draft-ietf-rtcweb-security-arch-02.txt   draft-ietf-rtcweb-security-arch-03.txt 
RTCWEB E. Rescorla RTCWEB E. Rescorla
Internet-Draft RTFM, Inc. Internet-Draft RTFM, Inc.
Intended status: Standards Track June 5, 2012 Intended status: Standards Track July 16, 2012
Expires: December 7, 2012 Expires: January 17, 2013
RTCWEB Security Architecture RTCWEB Security Architecture
draft-ietf-rtcweb-security-arch-02 draft-ietf-rtcweb-security-arch-03
Abstract Abstract
The Real-Time Communications on the Web (RTCWEB) working group is The Real-Time Communications on the Web (RTCWEB) working group is
tasked with standardizing protocols for enabling real-time tasked with standardizing protocols for enabling real-time
communications within user-agents using web technologies (e.g communications within user-agents using web technologies (e.g
JavaScript). The major use cases for RTCWEB technology are real-time JavaScript). The major use cases for RTCWEB technology are real-time
audio and/or video calls, Web conferencing, and direct data transfer. audio and/or video calls, Web conferencing, and direct data transfer.
Unlike most conventional real-time systems (e.g., SIP-based soft Unlike most conventional real-time systems (e.g., SIP-based soft
phones) RTCWEB communications are directly controlled by some Web phones) RTCWEB communications are directly controlled by some Web
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 7, 2012. This Internet-Draft will expire on January 17, 2013.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Trust Model . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Authenticated Entities . . . . . . . . . . . . . . . . . . 5 3.1. Authenticated Entities . . . . . . . . . . . . . . . . . . 5
3.2. Unauthenticated Entities . . . . . . . . . . . . . . . . . 5 3.2. Unauthenticated Entities . . . . . . . . . . . . . . . . . 5
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Initial Signaling . . . . . . . . . . . . . . . . . . . . 7 4.1. Initial Signaling . . . . . . . . . . . . . . . . . . . . 7
4.2. Media Consent Verification . . . . . . . . . . . . . . . . 9 4.2. Media Consent Verification . . . . . . . . . . . . . . . . 9
4.3. DTLS Handshake . . . . . . . . . . . . . . . . . . . . . . 10 4.3. DTLS Handshake . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Communications and Consent Freshness . . . . . . . . . . . 10 4.4. Communications and Consent Freshness . . . . . . . . . . . 10
5. Detailed Technical Description . . . . . . . . . . . . . . . . 10 5. Detailed Technical Description . . . . . . . . . . . . . . . . 10
5.1. Origin and Web Security Issues . . . . . . . . . . . . . . 10 5.1. Origin and Web Security Issues . . . . . . . . . . . . . . 10
5.2. Device Permissions Model . . . . . . . . . . . . . . . . . 11 5.2. Device Permissions Model . . . . . . . . . . . . . . . . . 11
5.3. Communications Consent . . . . . . . . . . . . . . . . . . 12 5.3. Communications Consent . . . . . . . . . . . . . . . . . . 12
5.4. IP Location Privacy . . . . . . . . . . . . . . . . . . . 13 5.4. IP Location Privacy . . . . . . . . . . . . . . . . . . . 13
5.5. Communications Security . . . . . . . . . . . . . . . . . 13 5.5. Communications Security . . . . . . . . . . . . . . . . . 14
5.6. Web-Based Peer Authentication . . . . . . . . . . . . . . 15 5.6. Web-Based Peer Authentication . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . . 16 5.6.1. Trust Relationships: IdPs, APs, and RPs . . . . . . . 16
6.1. Communications Security . . . . . . . . . . . . . . . . . 16 5.6.2. Overview of Operation . . . . . . . . . . . . . . . . 17
6.2. Privacy . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.6.3. Items for Standardization . . . . . . . . . . . . . . 19
6.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 17 5.6.4. Binding Identity Assertions to JSEP Offer/Answer
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18 Transactions . . . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.6.4.1. Input to Assertion Generation Process . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . . 18 5.6.4.2. Carrying Identity Assertions . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . . 19 5.6.5. IdP Interaction Details . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.6.5.1. General Message Structure . . . . . . . . . . . . 20
5.6.5.2. IdP Proxy Setup . . . . . . . . . . . . . . . . . 21
5.7. Security Considerations . . . . . . . . . . . . . . . . . 26
5.7.1. Communications Security . . . . . . . . . . . . . . . 26
5.7.2. Privacy . . . . . . . . . . . . . . . . . . . . . . . 27
5.7.3. Denial of Service . . . . . . . . . . . . . . . . . . 27
5.7.4. IdP Authentication Mechanism . . . . . . . . . . . . . 28
5.7.4.1. IdP Well-known URI . . . . . . . . . . . . . . . . 29
5.7.4.2. Privacy of IdP-generated identities and the
hosting site . . . . . . . . . . . . . . . . . . . 29
5.7.4.3. Security of Third-Party IdPs . . . . . . . . . . . 29
5.7.4.4. Web Security Feature Interactions . . . . . . . . 29
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 30
7. Changes since -02 . . . . . . . . . . . . . . . . . . . . . . 30
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.1. Normative References . . . . . . . . . . . . . . . . . . . 30
8.2. Informative References . . . . . . . . . . . . . . . . . . 31
Appendix A. Example IdP Bindings to Specific Protocols . . . . . 32
A.1. BrowserID . . . . . . . . . . . . . . . . . . . . . . . . 32
A.2. OAuth . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 36
1. Introduction 1. Introduction
The Real-Time Communications on the Web (RTCWEB) working group is The Real-Time Communications on the Web (RTCWEB) working group is
tasked with standardizing protocols for real-time communications tasked with standardizing protocols for real-time communications
between Web browsers. The major use cases for RTCWEB technology are between Web browsers. The major use cases for RTCWEB technology are
real-time audio and/or video calls, Web conferencing, and direct data real-time audio and/or video calls, Web conferencing, and direct data
transfer. Unlike most conventional real-time systems, (e.g., SIP- transfer. Unlike most conventional real-time systems, (e.g., SIP-
based[RFC3261] soft phones) RTCWEB communications are directly based[RFC3261] soft phones) RTCWEB communications are directly
controlled by some Web server, as shown in Figure 1. controlled by some Web server, as shown in Figure 1.
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categories: those which can be authenticated by the browser and thus categories: those which can be authenticated by the browser and thus
are partly trusted--though to the minimum extent necessary--and those are partly trusted--though to the minimum extent necessary--and those
which cannot be authenticated and thus are untrusted. This is a which cannot be authenticated and thus are untrusted. This is a
natural extension of the end-to-end principle. natural extension of the end-to-end principle.
3.1. Authenticated Entities 3.1. Authenticated Entities
There are two major classes of authenticated entities in the system: There are two major classes of authenticated entities in the system:
o Calling services: Web sites whose origin we can verify (optimally o Calling services: Web sites whose origin we can verify (optimally
via HTTPS). via HTTPS, but in some cases because we are on a topologically
restricted network, such as behind a firewall).
o Other users: RTCWEB peers whose origin we can verify o Other users: RTCWEB peers whose origin we can verify
cryptographically (optimally via DTLS-SRTP). cryptographically (optimally via DTLS-SRTP).
Note that merely being authenticated does not make these entities Note that merely being authenticated does not make these entities
trusted. For instance, just because we can verify that trusted. For instance, just because we can verify that
https://www.evil.org/ is owned by Dr. Evil does not mean that we can https://www.evil.org/ is owned by Dr. Evil does not mean that we can
trust Dr. Evil to access our camera an microphone. However, it gives trust Dr. Evil to access our camera and microphone. However, it
the user an opportunity to determine whether he wishes to trust Dr. gives the user an opportunity to determine whether he wishes to trust
Evil or not; after all, if he desires to contact Dr. Evil (perhaps to Dr. Evil or not; after all, if he desires to contact Dr. Evil
arrange for ransom payment), it's safe to temporarily give him access (perhaps to arrange for ransom payment), it's safe to temporarily
to the camera and microphone for the purpose of the call, but he give him access to the camera and microphone for the purpose of the
doesn't want Dr. Evil to be able to access his camera and microphone call, but he doesn't want Dr. Evil to be able to access his camera
other than during the call. The point here is that we must first and microphone other than during the call. The point here is that we
identify other elements before we can determine whether and how much must first identify other elements before we can determine whether
to trust them. and how much to trust them.
It's also worth noting that there are settings where authentication It's also worth noting that there are settings where authentication
is non-cryptographic, such as other machines behind a firewall. is non-cryptographic, such as other machines behind a firewall.
Naturally, the level of trust one can have in identities verified in Naturally, the level of trust one can have in identities verified in
this way depends on how strong the topology enforcement is. this way depends on how strong the topology enforcement is.
3.2. Unauthenticated Entities 3.2. Unauthenticated Entities
Other than the above entities, we are not generally able to identify Other than the above entities, we are not generally able to identify
other network elements, thus we cannot trust them. This does not other network elements, thus we cannot trust them. This does not
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two MediaStreams, one connected to an audio input and one connected two MediaStreams, one connected to an audio input and one connected
to a video input. At this point the first security check is to a video input. At this point the first security check is
required: untrusted origins are not allowed to access the camera and required: untrusted origins are not allowed to access the camera and
microphone. In this case, because Alice is a long-term user of the microphone. In this case, because Alice is a long-term user of the
calling service, she has made a permissions grant (i.e., a setting in calling service, she has made a permissions grant (i.e., a setting in
the browser) to allow the calling service to access her camera and the browser) to allow the calling service to access her camera and
microphone any time it wants. The browser checks this setting when microphone any time it wants. The browser checks this setting when
the camera and microphone requests are made and thus allows them. the camera and microphone requests are made and thus allows them.
In the current W3C API, once some streams have been added, Alice's In the current W3C API, once some streams have been added, Alice's
browser + JS generates a signaling message The format of this data is browser + JS generates a signaling message [I-D.ietf-rtcweb-jsep]
currently undefined. It may be a complete message as defined by ROAP contianing:
[I-D.jennings-rtcweb-signaling] or separate media description and
transport messages as defined in [I-D.ietf-rtcweb-jsep] or may be
assembled piecemeal by the JS. In either case, it will contain:
o Media channel information o Media channel information
o ICE candidates o ICE candidates
o A fingerprint attribute binding the communication to Alice's o A fingerprint attribute binding the communication to Alice's
public key [RFC5763] public key [RFC5763]
[Note that it is currently unclear where JSEP will eventually put Prior to sending out the signaling message, the PeerConnection code
this information, in the SDP or in the transport info.] Prior to contacts the identity service and obtains an assertion binding
sending out the signaling message, the PeerConnection code contacts Alice's identity to her fingerprint. The exact details depend on the
the identity service and obtains an assertion binding Alice's identity service (though as discussed in Section 5.6 PeerConnection
identity to her fingerprint. The exact details depend on the can be agnostic to them), but for now it's easiest to think of as a
identity service (though as discussed in BrowserID assertion. The assertion may bind other information to the
[I-D.rescorla-rtcweb-generic-idp] PeerConnection can be agnostic to identity besides the fingerprint, but at minimum it needs to bind the
them), but for now it's easiest to think of as a BrowserID assertion. fingerprint.
The assertion may bind other information to the identity besides the
fingerprint, but at minimum it needs to bind the fingerprint.
This message is sent to the signaling server, e.g., by XMLHttpRequest This message is sent to the signaling server, e.g., by XMLHttpRequest
[XmlHttpRequest] or by WebSockets [RFC6455] The signaling server [XmlHttpRequest] or by WebSockets [RFC6455] The signaling server
processes the message from Alice's browser, determines that this is a processes the message from Alice's browser, determines that this is a
call to Bob and sends a signaling message to Bob's browser (again, call to Bob and sends a signaling message to Bob's browser (again,
the format is currently undefined). The JS on Bob's browser the format is currently undefined). The JS on Bob's browser
processes it, and alerts Bob to the incoming call and to Alice's processes it, and alerts Bob to the incoming call and to Alice's
identity. In this case, Alice has provided an identity assertion and identity. In this case, Alice has provided an identity assertion and
so Bob's browser contacts Alice's identity provider (again, this is so Bob's browser contacts Alice's identity provider (again, this is
done in a generic way so the browser has no specific knowledge of the done in a generic way so the browser has no specific knowledge of the
IdP) to verity the assertion. This allows the browser to display a IdP) to verify the assertion. This allows the browser to display a
trusted element indicating that a call is coming in from Alice. If trusted element indicating that a call is coming in from Alice. If
Alice is in Bob's address book, then this interface might also Alice is in Bob's address book, then this interface might also
include her real name, a picture, etc. The calling site will also include her real name, a picture, etc. The calling site will also
provide some user interface element (e.g., a button) to allow Bob to provide some user interface element (e.g., a button) to allow Bob to
answer the call, though this is most likely not part of the trusted answer the call, though this is most likely not part of the trusted
UI. UI.
If Bob agrees [I am ignoring early media for now], a PeerConnection If Bob agrees [I am ignoring early media for now], a PeerConnection
is instantiated with the message from Alice's side. Then, a similar is instantiated with the message from Alice's side. Then, a similar
process occurs as on Alice's browser: Bob's browser verifies that process occurs as on Alice's browser: Bob's browser verifies that
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duration of the call. Implementations MAY choose to terminate the duration of the call. Implementations MAY choose to terminate the
call or display a warning at that point, but it is also permissible call or display a warning at that point, but it is also permissible
to ignore this condition. This is a deliberate implementation to ignore this condition. This is a deliberate implementation
complexity versus security tradeoff. [[ OPEN ISSUE:: Should we be complexity versus security tradeoff. [[ OPEN ISSUE:: Should we be
more aggressive about this?]] more aggressive about this?]]
5.2. Device Permissions Model 5.2. Device Permissions Model
Implementations MUST obtain explicit user consent prior to providing Implementations MUST obtain explicit user consent prior to providing
access to the camera and/or microphone. Implementations MUST at access to the camera and/or microphone. Implementations MUST at
minimum support the following two permissions models: minimum support the following two permissions models for HTTPS
origins.
o Requests for one-time camera/microphone access. o Requests for one-time camera/microphone access.
o Requests for permanent access. o Requests for permanent access.
Because HTTP origins cannot be securely established against network
attackers, implementations MUST NOT allow the setting of permanent
access permissions for HTTP origins. Implementations MAY also opt to
refuse all permissions grants for HTTP origins, but it is RECOMMENDED
that currently they support one-time camera/microphone access.
In addition, they SHOULD support requests for access to a single In addition, they SHOULD support requests for access to a single
communicating peer. E.g., "Call customerservice@ford.com". Browsers communicating peer. E.g., "Call customerservice@ford.com". Browsers
servicing such requests SHOULD clearly indicate that identity to the servicing such requests SHOULD clearly indicate that identity to the
user when asking for permission. user when asking for permission.
API Requirement: The API MUST provide a mechanism for the requesting API Requirement: The API MUST provide a mechanism for the requesting
JS to indicate which of these forms of permissions it is JS to indicate which of these forms of permissions it is
requesting. This allows the client to know what sort of user requesting. This allows the browser to know what sort of user
interface experience to provide. In particular, browsers might interface experience to provide to the user, including what
display a non-invasive door hanger ("some features of this site permissions to request from the user and hence that to enforce
may not work..." when asking for long-term permissions) but a more later. For instance, browsers might display a non-invasive door
invasive UI ("here is your own video") for single-call hanger ("some features of this site may not work..." when asking
permissions. The API MAY grant weaker permissions than the JS for long-term permissions) but a more invasive UI ("here is your
asked for if the user chooses to authorize only those permissions, own video") for single-call permissions. The API MAY grant weaker
but if it intends to grant stronger ones it SHOULD display the permissions than the JS asked for if the user chooses to authorize
appropriate UI for those permissions and MUST clearly indicate only those permissions, but if it intends to grant stronger ones
what permissions are being requested. it SHOULD display the appropriate UI for those permissions and
MUST clearly indicate what permissions are being requested.
API Requirement: The API MUST provide a mechanism for the requesting API Requirement: The API MUST provide a mechanism for the requesting
JS to relinquish the ability to see or modify the media (e.g., via JS to relinquish the ability to see or modify the media (e.g., via
MediaStream.record()). Combined with secure authentication of the MediaStream.record()). Combined with secure authentication of the
communicating peer, this allows a user to be sure that the calling communicating peer, this allows a user to be sure that the calling
site is not accessing or modifying their conversion. site is not accessing or modifying their conversion.
UI Requirement: The UI MUST clearly indicate when the user's camera UI Requirement: The UI MUST clearly indicate when the user's camera
and microphone are in use. This indication MUST NOT be and microphone are in use. This indication MUST NOT be
suppressable by the JS and MUST clearly indicate how to terminate suppressable by the JS and MUST clearly indicate how to terminate
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address book (this only works with address book integration, of address book (this only works with address book integration, of
course). course).
Implementations SHOULD also provide a different user interface Implementations SHOULD also provide a different user interface
indication when calls are in progress to users whose identities are indication when calls are in progress to users whose identities are
directly verifiable. Section 5.5 provides more on this. directly verifiable. Section 5.5 provides more on this.
5.3. Communications Consent 5.3. Communications Consent
Browser client implementations of RTCWEB MUST implement ICE. Server Browser client implementations of RTCWEB MUST implement ICE. Server
gateway implementations which operate only at public IP addresses may gateway implementations which operate only at public IP addresses
implement ICE-Lite. MUST implement either full ICE or ICE-Lite.
Browser implementations MUST verify reachability via ICE prior to Browser implementations MUST verify reachability via ICE prior to
sending any non-ICE packets to a given destination. Implementations sending any non-ICE packets to a given destination. Implementations
MUST NOT provide the ICE transaction ID to JavaScript during the MUST NOT provide the ICE transaction ID to JavaScript during the
lifetime of the transaction (i.e., during the period when the ICE lifetime of the transaction (i.e., during the period when the ICE
stack would accept a new response for that transaction). [Note: stack would accept a new response for that transaction). [Note:
this document takes no position on the split between ICE in JS and this document takes no position on the split between ICE in JS and
ICE in the browser. The above text is written the way it is for ICE in the browser. The above text is written the way it is for
editorial convenience and will be modified appropriately if the WG editorial convenience and will be modified appropriately if the WG
decides on ICE in the JS.] decides on ICE in the JS.] The JS MUST NOT be permitted to control
the local ufrag and password, though it of course knows it.
Implementations MUST send keepalives no less frequently than every 30 While continuing consent is required, that ICE [RFC5245]; Section 10
seconds regardless of whether traffic is flowing or not. If a keepalives STUN Binding Indications are one-way and therefore not
keepalive fails then the implementation MUST either attempt to find a sufficient. The current WG consensus is to use ICE Binding Requests
new valid path via ICE or terminate media for that ICE component. for continuing consent freshness. ICE already requires that
Note that ICE [RFC5245]; Section 10 keepalives use STUN Binding implementations respond to such requests, so this approach is
Indications which are one-way and therefore not sufficient. Instead, maximally compatible. A separate document will profile the ICE
the consent freshness mechanism [I-D.muthu-behave-consent-freshness] timers to be used [[TODO: insert REF here when available.]]
MUST be used.
5.4. IP Location Privacy 5.4. IP Location Privacy
A side effect of the default ICE behavior is that the peer learns A side effect of the default ICE behavior is that the peer learns
one's IP address, which leaks large amounts of location information, one's IP address, which leaks large amounts of location information,
especially for mobile devices. This has negative privacy especially for mobile devices. This has negative privacy
consequences in some circumstances. The following two API consequences in some circumstances. The API requirements in this
requirements are intended to mitigate this issue: section are intended to mitigate this issue. Note that these
requirements are NOT intended to protect the user's IP address from a
malicious site. In general, the site will learn at least a user's
server reflexive address from any HTTP transaction. Rather, these
requirements are intended to allow a site to cooperate with the user
to hide the user's IP address from the other side of the call.
Hiding the user's IP address from the server requires some sort of
explicit privacy preserving mechanism on the client (e.g., Torbutton
[https://www.torproject.org/torbutton/]) and is out of scope for this
specification.
API Requirement: The API MUST provide a mechanism to suppress ICE API Requirement: The API MUST provide a mechanism to allow the JS to
negotiation (though perhaps to allow candidate gathering) until suppress ICE negotiation (though perhaps to allow candidate
the user has decided to answer the call [note: determining when gathering) until the user has decided to answer the call [note:
the call has been answered is a question for the JS.] This determining when the call has been answered is a question for the
enables a user to prevent a peer from learning their IP address if JS.] This enables a user to prevent a peer from learning their IP
they elect not to answer a call and also from learning whether the address if they elect not to answer a call and also from learning
user is online. whether the user is online.
API Requirement: The API MUST provide a mechanism for the calling API Requirement: The API MUST provide a mechanism for the calling
application to indicate that only TURN candidates are to be used. application JS to indicate that only TURN candidates are to be
This prevents the peer from learning one's IP address at all. The used. This prevents the peer from learning one's IP address at
API MUST provide a mechanism for the calling application to all.
reconfigure an existing call to add non-TURN candidates. Taken
together, these requirements allow ICE negotiation to start API Requirement: The API MUST provide a mechanism for the calling
immediately on incoming call notification, thus reducing post-dial application to reconfigure an existing call to add non-TURN
delay, but also to avoid disclosing the user's IP address until candidates. Taken together, this and the previous requirement
they have decided to answer. allow ICE negotiation to start immediately on incoming call
notification, thus reducing post-dial delay, but also to avoid
disclosing the user's IP address until they have decided to
answer. They also allow users to completely hide their IP address
for the duration of the call. Finally, they allow a mechanism for
the user to optimize performance by reconfiguring to allow non-
turn candidates during an active call if the user decides they no
longer need to hide their IP address
5.5. Communications Security 5.5. Communications Security
Implementations MUST implement DTLS [RFC4347] and DTLS-SRTP Implementations MUST implement DTLS [RFC4347] and DTLS-SRTP
[RFC5763][RFC5764]. All data channels MUST be secured via DTLS. [RFC5763][RFC5764]. All data channels MUST be secured via DTLS.
DTLS-SRTP MUST be offered for every media channel and MUST be the DTLS-SRTP MUST be offered for every media channel and MUST be the
default; i.e., if an implementation receives an offer for DTLS-SRTP default; i.e., if an implementation receives an offer for DTLS-SRTP
and SDES, DTLS-SRTP MUST be selected. Media traffic MUST NOT be sent and SDES, DTLS-SRTP MUST be selected. Media traffic MUST NOT be sent
over plain (unencrypted) RTP. over plain (unencrypted) RTP.
[OPEN ISSUE: What should the settings be here? MUST?] [OPEN ISSUE: What should the settings be here? MUST?]
Implementations MAY support SDES and RTP for media traffic for Implementations MAY support SDES for media traffic for backward
backward compatibility purposes. compatibility purposes.
API Requirement: The API MUST provide a mechanism to indicate that a API Requirement: The API MUST provide a mechanism to indicate that a
fresh DTLS key pair is to be generated for a specific call. This fresh DTLS key pair is to be generated for a specific call. This
is intended to allow for unlinkability. Note that there are also is intended to allow for unlinkability. Note that there are also
settings where it is attractive to use the same keying material settings where it is attractive to use the same keying material
repeatedly, especially those with key continuity-based repeatedly, especially those with key continuity-based
authentication. authentication.
API Requirement: The API MUST provide a mechanism to indicate that a
fresh DTLS key pair is to be generated for a specific call. This
is intended to allow for unlinkability.
API Requirement: When DTLS-SRTP is used, the API MUST NOT permit the API Requirement: When DTLS-SRTP is used, the API MUST NOT permit the
JS to obtain the negotiated keying material. This requirement JS to obtain the negotiated keying material. This requirement
preserves the end-to-end security of the media. preserves the end-to-end security of the media.
UI Requirements: A user-oriented client MUST provide an UI Requirements: A user-oriented client MUST provide an
"inspector" interface which allows the user to determine the "inspector" interface which allows the user to determine the
security characteristics of the media. [largely derived from security characteristics of the media. [largely derived from
[I-D.kaufman-rtcweb-security-ui] [I-D.kaufman-rtcweb-security-ui]
The following properties SHOULD be displayed "up-front" in the The following properties SHOULD be displayed "up-front" in the
browser chrome, i.e., without requiring the user to ask for them: browser chrome, i.e., without requiring the user to ask for them:
skipping to change at page 14, line 47 skipping to change at page 15, line 16
determine the security characteristics for transmissions of determine the security characteristics for transmissions of
their microphone audio and camera video. their microphone audio and camera video.
* The "security characteristics" MUST include an indication as to * The "security characteristics" MUST include an indication as to
whether the cryptographic keys were delivered out-of-band (from whether the cryptographic keys were delivered out-of-band (from
a server) or were generated as a result of a pairwise a server) or were generated as a result of a pairwise
negotiation. negotiation.
* If the far endpoint was directly verified, either via a third- * If the far endpoint was directly verified, either via a third-
party verifiable X.509 certificate or via a Web IdP mechanism party verifiable X.509 certificate or via a Web IdP mechanism
(see Section 5.6) the "security characteristics" MUST include (see Section 5.6) the "security characteristics" MUST include
the verified information. the verified information.
The following properties are more likely to require some "drill- The following properties are more likely to require some "drill-
down" from the user: down" from the user:
* The cryptographic algorithms in use (For example: "AES-CBC" or * The "security characteristics" MUST indicate the cryptographic
"Null Cipher".) algorithms in use (For example: "AES-CBC" or "Null Cipher".)
* The "security characteristics" MUST indicate whether PFS is * The "security characteristics" MUST indicate whether PFS is
provided. provided.
* The "security characteristics" MUST include some mechanism to * The "security characteristics" MUST include some mechanism to
allow an out-of-band verification of the peer, such as a allow an out-of-band verification of the peer, such as a
certificate fingerprint or an SAS. certificate fingerprint or an SAS.
5.6. Web-Based Peer Authentication 5.6. Web-Based Peer Authentication
In a number of cases, it is desirable for the endpoint (i.e., the In a number of cases, it is desirable for the endpoint (i.e., the
browser) to be able to directly identity the endpoint on the other browser) to be able to directly identity the endpoint on the other
side without trusting only the signaling service to which they are side without trusting only the signaling service to which they are
connected. For instance, users may be making a call via a federated connected. For instance, users may be making a call via a federated
system where they wish to get direct authentication of the other system where they wish to get direct authentication of the other
side. Alternately, they may be making a call on a site which they side. Alternately, they may be making a call on a site which they
minimally trust (such as a poker site) but to someone who has an minimally trust (such as a poker site) but to someone who has an
identity on a site they do trust (such as a social network.) identity on a site they do trust (such as a social network.)
Recently, a number of Web-based identity technologies (OAuth, Recently, a number of Web-based identity technologies (OAuth,
BrowserID, Facebook Connect), etc. have been developed. While the BrowserID, Facebook Connect), etc. have been developed. While the
details vary, what these technologies share is that they have a Web- details vary, what these technologies share is that they have a Web-
based (i.e., HTTP/HTTPS identity provider) which attests to your based (i.e., HTTP/HTTPS) identity provider which attests to your
identity. For instance, if I have an account at example.org, I could identity. For instance, if I have an account at example.org, I could
use the example.org identity provider to prove to others that I was use the example.org identity provider to prove to others that I was
alice@example.org. The development of these technologies allows us alice@example.org. The development of these technologies allows us
to separate calling from identity provision: I could call you on to separate calling from identity provision: I could call you on
Poker Galaxy but identify myself as alice@example.org. Poker Galaxy but identify myself as alice@example.org.
Whatever the underlying technology, the general principle is that the Whatever the underlying technology, the general principle is that the
party which is being authenticated is NOT the signaling site but party which is being authenticated is NOT the signaling site but
rather the user (and their browser). Similarly, the relying party is rather the user (and their browser). Similarly, the relying party is
the browser and not the signaling site. Thus, the browser MUST the browser and not the signaling site. Thus, the browser MUST
securely generate the input to the IdP assertion process and MUST securely generate the input to the IdP assertion process and MUST
securely display the results of the verification process to the user securely display the results of the verification process to the user
in a way which cannot be imitated by the calling site. in a way which cannot be imitated by the calling site.
The mechanisms defined 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.
5.6.1. Trust Relationships: IdPs, APs, and RPs
Any federated identity 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
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.
5.6.2. 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. The details of the mechanism are described in the W3C API
specification, but the general idea is that the PeerConnection
component downloads JS from a specific location on the IdP dictated
by the IdP domain name. That JS (the "IdP proxy") runs in an
isolated security context within the browser and the PeerConnection
talks to it via a secure message passing channel.
+------------------------------------+
| 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 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
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
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.
5.6.3. Items for Standardization
In order to make this work, we must standardize the following items: In order to make this work, we must standardize the following items:
o The precise information from the signaling message that must be o The precise information from the signaling message that must be
cryptographically bound to the user's identity. At minimum this cryptographically bound to the user's identity and a mechanism for
MUST be the fingerprint, but we may choose to add other carrying assertions in JSEP messages. Section 5.6.4
information as the signaling protocol firms up. This will be o The interface to the IdP. Section 5.6.5 specifies a specific
defined in a future version of this document. protocol mechanism which allows the use of any identity protocol
o The interface to the IdP. [I-D.rescorla-rtcweb-generic-idp] without requiring specific further protocol support in the browser
specifies a specific protocol mechanism which allows the use of
any identity protocol without requiring specific further protocol
support in the browser.
o The JavaScript interfaces which the calling application can use to o The JavaScript interfaces which the calling application can use to
specify the IdP to use to generate assertions and to discover what specify the IdP to use to generate assertions and to discover what
assertions were received. These interfaces should be defined in assertions were received.
the W3C document.
6. Security Considerations The first two items are defined in this document. The final one is
defined in the companion W3C WebRTC API specification [TODO:REF]
5.6.4. Binding Identity Assertions to JSEP Offer/Answer Transactions
5.6.4.1. Input to Assertion Generation Process
As discussed above, an identity assertion binds the user's identity
(as asserted by the IdP) to the JSEP offer/exchange transaction and
specifically to the media. In order to achieve this, the
PeerConnection must provide the DTLS-SRTP fingerprint to be bound to
the identity. This is provided in a JSON structure for
extensibility, as shown below:
{
"fingerprint" :
{
"algorithm":"SHA-1",
"digest":"4A:AD:B9:B1:3F:...:E5:7C:AB"
}
}
The "algorithm" and digest values correspond directly to the
algorithm and digest in the a=fingerprint line of the SDP.
Note: this structure does not need to be interpreted by the IdP or
the IdP proxy. It is consumed solely by the RP's browser. The IdP
merely treats it as an opaque value to be attested to. Thus, new
parameters can be added to the assertion without modifying the IdP.
5.6.4.2. Carrying Identity Assertions
Once an IdP has generated an assertion, the JSEP message. This is
done by adding a new a-line to the SDP, of the form a=identity. The
sole contents of this value are a base-64-encoded version of the
identity assertion. For example:
v=0
o=- 1181923068 1181923196 IN IP4 ua1.example.com
s=example1
c=IN IP4 ua1.example.com
a=setup:actpass
a=fingerprint: SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=identity: \
ImlkcCI6eyJkb21haW4iOiAiZXhhbXBsZS5vcmciLCAicHJvdG9jb2wiOiAiYm9n \
dXMifSwiYXNzZXJ0aW9uIjpcIntcImlkZW50aXR5XCI6XCJib2JAZXhhbXBsZS5v \
cmdcIixcImNvbnRlbnRzXCI6XCJhYmNkZWZnaGlqa2xtbm9wcXJzdHV2d3l6XCIs \
XCJzaWduYXR1cmVcIjpcIjAxMDIwMzA0MDUwNlwifSJ9Cg==
t=0 0
m=audio 6056 RTP/AVP 0
a=sendrecv
a=tcap:1 UDP/TLS/RTP/SAVP RTP/AVP
a=pcfg:1 t=1
Each identity attribute should be paired (and attests to) with an
a=fingerprint attribute and therefore can exist either at the session
or media level. Multiple identity attributes may appear at either
level, though implementations are discouraged from doing this unless
they have a clear idea of what security claim they intend to be
making.
5.6.5. IdP Interaction Details
5.6.5.1. 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",
"origin":"https://calling-site.example.com",
"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.
All requests from the PeerConnection object MUST contain an "origin"
field containing the origin of the JS which initiated the PC (i.e.,
the URL of the calling site). This origin value can be used by the
IdP to make access control decisions. For instance, an IdP might
only issue identity assertions for certain calling services in the
same way that some IdPs require that relying Web sites have an API
key before learning user identity.
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.
5.6.5.1.1. 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 3: Example error
5.6.5.2. IdP Proxy Setup
In order to perform an identity transaction, the PeerConnection must
first create an IdP proxy. While the details of this are specified
in the W3C API document, from the perspective of this specification,
however, the relevant facts are:
o The JS runs in the IdP's security context with the base page
retrieved from the URL specified in Section 5.6.5.2.1
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 and the PC and IdP proxy are able
to verify the source and destination of these messages.
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. When the IdP proxy is ready to receive
commands, it delivers a "ready" message. As this message is
unsolicited, it simply contains:
{ "type":"READY" }
[[ OPEN ISSUE: if the W3C half of this converges on WebIntents, then
the READY message will not be necessary.]]
Once the PeerConnection object receives the ready message, it can
send commands to the IdP proxy.
5.6.5.2.1. 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
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
5.6.5.2.1.1. 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"
5.6.5.2.1.2. 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 5.6.5.2.3.1.
5.6.5.2.2. 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 above, but are opaque from the perspective of the IdP.
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 4 shows an example transaction, with the message "abcde..."
(remember, the messages are opaque at this layer) being signed and
bound to identity "ekr@example.org". In this case, 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,
"origin":"https://calling-service.example.com/",
"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 4: Example assertion request
5.6.5.2.3. 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:
identity The identity of the AP from the IdP's perspective. Details
of this are provided in Section 5.6.5.2.3.1
contents The original unmodified string provided by the AP in the
original SIGN request.
Figure 5 shows an example transaction. Line breaks are inserted
solely for readability.
PeerConnection -> IdP Proxy:
{
"type":"VERIFY",
"id":2,
"origin":"https://calling-service.example.com/",
"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 5: Example verification request
5.6.5.2.3.1. 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
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. [[OPEN
ISSUE: Should this field exist? Is it confusing? ]]
5.7. Security Considerations
Much of the security analysis of this problem is contained in Much of the security analysis of this problem is contained in
[I-D.ietf-rtcweb-security] or in the discussion of the particular [I-D.ietf-rtcweb-security] or in the discussion of the particular
issues above. In order to avoid repetition, this section focuses on issues above. In order to avoid repetition, this section focuses on
(a) residual threats that are not addressed by this document and (b) (a) residual threats that are not addressed by this document and (b)
threats produced by failure/misbehavior of one of the components in threats produced by failure/misbehavior of one of the components in
the system. the system.
6.1. Communications Security 5.7.1. Communications Security
While this document favors DTLS-SRTP, it permits a variety of While this document favors DTLS-SRTP, it permits a variety of
communications security mechanisms and thus the level of communications security mechanisms and thus the level of
communications security actually provided varies considerably. Any communications security actually provided varies considerably. Any
pair of implementations which have multiple security mechanisms in pair of implementations which have multiple security mechanisms in
common are subject to being downgraded to the weakest of those common common are subject to being downgraded to the weakest of those common
mechanisms by any attacker who can modify the signaling traffic. If mechanisms by any attacker who can modify the signaling traffic. If
communications are over HTTP, this means any on-path attacker. If communications are over HTTP, this means any on-path attacker. If
communications are over HTTPS, this means the signaling server. communications are over HTTPS, this means the signaling server.
Implementations which wish to avoid downgrade attack should only Implementations which wish to avoid downgrade attack should only
skipping to change at page 16, line 46 skipping to change at page 27, line 9
have some mechanism for independently verifying keys. The UI have some mechanism for independently verifying keys. The UI
requirements in Section 5.5 are designed to provide such a mechanism requirements in Section 5.5 are designed to provide such a mechanism
for motivated/security conscious users, but are not suitable for for motivated/security conscious users, but are not suitable for
general use. The identity service mechanisms in Section 5.6 are more general use. The identity service mechanisms in Section 5.6 are more
suitable for general use. Note, however, that a malicious signaling suitable for general use. Note, however, that a malicious signaling
service can strip off any such identity assertions, though it cannot service can strip off any such identity assertions, though it cannot
forge new ones. Note that all of the third-party security mechanisms forge new ones. Note that all of the third-party security mechanisms
available (whether X.509 certificates or a third-party IdP) rely on available (whether X.509 certificates or a third-party IdP) rely on
the security of the third party--this is of course also true of your the security of the third party--this is of course also true of your
connection to the Web site itself. Users who wish to assure connection to the Web site itself. Users who wish to assure
themselves of security against a malicious identity provider MUST themselves of security against a malicious identity provider can only
verify peer credentials directly, e.g., by checking the peer's do so by verifying peer credentials directly, e.g., by checking the
fingerprint against a value delivered out of band. peer's fingerprint against a value delivered out of band.
6.2. Privacy 5.7.2. Privacy
The requirements in this document are intended to allow: The requirements in this document are intended to allow:
o Users to participate in calls without revealing their location. o Users to participate in calls without revealing their location.
o Potential callees to avoid revealing their location and even o Potential callees to avoid revealing their location and even
presence status prior to agreeing to answer a call. presence status prior to agreeing to answer a call.
However, these privacy protections come at a performance cost in However, these privacy protections come at a performance cost in
terms of using TURN relays and, in the latter case, delaying ICE. terms of using TURN relays and, in the latter case, delaying ICE.
Sites SHOULD make users aware of these tradeoffs. Sites SHOULD make users aware of these tradeoffs.
skipping to change at page 17, line 27 skipping to change at page 27, line 35
Note that the protections provided here assume a non-malicious Note that the protections provided here assume a non-malicious
calling service. As the calling service always knows the users calling service. As the calling service always knows the users
status and (absent the use of a technology like Tor) their IP status and (absent the use of a technology like Tor) their IP
address, they can violate the users privacy at will. Users who wish address, they can violate the users privacy at will. Users who wish
privacy against the calling sites they are using must use separate privacy against the calling sites they are using must use separate
privacy enhancing technologies such as Tor. Combined RTCWEB/Tor privacy enhancing technologies such as Tor. Combined RTCWEB/Tor
implementations SHOULD arrange to route the media as well as the implementations SHOULD arrange to route the media as well as the
signaling through Tor. [Currently this will produce very suboptimal signaling through Tor. [Currently this will produce very suboptimal
performance.] performance.]
6.3. Denial of Service 5.7.3. Denial of Service
The consent mechanisms described in this document are intended to The consent mechanisms described in this document are intended to
mitigate denial of service attacks in which an attacker uses clients mitigate denial of service attacks in which an attacker uses clients
to send large amounts of traffic to a victim without the consent of to send large amounts of traffic to a victim without the consent of
the victim. While these mechanisms are sufficient to protect victims the victim. While these mechanisms are sufficient to protect victims
who have not implemented RTCWEB at all, RTCWEB implementations need who have not implemented RTCWEB at all, RTCWEB implementations need
to be more careful. to be more careful.
Consider the case of a call center which accepts calls via RTCWeb. Consider the case of a call center which accepts calls via RTCWeb.
An attacker proxies the call center's front-end and arranges for An attacker proxies the call center's front-end and arranges for
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which are behaving badly, and especially to be prepared to remotely which are behaving badly, and especially to be prepared to remotely
throttle the data channel in the absence of plausible audio and video throttle the data channel in the absence of plausible audio and video
(which the attacker cannot control). (which the attacker cannot control).
Another related attack is for the signaling service to swap the ICE Another related attack is for the signaling service to swap the ICE
candidates for the audio and video streams, thus forcing a browser to candidates for the audio and video streams, thus forcing a browser to
send video to the sink that the other victim expects will contain send video to the sink that the other victim expects will contain
audio (perhaps it is only expecting audio!) potentially causing audio (perhaps it is only expecting audio!) potentially causing
overload. Muxing multiple media flows over a single transport makes overload. Muxing multiple media flows over a single transport makes
it harder to individually suppress a single flow by denying ICE it harder to individually suppress a single flow by denying ICE
keepalives. Media-level (RTCP) mechanisms must be used in this case. keepalives. Either media-level (RTCP) mechanisms must be used or the
implementation must deny responses entirely, thus termnating the
call.
Yet another attack, suggested by Magnus Westerlund, is for the Yet another attack, suggested by Magnus Westerlund, is for the
attacker to cross-connect offers and answers as follows. It induces attacker to cross-connect offers and answers as follows. It induces
the victim to make a call and then uses its control of other users the victim to make a call and then uses its control of other users
browsers to get them to attempt a call to someone. It then browsers to get them to attempt a call to someone. It then
translates their offers into apparent answers to the victim, which translates their offers into apparent answers to the victim, which
looks like large-scale parallel forking. The victim still responds looks like large-scale parallel forking. The victim still responds
to ICE responses and now the browsers all try to send media to the to ICE responses and now the browsers all try to send media to the
victim. [[ OPEN ISSUE: How do we address this? ]] victim. Implementations can defend themselves from this attack by
only responding to ICE Binding Requests for a limited number of
[TODO: Should we have a mechanism for verifying total expected remote ufrags (this is the reason for the requirement that the JS not
bandwidth] be able to control the ufrag and password).
Note that attacks based on confusing one end or the other about Note that attacks based on confusing one end or the other about
consent are possible primarily even in the face of the third-party consent are possible even in the face of the third-party identity
identity mechanism as long as major parts of the signaling messages mechanism as long as major parts of the signaling messages are not
are not signed. On the other hand, signing the entire message signed. On the other hand, signing the entire message severely
severely restricts the capabilities of the calling application, so restricts the capabilities of the calling application, so there are
there are difficult tradeoffs here. difficult tradeoffs here.
7. Acknowledgements 5.7.4. IdP Authentication Mechanism
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.
5.7.4.1. IdP Well-known URI
As described in Section 5.6.5.2.1 the IdP proxy HTML/JS landing page
is located at a well-known URI based on the IdP's domain name. This
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.
5.7.4.2. Privacy of IdP-generated identities and the hosting site
Depending on the structure of the IdP's assertions, the calling site
may learn the user's identity from the perspective of the IdP. In
many cases this is not an issue because the user is authenticating to
the site via the IdP in any case, for instance when the user has
logged in with Facebook Connect and is then authenticating their call
with a Facebook identity. However, in other case, the user may not
have already revealed their identity to the site. In general, IdPs
SHOULD either verify that the user is willing to have their identity
revealed to the site (e.g., through the usual IdP permissions dialog)
or arrange that the identity information is only available to known
RPs (e.g., social graph adjacencies) but not to the calling site.
The "origin" field of the signature request can be used to check that
the user has agreed to disclose their identity to the calling site;
because it is supplied by the PeerConnection it can be trusted to be
correct.
5.7.4.3. 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.
5.7.4.4. Web Security Feature Interactions
A number of optional Web security features have the potential to
cause issues for this mechanism, as discussed below.
5.7.4.4.1. 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.
5.7.4.4.2. 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.
6. Acknowledgements
Bernard Aboba, Harald Alvestrand, Dan Druta, Cullen Jennings, Hadriel Bernard Aboba, Harald Alvestrand, Dan Druta, Cullen Jennings, Hadriel
Kaplan, Matthew Kaufman, Martin Thomson, Magnus Westerland. Kaplan, Matthew Kaufman, Jim McEachem, Martin Thomson, Magnus
Westerland.
7. Changes since -02
The following changes have been made since the -02 draft.
o Forbid persistent HTTP permissions.
o Clarified the text in S 5.4 to clearly refer to requirements on
the API to provide functionality to the site.
o Fold in the IETF portion of draft-rescorla-rtcweb-generic-idp
o Retarget the continuing consent section to assume Binding Requests
o Editorial improvements
8. References 8. References
8.1. Normative References 8.1. Normative References
[I-D.ietf-rtcweb-security] [I-D.ietf-rtcweb-security]
Rescorla, E., "Security Considerations for RTC-Web", Rescorla, E., "Security Considerations for RTC-Web",
draft-ietf-rtcweb-security-02 (work in progress), draft-ietf-rtcweb-security-03 (work in progress),
March 2012. June 2012.
[I-D.muthu-behave-consent-freshness] [I-D.muthu-behave-consent-freshness]
Perumal, M., Wing, D., and H. Kaplan, "STUN Usage for Perumal, M., Wing, D., and H. Kaplan, "STUN Usage for
Consent Freshness", Consent Freshness and Session Liveness",
draft-muthu-behave-consent-freshness-00 (work in draft-muthu-behave-consent-freshness-01 (work in
progress), March 2012. progress), July 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006. Security", RFC 4347, April 2006.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT) (ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, Traversal for Offer/Answer Protocols", RFC 5245,
April 2010. April 2010.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework [RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer (SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, May 2010. Security (DTLS)", RFC 5763, May 2010.
skipping to change at page 19, line 31 skipping to change at page 31, line 44
Security (DTLS) Extension to Establish Keys for the Secure Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010. Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011. December 2011.
8.2. Informative References 8.2. Informative References
[I-D.ietf-rtcweb-jsep] [I-D.ietf-rtcweb-jsep]
Uberti, J. and C. Jennings, "Javascript Session Uberti, J. and C. Jennings, "Javascript Session
Establishment Protocol", draft-ietf-rtcweb-jsep-00 (work Establishment Protocol", draft-ietf-rtcweb-jsep-01 (work
in progress), March 2012. in progress), June 2012.
[I-D.jennings-rtcweb-signaling] [I-D.jennings-rtcweb-signaling]
Jennings, C., Rosenberg, J., and R. Jesup, "RTCWeb Offer/ Jennings, C., Rosenberg, J., and R. Jesup, "RTCWeb Offer/
Answer Protocol (ROAP)", Answer Protocol (ROAP)",
draft-jennings-rtcweb-signaling-01 (work in progress), draft-jennings-rtcweb-signaling-01 (work in progress),
October 2011. October 2011.
[I-D.kaufman-rtcweb-security-ui] [I-D.kaufman-rtcweb-security-ui]
Kaufman, M., "Client Security User Interface Requirements Kaufman, M., "Client Security User Interface Requirements
for RTCWEB", draft-kaufman-rtcweb-security-ui-00 (work in for RTCWEB", draft-kaufman-rtcweb-security-ui-00 (work in
progress), June 2011. progress), June 2011.
[I-D.rescorla-rtcweb-generic-idp]
Rescorla, E., "RTCWeb Generic Identity Provider
Interface", draft-rescorla-rtcweb-generic-idp-00 (work in
progress), January 2012.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261, Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002. June 2002.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport [RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, March 2010. Layer Security (TLS)", RFC 5705, March 2010.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", [RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011. RFC 6455, December 2011.
[XmlHttpRequest] [XmlHttpRequest]
van Kesteren, A., "XMLHttpRequest Level 2". van Kesteren, A., "XMLHttpRequest Level 2".
Appendix A. Example IdP 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).
A.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.
+----------------------+ +----------------------+
| | | |
| 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.
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.
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.
A.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.
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.
Author's Address Author's Address
Eric Rescorla Eric Rescorla
RTFM, Inc. RTFM, Inc.
2064 Edgewood Drive 2064 Edgewood Drive
Palo Alto, CA 94303 Palo Alto, CA 94303
USA USA
Phone: +1 650 678 2350 Phone: +1 650 678 2350
Email: ekr@rtfm.com Email: ekr@rtfm.com
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