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Versions: 00 01

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

   THIS DOCUMENT AND THE INFORMATION CONTAINED THEREIN ARE PROVIDED ON
   AN "AS IS" BASIS AND THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
   IETF TRUST, AND THE INTERNET ENGINEERING TASK FORCE, DISCLAIM ALL
   WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
   WARRANTY THAT THE USE OF THE INFORMATION THEREIN WILL NOT INFRINGE
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   FOR A PARTICULAR PURPOSE.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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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
   document authors.  All rights reserved.

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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.



























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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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