notice
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Copyright (c) 2012 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Trust Relationships: IdPs, APs, and RPs . . . . . . . . . . . 6
4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 7
5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. General Message Structure . . . . . . . . . . . . . . . . 9
5.1.1. Errors . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. IdP Proxy Setup . . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Determining the IdP URI . . . . . . . . . . . . . . . 10
5.2.1.1. Authenticating Party . . . . . . . . . . . . . . . 11
5.2.1.2. Relying Party . . . . . . . . . . . . . . . . . . 11
5.3. Requesting Assertions . . . . . . . . . . . . . . . . . . 11
5.4. Verifying Assertions . . . . . . . . . . . . . . . . . . . 12
5.4.1. Identity Formats . . . . . . . . . . . . . . . . . . . 13
5.4.2. PostMessage Checks . . . . . . . . . . . . . . . . . . 14
5.4.3. PeerConnection API Extensions . . . . . . . . . . . . 14
5.4.3.1. Authenticating Party . . . . . . . . . . . . . . . 14
5.4.3.2. Relying Party . . . . . . . . . . . . . . . . . . 15
5.5. Example Bindings to Specific Protocols . . . . . . . . . . 16
5.5.1. BrowserID . . . . . . . . . . . . . . . . . . . . . . 16
5.5.2. OAuth . . . . . . . . . . . . . . . . . . . . . . . . 19
5.6. Security Considerations . . . . . . . . . . . . . . . . . 20
5.6.1. PeerConnection Origin Check . . . . . . . . . . . . . 20
5.6.2. IdP Well-known URI . . . . . . . . . . . . . . . . . . 20
5.6.3. Security of Third-Party IdPs . . . . . . . . . . . . . 21
5.7. Web Security Feature Interactions . . . . . . . . . . . . 21
5.7.1. Popup Blocking . . . . . . . . . . . . . . . . . . . . 21
5.7.2. Third Party Cookies . . . . . . . . . . . . . . . . . 21
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6.1. Normative References . . . . . . . . . . . . . . . . . . . 22
6.2. Informative References . . . . . . . . . . . . . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
Security for RTCWEB communications requires that the communicating
endpoints be able to authenticate each other. While authentication
may be mediated by the calling service, there are settings in which
this is undesirable. This document describes a mechanism for
leveraging existing identity providers (IdPs) such as BrowserID or
OAuth to provide this authentication service.
Specifically, Alice and Bob have relationships with some Identity
Provider (IdP) that supports a protocol such OpenID or BrowserID)
that can be used to attest to their identity. While they are making
calls through the signaling service, their identities (and the
cryptographic keying material used to make the call) is authenticated
via the IdP. This separation isn't particularly important in "closed
world" cases where Alice and Bob are users on the same social
network, have identities based on that network, and are calling using
that network's signaling service. However, there are important
settings where that is not the case, such as federation (calls from
one network to another) and calling on untrusted sites, such as where
two users who have a relationship via a given social network want to
call each other on another, untrusted, site, such as a poker site.
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+----------------+
| |
| Signaling |
| Server |
| |
+----------------+
^ ^
/ \
HTTPS / \ HTTPS
/ \
/ \
v v
JS API JS API
+-----------+ +-----------+
| | Media | |
Alice | Browser |<---------->| Browser | Bob
| | (DTLS-SRTP)| |
+-----------+ +-----------+
^ ^--+ +--^ ^
| | | |
v | | v
+-----------+ | | +-----------+
| |<--------+ | |
| IdP A | | | IdP B |
| | +------->| |
+-----------+ +-----------+
Figure 1: A call with IdP-based identity
Figure 1 shows the basic topology. Alice and Bob are on the same
signaling server, but they additionally have relationships with their
own IdPs. Alice has registered with IdP A and Bob has registered
with IdP B. Note that nothing stops these IdPs from being the same,
or indeed from being the same as the signaling server, but they can
also be totally distinct. In particular, Alice and Bob need not have
identities from the same IdP.
Starting from this point, the mechanisms described in this document
allow Alice and Bob to establish a mutually authenticated phone call.
In the interest of clarity the remainder of this section provides a
brief overview of how these mechanisms fit into the bigger RTCWEB
calling picture. For a detailed description of the relevant protocol
elements and their interaction with the larger signaling protocol see
[I-D.ietf-rtcweb-security]. When Alice goes to call Bob, her browser
(specifically her PeerConnection object) contacts her IdP on her
behalf and obtains an assertion of her identity bound to her
certificate fingerprint. This assertion is carried with her
signaling messages to the signaling server and then down to Bob.
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Bob's browser verifies the assertion, possibly with the cooperation
of the IdP, and can then display Alice's identity to Bob in a trusted
user interface element. If Alice is in Bob's address book, then this
interface might also include her real name, a picture, etc.
When/If Bob agrees to answer the call, his browser contacts his IdP
and gets a similar assertion. This assertion is sent to the
signaling server as part of Bob's answer which is then forwarded to
Alice. Alice's browser verifies Bob's identity and can display the
result in a trusted UI element. At this point Alice and Bob know
each other's fingerprints and so they can transitively verify the
keys used to authenticate the DTLS-SRTP handshake and hence the
security of the media.
The mechanisms in this document do not require the browser to
implement any particular identity protocol or to support any
particular IdP. Instead, this document provides a generic interface
which any IdP can implement. Thus, new IdPs and protocols can be
introduced without change to either the browser or the calling
service. This avoids the need to make a commitment to any particular
identity protocol, although browsers may opt to directly implement
some identity protocols in order to provide superior performance or
UI properties.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Trust Relationships: IdPs, APs, and RPs
Any authentication protocol has three major participants:
Authenticating Party (AP): The entity which is trying to establish
its identity.
Identity Provider (IdP): The entity which is vouching for the AP's
identity.
Relying Party (RP): The entity which is trying to verify the AP's
identity.
The AP and the IdP have an account relationship of some kind: the AP
registers with the IdP and is able to subsequently authenticate
directly to the IdP (e.g., with a password). This means that the
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browser must somehow know which IdP(s) the user has an account
relationship with. This can either be something that the user
configures into the browser or that is configured at the calling site
and then provided to the PeerConnection by the calling site.
At a high level there are two kinds of IdPs:
Authoritative: IdPs which have verifiable control of some section
of the identity space. For instance, in the realm of e-mail, the
operator of "example.com" has complete control of the namespace
ending in "@example.com". Thus, "alice@example.com" is whoever
the operator says it is. Examples of systems with authoritative
identity providers include DNSSEC, RFC 4474, and Facebook Connect
(Facebook identities only make sense within the context of the
Facebook system).
Third-Party: IdPs which don't have control of their section of the
identity space but instead verify user's identities via some
unspecified mechanism and then attest to it. Because the IdP
doesn't actually control the namespace, RPs need to trust that the
IdP is correctly verifying AP identities, and there can
potentially be multiple IdPs attesting to the same section of the
identity space. Probably the best-known example of a third-party
identity provider is SSL certificates, where there are a large
number of CAs all of whom can attest to any domain name.
If an AP is authenticating via an authoritative IdP, then the RP does
not need to explicitly trust the IdP at all: as long as the RP knows
how to verify that the IdP indeed made the relevant identity
assertion (a function provided by the mechanisms in this document),
then any assertion it makes about an identity for which it is
authoritative is directly verifiable.
By contrast, if an AP is authenticating via a third-party IdP, the RP
needs to explicitly trust that IdP (hence the need for an explicit
trust anchor list in PKI-based SSL/TLS clients). The list of
trustable IdPs needs to be configured directly into the browser,
either by the user or potentially by the browser manufacturer. This
is a significant advantage of authoritative IdPs and implies that if
third-party IdPs are to be supported, the potential number needs to
be fairly small.
4. Overview of Operation
In order to provide security without trusting the calling site, the
PeerConnection component of the browser must interact directly with
the IdP. In this section, we describe a standalone mechanism based
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on IFRAMEs and postMessage(), however, most likely this will
eventually be superceded by WebIntents <http://www.webintents.com/>.
[[ OPEN ISSUE: I've been looking at WebIntents and I believe that it
can be made to work but may require some modifications. I am
currently studying the problem. More analysis to come.]] ]].
+------------------------------------+
| https://calling-site.example.com |
| |
| |
| |
| Calling JS Code |
| ^ |
| | API Calls |
| v |
| PeerConnection |
| ^ |
| | postMessage() |
| v |
| +-------------------------+ | +---------------+
| | https://idp.example.org | | | |
| | |<--------->| Identity |
| | IdP JS | | | Provider |
| | | | | |
| +-------------------------+ | +---------------+
| |
+------------------------------------+
When the PeerConnection object wants to interact with the IdP, the
sequence of events is as follows:
1. The browser (the PeerConnection component) instantiates an IdP
proxy (typically a hidden IFRAME) with its source at the IdP.
This allows the IdP to load whatever JS is necessary into the
proxy, which runs in the IdP's security context.
2. If the user is not already logged in, the IdP does whatever is
required to log them in, such as soliciting a username and
password.
3. Once the user is logged in, the IdP proxy notifies the browser
(via postMessage()) that it is ready.
4. The browser and the IdP proxy communicate via a standardized
series of messages delivered via postMessage. For instance, the
browser might request the IdP proxy to sign or verify a given
identity assertion.
This approach allows us to decouple the browser from any particular
identity provider; the browser need only know how to load the IdP's
JavaScript--which is deterministic from the IdP's identity--and the
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generic protocol for requesting and verifying assertions. The IdP
provides whatever logic is necessary to bridge the generic protocol
to the IdP's specific requirements. Thus, a single browser can
support any number of identity protocols, including being forward
compatible with IdPs which did not exist at the time the browser was
written.
5. Protocol Details
5.1. General Message Structure
Messages between the PeerConnection object and the IdP proxy are
formatted using JSON [RFC4627]. For instance, the PeerConnection
would request a signature with the following "SIGN" message:
{
"type":"SIGN",
"id": "1",
"message":"012345678abcdefghijkl"
}
All messages MUST contain a "type" field which indicates the general
meaning of the message.
All requests from the PeerConnection object MUST contain an "id"
field which MUST be unique for that PeerConnection object. Any
responses from the IdP proxy MUST contain the same id in response,
which allows the PeerConnection to correlate requests and responses.
Any message-specific data is carried in a "message" field. Depending
on the message type, this may either be a string or a richer JSON
object.
5.1.1. Errors
If an error occurs, the IdP sends a message of type "ERROR". The
message MAY have an "error" field containing freeform text data which
containing additional information about what happened. For instance:
{
"type":"ERROR",
"error":"Signature verification failed"
}
Figure 2: Example error
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5.2. IdP Proxy Setup
In order to perform an identity transaction, the PeerConnection must
first create the IdP proxy. While the specific technical mechanism
used is left up to the implementation, the following requirements
MUST be met for security and interoperability.
o Any JS MUST run in the IdP's security context.
o The usual browser sandbox isolation mechanisms MUST be enforced
with respect to the IdP proxy.
o JS running in the IdP proxy MUST be able to send and receive
messages to the PeerConnection object using postMessage.
o Either window.parent or window.opener MUST be set such that
messages sent with postMessage() arrive at the PeerConnection
object. If both variables are set, they MUST be the same.
o Messages sent by the PeerConnection object MUST have their .origin
value set to "rtcweb:://idp-interface". [TBD]
One mechanism for implementing the IdP proxy is as a hidden (CSS
"display=none") IFRAME with a URI as determined in Section 5.2.1.
The PeerConnection component will of course need to specially arrange
for the origin value to be set correctly; as dicussed in Section 5.6,
the fact that ordinary Web pages cannot set their origins to
"rtcweb://..." is an essential security feature.
Initially the IdP proxy is in an unready state; the IdP JS must be
loaded and there may be several round trips to the IdP server, for
instance to log the user in. Thus, the IFRAME's "onready" property
is not a reliable indicator of when the IdP IFRAME is ready to
receive commands. Instead, when the IdP proxy is ready to receive
commands, it delivers a "ready" message via postMessage(). As this
message is unsolicited, it simply contains:
{ "type":"READY" }
Once the PeerConnection object receives the ready message, it can
send commands to the IdP proxy.
5.2.1. Determining the IdP URI
Each IdP proxy instance is associated with two values:
domain name: The IdP's domain name
protocol: The specific IdP protocol which the IdP is using. This is
a completely IdP-specific string, but allows an IdP to implement
two protocols in parallel. This value may be the empty string.
Each IdP MUST serve its initial entry page (i.e., the one loaded by
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the IdP proxy) from the well-known URI specified in "/.well-known/
idp-proxy/<protocol>" on the IdP's web site. This URI MUST be loaded
via HTTPS [RFC2818]. For example, for the IdP "identity.example.com"
and the protocol "example", the URL would be:
https://example.com/.well-known/idp-proxy/example
5.2.1.1. Authenticating Party
How an AP determines the appropriate IdP domain is out of scope of
this specification. In general, however, the AP has some actual
account relationship with the IdP, as this identity is what the IdP
is attesting to. Thus, the AP somehow supplies the IdP information
to the browser. Some potential mechanisms include:
o Provided by the user directly.
o Selected from some set of IdPs known to the calling site. E.g., a
button that shows "Authenticate via Facebook Connect"
5.2.1.2. Relying Party
Unlike the AP, the RP need not have any particular relationship with
the IdP. Rather, it needs to be able to process whatever assertion
is provided by the AP. As the assertion contains the IdP's identity,
the URI can be constructed directly from the assertion, and thus the
RP can directly verify the technical validity of the assertion with
no user interaction. Authoritative assertions need only be
verifiable. Third-party assertions also MUST be verified against
local policy, as described in Section 5.4.1.
5.3. Requesting Assertions
In order to request an assertion, the PeerConnection sends a "SIGN"
message. Aside from the mandatory fields, this message has a
"message" field containing a string. The contents of this string are
defined in [I-D.ietf-rtcweb-security], but are opaque from the
perspective of this protocol.
A successful response to a "SIGN" message contains a message field
which is a JS dictionary dictionary consisting of two fields:
idp: A dictionary containing the domain name of the provider and the
protocol string
assertion: An opaque field containing the assertion itself. This is
only interpretable by the idp or its proxy.
Figure 3 shows an example transaction, with the message "abcde..."
being signed and bound to identity "ekr@example.org". In this case,
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the message has presumably been digitally signed/MACed in some way
that the IdP can later verify it, but this is an implementation
detail and out of scope of this document. Line breaks are inserted
solely for readability.
PeerConnection -> IdP proxy:
{
"type":"SIGN",
"id":1,
"message":"abcdefghijklmnopqrstuvwyz"
}
IdPProxy -> PeerConnection:
{
"type":"SUCCESS",
"id":1,
"message": {
"idp":{
"domain": "example.org"
"protocol": "bogus"
},
"assertion":\"{\"identity\":\"bob@example.org\",
\"contents\":\"abcdefghijklmnopqrstuvwyz\",
\"signature\":\"010203040506\"}"
}
}
Figure 3: Example assertion request
5.4. Verifying Assertions
In order to verify an assertion, an RP sends a "VERIFY" message to
the IdP proxy containing the assertion supplied by the AP in the
"message" field.
The IdP proxy verifies the assertion. Depending on the identity
protocol, this may require one or more round trips to the IdP. For
instance, an OAuth-based protocol will likely require using the IdP
as an oracle, whereas with BrowserID the IdP proxy can likely verify
the signature on the assertion without contacting the IdP, provided
that it has cached the IdP's public key.
Regardless of the mechanism, if verification succeeds, a successful
response from the IdP proxy MUST contain a message field consisting
of a dictionary/hash with the following fields:
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identity The identity of the AP from the IdP's perspective. Details
of this are provided in Section 5.4.1
contents The original unmodified string provided by the AP in the
original SIGN request.
Figure 4 shows an example transaction. Line breaks are inserted
solely for readability.
PeerConnection -> IdP Proxy:
{
"type":"VERIFY",
"id":2,
"message":\"{\"identity\":\"bob@example.org\",
\"contents\":\"abcdefghijklmnopqrstuvwyz\",
\"signature\":\"010203040506\"}"
}
IdP Proxy -> PeerConnection:
{
"type":"SUCCESS",
"id":2,
"message": {
"identity" : {
"name" : "bob@example.org",
"displayname" : "Bob"
},
"contents":"abcdefghijklmnopqrstuvwyz"
}
}
Figure 4: Example assertion request
5.4.1. Identity Formats
Identities passed from the IdP proxy to the PeerConnection are
structured as JSON dictionaries with one mandatory field: "name".
This field MUST consist of an RFC822-formatted string representing
the user's identity. [[ OPEN ISSUE: Would it be better to have a
typed field? ]] The PeerConnection API MUST check this string as
follows:
1. If the RHS of the string is equal to the domain name of the IdP
proxy, then the assertion is valid, as the IdP is authoritative
for this domain.
2. If the RHS of the string is not equal to the domain name of the
IdP proxy, then the PeerConnection object MUST reject the
assertion unless (a) the IdP domain is listed as an acceptable
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third-party IdP and (b) local policy is configured to trust this
IdP domain for the RHS of the identity string.
Sites which have identities that do not fit into the RFC822 style
(for instance, Facebook ids are simple numeric values) SHOULD convert
them to this form by appending their IdP domain (e.g.,
12345@identity.facebook.com), thus ensuring that they are
authoritative for the identity.
The IdP proxy MAY also include a "displayname" field which contains a
more user-friendly identity assertion. Browsers SHOULD take care in
the UI to distinguish the "name" assertion which is verifiable
directly from the "displayname" which cannot be verified and thus
relies on trust in the IdP. In future, we may define other fields to
allow the IdP to provide more information to the browser.
5.4.2. PostMessage Checks
Because the PeerConnect object and the IdP proxy communicate via
postMessage(), it is essential to verify that the origin of any
message (contained in the event.origin property) and source
(contained in the event.source) property are as expected:
o For messages from the PeerConnection object, the IdP proxy MUST
verify that the origin is "rtcweb://idp-interface" and that the
source matches either window.opener or window.parent. If both are
non-falsey, they MUST be equal. If any of these checks fail, the
message MUST be rejected. [[ OPEN ISSUE: An alternate (more
generic) design would be to not check the origin here but rather
to include the origin in the assertion and have it checked at the
RP. Comments? ]]
o For messages from the IdP proxy, the PeerConnection object MUST
verify that the origin matches the IdP's origin and that the
source matches the window/IFRAME opened for the IdP proxy.
If any of these checks fail, the message MUST be rejected. In
general, mismatches SHOULD NOT cause transaction failure, since
malicious JS might use bogus messages as a form of DoS attack.
5.4.3. PeerConnection API Extensions
5.4.3.1. Authenticating Party
As discussed in Section 3, the AP's IdP can either be configured
directly into the browser or selected from a list known to the
calling site. We anticipate that some browsers will allow
configuration of IdPs in the browser UI but allow the calling
application to provide new candidate IdPs or to direct the selection
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of a known one. Thus, one model would be:
o If a IdP is provided by the calling application use that.
o If no IdP is provided, and one is configured, use that.
o If no IdP is provided or configured, do nothing.
Implementations MAY also wish to have configuration settings override
the calling application's preferences.
APIs for PeerConnection configuration are as-yet unsettled, but it
MUST be possible to specify the following parameters to the
PeerConnection.
o The IdP domain.
o The users expected identity (if known) [this allows selection
between multiple candidate identities with the same IdP.]
5.4.3.2. Relying Party
Because the browser UI must be responsible for displaying the user's
identity, it isn't strictly necessary to have new JS interfaces on
the relying party side. However, two new interfaces are RECOMMENDED.
When a message is provided to the PeerConnection API with
processSignalingMessage() with an assertion that cannot be verified,
there is a need for some sort of error indicating verification
failure. [Note: I don't see an interface for any other kind of
parse error, so I'm not sure what to imitate here.]
A new attribute should be added to indicate the verification status.
For instance:
readonly attribute DOMString verifiedIdentity;
The attribute value should be a JS dictionary indicating the identity
and the domain name of the IdP, such as:
{
"identity" : "ekr@example.org",
"idp": "example.org"
}
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5.5. Example Bindings to Specific Protocols
This section provides some examples of how the mechanisms described
in this document could be used with existing authentication protocols
such as BrowserID or OAuth. Note that this does not require browser-
level support for either protocol. Rather, the protocols can be fit
into the generic framework. (Though BrowserID in particular works
better with some client side support).
5.5.1. BrowserID
BrowserID [https://browserid.org/] is a technology which allows a
user with a verified email address to generate an assertion
(authenticated by their identity provider) attesting to their
identity (phrased as an email address). The way that this is used in
practice is that the relying party embeds JS in their site which
talks to the BrowserID code (either hosted on a trusted intermediary
or embedded in the browser). That code generates the assertion which
is passed back to the relying party for verification. The assertion
can be verified directly or with a Web service provided by the
identity provider. It's relatively easy to extend this functionality
to authenticate RTCWEB calls, as shown below.
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+----------------------+ +----------------------+
| | | |
| Alice's Browser | | Bob's Browser |
| | OFFER ------------> | |
| Calling JS Code | | Calling JS Code |
| ^ | | ^ |
| | | | | |
| v | | v |
| PeerConnection | | PeerConnection |
| | ^ | | | ^ |
| Finger| |Signed | |Signed | | |
| print | |Finger | |Finger | |"Alice"|
| | |print | |print | | |
| v | | | v | |
| +--------------+ | | +---------------+ |
| | IdP Proxy | | | | IdP Proxy | |
| | to | | | | to | |
| | BrowserID | | | | BrowserID | |
| | Signer | | | | Verifier | |
| +--------------+ | | +---------------+ |
| ^ | | ^ |
+-----------|----------+ +----------|-----------+
| |
| Get certificate |
v | Check
+----------------------+ | certificate
| | |
| Identity |/-------------------------------+
| Provider |
| |
+----------------------+
The way this mechanism works is as follows. On Alice's side, Alice
goes to initiate a call.
1. The calling JS instantiates a PeerConnection and tells it that it
is interested in having it authenticated via BrowserID (i.e., it
provides "browserid.org" as the IdP name.)
2. The PeerConnection instantiates the BrowserID signer in the IdP
proxy
3. The BrowserID signer contacts Alice's identity provider,
authenticating as Alice (likely via a cookie).
4. The identity provider returns a short-term certificate attesting
to Alice's identity and her short-term public key.
5. The Browser-ID code signs the fingerprint and returns the signed
assertion + certificate to the PeerConnection.
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6. The PeerConnection returns the signed information to the calling
JS code.
7. The signed assertion gets sent over the wire to Bob's browser
(via the signaling service) as part of the call setup.
Obviously, the format of the signed assertion varies depending on
what signaling style the WG ultimately adopts. However, for
concreteness, if something like ROAP were adopted, then the entire
message might look like:
{
"messageType":"OFFER",
"callerSessionId":"13456789ABCDEF",
"seq": 1
"sdp":"
v=0\n
o=- 2890844526 2890842807 IN IP4 192.0.2.1\n
s= \n
c=IN IP4 192.0.2.1\n
t=2873397496 2873404696\n
m=audio 49170 RTP/AVP 0\n
a=fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB\n",
"identity":{
"idp":{ // Standardized
"domain":"browserid.org",
"method":"default"
},
"assertion": // Contents are browserid-specific
"\"assertion\": {
\"digest\":\"<hash of the contents from the browser>\",
\"audience\": \"[TBD]\"
\"valid-until\": 1308859352261,
},
\"certificate\": {
\"email\": \"rescorla@example.org\",
\"public-key\": \"<ekrs-public-key>\",
\"valid-until\": 1308860561861,
}" // certificate is signed by example.org
}
}
Note that while the IdP here is specified as "browserid.org", the
actual certificate is signed by example.org. This is because
BrowserID is a combined authoritative/third-party system in which
browserid.org delegates the right to be authoritative (what BrowserID
calls primary) to individual domains.
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On Bob's side, he receives the signed assertion as part of the call
setup message and a similar procedure happens to verify it.
1. The calling JS instantiates a PeerConnection and provides it the
relevant signaling information, including the signed assertion.
2. The PeerConnection instantiates the IdP proxy which examines the
IdP name and brings up the BrowserID verification code.
3. The BrowserID verifier contacts the identity provider to verify
the certificate and then uses the key to verify the signed
fingerprint.
4. Alice's verified identity is returned to the PeerConnection (it
already has the fingerprint).
5. At this point, Bob's browser can display a trusted UI indication
that Alice is on the other end of the call.
When Bob returns his answer, he follows the converse procedure, which
provides Alice with a signed assertion of Bob's identity and keying
material.
5.5.2. OAuth
While OAuth is not directly designed for user-to-user authentication,
with a little lateral thinking it can be made to serve. We use the
following mapping of OAuth concepts to RTCWEB concepts:
+----------------------+----------------------+
| OAuth | RTCWEB |
+----------------------+----------------------+
| Client | Relying party |
| Resource owner | Authenticating party |
| Authorization server | Identity service |
| Resource server | Identity service |
+----------------------+----------------------+
Table 1
The idea here is that when Alice wants to authenticate to Bob (i.e.,
for Bob to be aware that she is calling). In order to do this, she
allows Bob to see a resource on the identity provider that is bound
to the call, her identity, and her public key. Then Bob retrieves
the resource from the identity provider, thus verifying the binding
between Alice and the call.
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Alice IdP Bob
---------------------------------------------------------
Call-Id, Fingerprint ------->
<------------------- Auth Code
Auth Code ---------------------------------------------->
<----- Get Token + Auth Code
Token --------------------->
<------------- Get call-info
Call-Id, Fingerprint ------>
This is a modified version of a common OAuth flow, but omits the
redirects required to have the client point the resource owner to the
IdP, which is acting as both the resource server and the
authorization server, since Alice already has a handle to the IdP.
Above, we have referred to "Alice", but really what we mean is the
PeerConnection. Specifically, the PeerConnection will instantiate an
IFRAME with JS from the IdP and will use that IFRAME to communicate
with the IdP, authenticating with Alice's identity (e.g., cookie).
Similarly, Bob's PeerConnection instantiates an IFRAME to talk to the
IdP.
5.6. Security Considerations
This mechanism relies for its security on the IdP and on the
PeerConnection correctly enforcing the security invariants described
above. At a high level, the IdP is attesting that the user
identified in the assertion wishes to be associated with the
assertion. Thus, it must not be possible for arbitrary third parties
to get assertions tied to a user or to produce assertions that RPs
will accept.
5.6.1. PeerConnection Origin Check
Fundamentally, the IdP proxy is just a piece of HTML and JS loaded by
the browser, so nothing stops a Web attacker o from creating their
own IFRAME, loading the IdP proxy HTML/JS, and requesting a
signature. In order to prevent this attack, we require that all
signatures be tied to a specific origin ("rtcweb://...") which cannot
be produced by a page tied to a Web attacker. Thus, while an
attacker can instantiate the IdP proxy, they cannot send messages
from an appropriate origin and so cannot create acceptable
assertions. [[OPEN ISSUE: Where is this enforced? ]]
5.6.2. IdP Well-known URI
As described in Section 5.2.1 the IdP proxy HTML/JS landing page is
located at a well-known URI based on the IdP's domain name. This
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requirement prevents an attacker who can write some resources at the
IdP (e.g., on one's Facebook wall) from being able to impersonate the
IdP.
5.6.3. Security of Third-Party IdPs
As discussed above, each third-party IdP represents a new universal
trust point and therefore the number of these IdPs needs to be quite
limited. Most IdPs, even those which issue unqualified identities
such as Facebook, can be recast as authoritative IdPs (e.g.,
123456@facebook.com). However, in such cases, the user interface
implications are not entirely desirable. One intermediate approach
is to have special (potentially user configurable) UI for large
authoritative IdPs, thus allowing the user to instantly grasp that
the call is being authenticated by Facebook, Google, etc.
5.7. Web Security Feature Interactions
A number of optional Web security features have the potential to
cause issues for this mechanism, as discussed below.
5.7.1. Popup Blocking
If the user is not already logged into the IdP, the IdP proxy may
need to pop up a top level window in order to prompt the user for
their authentication information (it is bad practice to do this in an
IFRAME inside the window because then users have no way to determine
the destination for their password). If the user's browser is
configured to prevent popups, this may fail (depending on the exact
algorithm that the popup blocker uses to suppress popups). It may be
necessary to provide a standardized mechanism to allow the IdP proxy
to request popping of a login window. Note that care must be taken
here to avoid PeerConnection becoming a general escape hatch from
popup blocking. One possibility would be to only allow popups when
the user has explicitly registered a given IdP as one of theirs (this
is only relevant at the AP side in any case). This is what
WebIntents does, and the problem would go away if WebIntents is used.
5.7.2. Third Party Cookies
Some browsers allow users to block third party cookies (cookies
associated with origins other than the top level page) for privacy
reasons. Any IdP which uses cookies to persist logins will be broken
by third-party cookie blocking. One option is to accept this as a
limitation; another is to have the PeerConnection object disable
third-party cookie blocking for the IdP proxy.
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6. References
6.1. Normative References
[I-D.ietf-rtcweb-security]
Rescorla, E., "Security Considerations for RTC-Web",
draft-ietf-rtcweb-security-01 (work in progress),
October 2011.
[I-D.ietf-rtcweb-security-arch]
Rescorla, E., "RTCWEB Security Architecture",
draft-ietf-rtcweb-security-arch-00 (work in progress),
January 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
6.2. Informative References
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011.
Author's Address
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Phone: +1 650 678 2350
Email: ekr@rtfm.com
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