| RTCWEB | E. Rescorla |
| Internet-Draft | RTFM, Inc. |
| Intended status: Standards Track | July 22, 2019 |
| Expires: January 23, 2020 |
WebRTC Security Architecture
draft-ietf-rtcweb-security-arch-latest
This document defines the security architecture for WebRTC, a protocol suite intended for use with real-time applications that can be deployed in browsers - "real time communication on the Web".
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The Real-Time Communications on the Web (RTCWEB) working group standardized protocols for real-time communications between Web browsers, generally called "WebRTC" [I-D.ietf-rtcweb-overview]. The major use cases for WebRTC technology are real-time audio and/or video calls, Web conferencing, and direct data transfer. Unlike most conventional real-time systems, (e.g., SIP-based [RFC3261] soft phones) WebRTC communications are directly controlled by some Web server, via a JavaScript (JS) API as shown in Figure 1.
+----------------+
| |
| Web Server |
| |
+----------------+
^ ^
/ \
HTTP / \ HTTP
/ \
/ \
v v
JS API JS API
+-----------+ +-----------+
| | Media | |
| Browser |<---------->| Browser |
| | | |
+-----------+ +-----------+
Figure 1: A simple WebRTC system
A more complicated system might allow for interdomain calling, as shown in Figure 2. The protocol to be used between the domains is not standardized by WebRTC, but given the installed base and the form of the WebRTC API is likely to be something SDP-based like SIP or something like Extensible Messaging and Presence Protocol (XMPP) [RFC6120].
+--------------+ +--------------+
| | SIP,XMPP,...| |
| Web Server |<----------->| Web Server |
| | | |
+--------------+ +--------------+
^ ^
| |
HTTP | | HTTP
| |
v v
JS API JS API
+-----------+ +-----------+
| | Media | |
| Browser |<---------------->| Browser |
| | | |
+-----------+ +-----------+
Figure 2: A multidomain WebRTC system
This system presents a number of new security challenges, which are analyzed in [I-D.ietf-rtcweb-security]. This document describes a security architecture for WebRTC which addresses the threats and requirements described in that document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
The basic assumption of this architecture is that network resources exist in a hierarchy of trust, rooted in the browser, which serves as the user's Trusted Computing Base (TCB). Any security property which the user wishes to have enforced must be ultimately guaranteed by the browser (or transitively by some property the browser verifies). Conversely, if the browser is compromised, then no security guarantees are possible. Note that there are cases (e.g., Internet kiosks) where the user can't really trust the browser that much. In these cases, the level of security provided is limited by how much they trust the browser.
Optimally, we would not rely on trust in any entities other than the browser. However, this is unfortunately not possible if we wish to have a functional system. Other network elements fall into two categories: those which can be authenticated by the browser and thus can be granted permissions to access sensitive resources, and those which cannot be authenticated and thus are untrusted.
There are two major classes of authenticated entities in the system:
Note that merely being authenticated does not make these entities trusted. For instance, just because we can verify that https://www.example.org/ is owned by Dr. Evil does not mean that we can trust Dr. Evil to access our camera and microphone. However, it gives the user an opportunity to determine whether he wishes to trust Dr. Evil or not; after all, if he desires to contact Dr. Evil (perhaps to arrange for ransom payment), it's safe to temporarily give him access to the camera and microphone for the purpose of the call, but he doesn't want Dr. Evil to be able to access his camera and microphone other than during the call. The point here is that we must first identify other elements before we can determine whether and how much to trust them. Additionally, sometimes we need to identify the communicating peer before we know what policies to apply.
Other than the above entities, we are not generally able to identify other network elements, thus we cannot trust them. This does not mean that it is not possible to have any interaction with them, but it means that we must assume that they will behave maliciously and design a system which is secure even if they do so.
This section describes a typical WebRTC session and shows how the various security elements interact and what guarantees are provided to the user. The example in this section is a "best case" scenario in which we provide the maximal amount of user authentication and media privacy with the minimal level of trust in the calling service. Simpler versions with lower levels of security are also possible and are noted in the text where applicable. It's also important to recognize the tension between security (or performance) and privacy. The example shown here is aimed towards settings where we are more concerned about secure calling than about privacy, but as we shall see, there are settings where one might wish to make different tradeoffs--this architecture is still compatible with those settings.
For the purposes of this example, we assume the topology shown in the figures below. This topology is derived from the topology shown in Figure 1, but separates Alice and Bob's identities from the process of signaling. Specifically, Alice and Bob have relationships with some Identity Provider (IdP) that supports a protocol (such as OpenID Connect) that can be used to demonstrate their identity to other parties. For instance, Alice might have an account with a social network which she can then use to authenticate to other web sites without explicitly having an account with those sites; this is a fairly conventional pattern on the Web. Section 7.1 provides an overview of Identity Providers and the relevant terminology. Alice and Bob might have relationships with different IdPs as well.
This separation of identity provision and signaling isn't particularly important in "closed world" cases where Alice and Bob are users on the same social network and have identities based on that domain (Figure 3). However, there are important settings where that is not the case, such as federation (calls from one domain to another; Figure 4) 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.
Note that the servers themselves are also authenticated by an external identity service, the SSL/TLS certificate infrastructure (not shown). As is conventional in the Web, all identities are ultimately rooted in that system. For instance, when an IdP makes an identity assertion, the Relying Party consuming that assertion is able to verify because it is able to connect to the IdP via HTTPS.
+----------------+
| |
| Signaling |
| Server |
| |
+----------------+
^ ^
/ \
HTTPS / \ HTTPS
/ \
/ \
v v
JS API JS API
+-----------+ +-----------+
| | Media | |
Alice | Browser |<---------->| Browser | Bob
| | (DTLS+SRTP)| |
+-----------+ +-----------+
^ ^--+ +--^ ^
| | | |
v | | v
+-----------+ | | +-----------+
| |<--------+ | |
| IdP1 | | | IdP2 |
| | +------->| |
+-----------+ +-----------+
Figure 3: A call with IdP-based identity
Figure 4 shows essentially the same calling scenario but with a call between two separate domains (i.e., a federated case), as in Figure 2. As mentioned above, the domains communicate by some unspecified protocol and providing separate signaling and identity allows for calls to be authenticated regardless of the details of the inter-domain protocol.
+----------------+ Unspecified +----------------+
| | protocol | |
| Signaling |<----------------->| Signaling |
| Server | (SIP, XMPP, ...) | Server |
| | | |
+----------------+ +----------------+
^ ^
| |
HTTPS | | HTTPS
| |
| |
v v
JS API JS API
+-----------+ +-----------+
| | Media | |
Alice | Browser |<--------------------------->| Browser | Bob
| | DTLS+SRTP | |
+-----------+ +-----------+
^ ^--+ +--^ ^
| | | |
v | | v
+-----------+ | | +-----------+
| |<-------------------------+ | |
| IdP1 | | | IdP2 |
| | +------------------------>| |
+-----------+ +-----------+
Figure 4: A federated call with IdP-based identity
For simplicity, assume the topology in Figure 3. Alice and Bob are both users of a common calling service; they both have approved the calling service to make calls (we defer the discussion of device access permissions until later). They are both connected to the calling service via HTTPS and so know the origin with some level of confidence. They also have accounts with some identity provider. This sort of identity service is becoming increasingly common in the Web environment (with technologies such as Federated Google Login, Facebook Connect, OAuth, OpenID, WebFinger), and is often provided as a side effect service of a user's ordinary accounts with some service. In this example, we show Alice and Bob using a separate identity service, though the identity service may be the same entity as the calling service or there may be no identity service at all.
Alice is logged onto the calling service and decides to call Bob. She can see from the calling service that he is online and the calling service presents a JS UI in the form of a button next to Bob's name which says "Call". Alice clicks the button, which initiates a JS callback that instantiates a PeerConnection object. This does not require a security check: JS from any origin is allowed to get this far.
Once the PeerConnection is created, the calling service JS needs to set up some media. Because this is an audio/video call, it creates a MediaStream with two MediaStreamTracks, one connected to an audio input and one connected to a video input. At this point the first security check is required: untrusted origins are not allowed to access the camera and microphone, so the browser prompts Alice for permission.
In the current W3C API, once some streams have been added, Alice's browser + JS generates a signaling message [I-D.ietf-rtcweb-jsep] containing:
Prior to sending out the signaling message, the PeerConnection code contacts the identity service and obtains an assertion binding Alice's identity to her fingerprint. The exact details depend on the identity service (though as discussed in Section 7 PeerConnection can be agnostic to them), but for now it's easiest to think of as an OAuth token. 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 [XmlHttpRequest] or by WebSockets [RFC6455], over TLS [RFC5246]. The signaling server 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, the format is currently undefined). The JS on Bob's browser processes it, and alerts Bob to the incoming call and to Alice's identity. In this case, Alice has provided an identity assertion and 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 IdP) to verify the assertion. It is also possible to have IdPs with which the browser has a specific trustrelationship, as described in Section 7.1. This allows the browser to display a trusted element in the browser chrome indicating that a call is coming in from Alice. If Alice is in Bob's address book, then this interface might 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 answer the call, though this is most likely not part of the trusted UI.
If Bob agrees a PeerConnection is instantiated with the message from Alice's side. Then, a similar process occurs as on Alice's browser: Bob's browser prompts him for device permission, the media streams are created, and a return signaling message containing media information, ICE candidates, and a fingerprint is sent back to Alice via the signaling service. If Bob has a relationship with an IdP, the message will also come with an identity assertion.
At this point, Alice and Bob each know that the other party wants to have a secure call with them. Based purely on the interface provided by the signaling server, they know that the signaling server claims that the call is from Alice to Bob. This level of security is provided merely by having the fingerprint in the message and having that message received securely from the signaling server. Because the far end sent an identity assertion along with their message, they know that this is verifiable from the IdP as well. Note that if the call is federated, as shown in Figure 4 then Alice is able to verify Bob's identity in a way that is not mediated by either her signaling server or Bob's. Rather, she verifies it directly with Bob's IdP.
Of course, the call works perfectly well if either Alice or Bob doesn't have a relationship with an IdP; they just get a lower level of assurance. I.e., they simply have whatever information their calling site claims about the caller/callee's identity. Moreover, Alice might wish to make an anonymous call through an anonymous calling site, in which case she would of course just not provide any identity assertion and the calling site would mask her identity from Bob.
As described in ([I-D.ietf-rtcweb-security]; Section 4.2) media consent verification is provided via ICE. Thus, Alice and Bob perform ICE checks with each other. At the completion of these checks, they are ready to send non-ICE data.
At this point, Alice knows that (a) Bob (assuming he is verified via his IdP) or someone else who the signaling service is claiming is Bob is willing to exchange traffic with her and (b) that either Bob is at the IP address which she has verified via ICE or there is an attacker who is on-path to that IP address detouring the traffic. Note that it is not possible for an attacker who is on-path between Alice and Bob but not attached to the signaling service to spoof these checks because they do not have the ICE credentials. Bob has the same security guarantees with respect to Alice.
Once the requisite ICE checks have completed, Alice and Bob can set up a secure channel or channels. This is performed via DTLS [RFC6347] and DTLS-SRTP [RFC5763] keying for SRTP [RFC3711] for the media channel and SCTP over DTLS [RFC8261] for data channels. Specifically, Alice and Bob perform a DTLS handshake on every component which has been established by ICE. The total number of channels depends on the amount of muxing; in the most likely case we are using both RTP/RTCP mux and muxing multiple media streams on the same channel, in which case there is only one DTLS handshake. Once the DTLS handshake has completed, the keys are exported [RFC5705] and used to key SRTP for the media channels.
At this point, Alice and Bob know that they share a set of secure data and/or media channels with keys which are not known to any third-party attacker. If Alice and Bob authenticated via their IdPs, then they also know that the signaling service is not mounting a man-in-the-middle attack on their traffic. Even if they do not use an IdP, as long as they have minimal trust in the signaling service not to perform a man-in-the-middle attack, they know that their communications are secure against the signaling service as well (i.e., that the signaling service cannot mount a passive attack on the communications).
From a security perspective, everything from here on in is a little anticlimactic: Alice and Bob exchange data protected by the keys negotiated by DTLS. Because of the security guarantees discussed in the previous sections, they know that the communications are encrypted and authenticated.
The one remaining security property we need to establish is "consent freshness", i.e., allowing Alice to verify that Bob is still prepared to receive her communications so that Alice does not continue to send large traffic volumes to entities which went abruptly offline. ICE specifies periodic STUN keepalives but only if media is not flowing. Because the consent issue is more difficult here, we require WebRTC implementations to periodically send keepalives. As described in Section 5.3, these keepalives MUST be based on the consent freshness mechanism specified in [RFC7675]. If a keepalive fails and no new ICE channels can be established, then the session is terminated.
The SDP 'identity' attribute is a session-level attribute that is used by an endpoint to convey its identity assertion to its peer. The identity assertion value is encoded as Base-64, as described in Section 4 of [RFC4648].
The procedures in this section are based on the assumption that the identity assertion of an endpoint is bound to the fingerprints of the endpoint. This does not preclude the definition of alternative means of binding an assertion to the endpoint, but such means are outside the scope of this specification.
The semantics of multiple 'identity' attributes within an offer or answer are undefined. Implementations SHOULD only include a single 'identity' attribute in an offer or answer and relying parties MAY elect to ignore all but the first 'identity' attribute.
Syntax:
identity-assertion = identity-assertion-value
*(SP identity-extension)
identity-assertion-value = base64
identity-extension = extension-name [ "=" extension-value ]
extension-name = token
extension-value = 1*(%x01-09 / %x0b-0c / %x0e-3a / %x3c-ff)
; byte-string from [RFC4566]
<ALPHA and DIGIT as defined in [RFC4566]>
<base64 as defined in [RFC4566]>
Example:
a=identity:\
eyJpZHAiOnsiZG9tYWluIjoiZXhhbXBsZS5vcmciLCJwcm90b2NvbCI6ImJvZ3Vz\
In0sImFzc2VydGlvbiI6IntcImlkZW50aXR5XCI6XCJib2JAZXhhbXBsZS5vcmdc\
IixcImNvbnRlbnRzXCI6XCJhYmNkZWZnaGlqa2xtbm9wcXJzdHV2d3l6XCIsXCJz\
aWduYXR1cmVcIjpcIjAxMDIwMzA0MDUwNlwifSJ9
Note that long lines in the example are folded to meet the column
width constraints of this document; the backslash ("\") at the end of
a line, the carriage return that follows, and whitespace shall be ignored.
This specification does not define any extensions for the attribute.
The identity-assertion value is a JSON [RFC8259] encoded string. The JSON object contains two keys: "assertion" and "idp". The assertion key value contains an opaque string that is consumed by the IdP. The idp key value contains a dictionary with one or two further values that identify the IdP. See Section 7.6 for more details.
This section defines the SDP Offer/Answer [RFC3264] considerations for the SDP 'identity' attribute.
Within this section, 'initial offer' refers to the first offer in the SDP session that contains an SDP identity attribute.
When an offerer sends an offer, in order to provide its identity assertion to the peer, it includes an 'identity' attribute in the offer. In addition, the offerer includes one or more SDP 'fingerprint' attributes. The 'identity' attribute MUST be bound to all the 'fingerprint' attributes in the session description.
If the answerer elects to include an 'identity' attribute, it follows the same steps as those in Section 5.1.1. The answerer can choose to include or omit an 'identity' attribute independently, regardless of whether the offerer did so.
When an endpoint receives an offer or answer that contains an 'identity' attribute, the answerer can use the the attribute information to contact the IdP and verify the identity of the peer. If the identity requires a third-party IdP as described in Section 7.1 then that IdP will need to have been specifically configured. If the identity verification fails, the answerer MUST discard the offer or answer as malformed.
When modifying a session, if the set of fingerprints is unchanged, then the sender MAY send the same 'identity' attribute. In this case, the established identity MUST be applied to existing DTLS connections as well as new connections established using one of those fingerprints. Note that [I-D.ietf-rtcweb-jsep], Section 5.2.1 requires that each media section use the same set of fingerprints for every media section. If a new identity attribute is received, then the receiver MUST apply that identity to all existing connections.
If the set of fingerprints changes, then the sender MUST either send a new 'identity' attribute or none at all. Because a change in fingerprints also causes a new DTLS connection to be established, the receiver MUST discard all previously established identities.
The basic unit of permissions for WebRTC is the origin [RFC6454]. Because the security of the origin depends on being able to authenticate content from that origin, the origin can only be securely established if data is transferred over HTTPS [RFC2818]. Thus, clients MUST treat HTTP and HTTPS origins as different permissions domains. Note: this follows directly from the origin security model and is stated here merely for clarity.
Many web browsers currently forbid by default any active mixed content on HTTPS pages. That is, when JavaScript is loaded from an HTTP origin onto an HTTPS page, an error is displayed and the HTTP content is not executed unless the user overrides the error. Any browser which enforces such a policy will also not permit access to WebRTC functionality from mixed content pages (because they never display mixed content). Browsers which allow active mixed content MUST nevertheless disable WebRTC functionality in mixed content settings.
Note that it is possible for a page which was not mixed content to become mixed content during the duration of the call. The major risk here is that the newly arrived insecure JS might redirect media to a location controlled by the attacker. Implementations MUST either choose to terminate the call or display a warning at that point.
Also note that the security architecture depends on the keying material not being available to move between origins. But, it is assumed that the identity assertion can be passed to anyone that the page cares to.
Implementations MUST obtain explicit user consent prior to providing access to the camera and/or microphone. Implementations MUST at minimum support the following two permissions models for HTTPS origins.
Because HTTP origins cannot be securely established against network attackers, implementations MUST refuse all permissions grants for HTTP origins.
In addition, they SHOULD support requests for access that promise that media from this grant will be sent to a single communicating peer (obviously there could be other requests for other peers), eE.g., "Call customerservice@example.org". The semantics of this request are that the media stream from the camera and microphone will only be routed through a connection which has been cryptographically verified (through the IdP mechanism or an X.509 certificate in the DTLS-SRTP handshake) as being associated with the stated identity. Note that it is unlikely that browsers would have X.509 certificates, but servers might. Browsers servicing such requests SHOULD clearly indicate that identity to the user when asking for permission. The idea behind this type of permissions is that a user might have a fairly narrow list of peers he is willing to communicate with, e.g., "my mother" rather than "anyone on Facebook". Narrow permissions grants allow the browser to do that enforcement.
Browsers MAY permit the formation of data channels without any direct user approval. Because sites can always tunnel data through the server, further restrictions on the data channel do not provide any additional security. (See Section 6.3 for a related issue).
Implementations which support some form of direct user authentication SHOULD also provide a policy by which a user can authorize calls only to specific communicating peers. Specifically, the implementation SHOULD provide the following interfaces/controls:
Implementations SHOULD also provide a different user interface indication when calls are in progress to users whose identities are directly verifiable. Section 6.5 provides more on this.
Browser client implementations of WebRTC MUST implement ICE. Server gateway implementations which operate only at public IP addresses MUST implement either full ICE or ICE-Lite [RFC8445].
Browser implementations MUST verify reachability via ICE prior to sending any non-ICE packets to a given destination. Implementations MUST NOT provide the ICE transaction ID to JavaScript during the lifetime of the transaction (i.e., during the period when the ICE stack would accept a new response for that transaction). The JS MUST NOT be permitted to control the local ufrag and password, though it of course knows it.
While continuing consent is required, the ICE [RFC8445]; Section 10 keepalives use STUN Binding Indications which are one-way and therefore not sufficient. The current WG consensus is to use ICE Binding Requests for continuing consent freshness. ICE already requires that implementations respond to such requests, so this approach is maximally compatible. A separate document will profile the ICE timers to be used; see [RFC7675].
A side effect of the default ICE behavior is that the peer learns one's IP address, which leaks large amounts of location information. This has negative privacy consequences in some circumstances. The API requirements in this 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., Tor Browser [https://www.torproject.org/projects/torbrowser.html.en]) and is out of scope for this specification.
Note that some enterprises may operate proxies and/or NATs designed to hide internal IP addresses from the outside world. WebRTC provides no explicit mechanism to allow this function. Either such enterprises need to proxy the HTTP/HTTPS and modify the SDP and/or the JS, or there needs to be browser support to set the "TURN-only" policy regardless of the site's preferences.
Implementations MUST support SRTP [RFC3711]. Implementations MUST support DTLS [RFC6347] and DTLS-SRTP [RFC5763][RFC5764] for SRTP keying. Implementations MUST support SCTP over DTLS [RFC8261].
All media channels MUST be secured via SRTP and SRTCP. Media traffic MUST NOT be sent over plain (unencrypted) RTP or RTCP; that is, implementations MUST NOT negotiate cipher suites with NULL encryption modes. DTLS-SRTP MUST be offered for every media channel. WebRTC implementations MUST NOT offer SDP Security Descriptions [RFC4568] or select it if offered. A SRTP MKI MUST NOT be used.
All data channels MUST be secured via DTLS.
All Implementations MUST support DTLS 1.2 with the TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 cipher suite and the P-256 curve. Earlier drafts of this specification required DTLS 1.0 with the cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, and at the time of this writing some implementations do not support DTLS 1.2; endpoints which support only DTLS 1.2 might encounter interoperability issues. The DTLS-SRTP protection profile SRTP_AES128_CM_HMAC_SHA1_80 MUST be supported for SRTP. Implementations MUST favor cipher suites which support (Perfect Forward Secrecy) PFS over non-PFS cipher suites and SHOULD favor AEAD over non-AEAD cipher suites.
Implementations MUST NOT implement DTLS renegotiation and MUST reject it with a "no_renegotiation" alert if offered.
Endpoints MUST NOT implement TLS False Start [RFC7918].
In a number of cases, it is desirable for the endpoint (i.e., the browser) to be able to directly identify the endpoint on the other side without trusting the signaling service to which they are connected. For instance, users may be making a call via a federated system where they wish to get direct authentication of the other 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 identity on a site they do trust (such as a social network.)
Recently, a number of Web-based identity technologies (OAuth, Facebook Connect etc.) have been developed. While the details vary, what these technologies share is that they have a Web-based (i.e., HTTP/HTTPS) identity provider which attests to Alice's identity. For instance, if Alice has an account at example.org, Alice could use the example.org identity provider to prove to others that Alice is alice@example.org. The development of these technologies allows us to separate calling from identity provision: Alice could call you on a poker site but identify herself as alice@example.org.
Whatever the underlying technology, the general principle is that the party which is being authenticated is NOT the signaling site but rather the user (and their browser). Similarly, the relying party is the browser and not the signaling site. Thus, the browser MUST generate the input to the IdP assertion process and display the results of the verification process to the user 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.
Any federated identity protocol has three major participants:
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 Web application at the calling site. The use case for having this information configured into the browser is that the user may "log into" the browser to bind it to some identity. This is becoming common in new browsers. However, it should also be possible for the IdP information to simply be provided by the calling application.
At a high level there are two kinds of IdPs:
If an AP is authenticating via an authoritative IdP, then the RP does not need to explicitly configure trust in the IdP at all. The identity mechanism can directly verify that the IdP indeed made the relevant identity assertion (a function provided by the mechanisms in this document), and any assertion it makes about an identity for which it is authoritative is directly verifiable. Note that this does not mean that the IdP might not lie, but that is a trustworthiness judgement that the user can make at the time he looks at the identity.
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.
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.
Note that there are two logically separate functions here:
The same IdP JS "endpoint" is used for both functions but of course a given IdP might behave differently and load new JS to perform one function or the other.
+--------------------------------------+
| Browser |
| |
| +----------------------------------+ |
| | https://calling-site.example.com | |
| | | |
| | Calling JS Code | |
| | ^ | |
| +---------------|------------------+ |
| | API Calls |
| v |
| PeerConnection |
| ^ |
| | API Calls |
| +-----------|-------------+ | +---------------+
| | v | | | |
| | IdP Proxy |<-------->| Identity |
| | | | | Provider |
| | https://idp.example.org | | | |
| +-------------------------+ | +---------------+
| |
+--------------------------------------+
When the PeerConnection object wants to interact with the IdP, the sequence of events is as follows:
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--the location of which is determined based on the IdP's identity--and to call the generic API for requesting and verifying identity 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.
There are two parts to this work:
The WebRTC API specification also defines JavaScript interfaces that the calling application can use to specify which IdP to use. That API also provides access to the assertion-generation capability and the status of the validation process.
An identity assertion binds the user's identity (as asserted by the IdP) to the SDP offer/answer exchange 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 as a JavaScript object (also known as a dictionary or hash) with a single fingerprint key, as shown below:
{
"fingerprint":
[
{ "algorithm": "sha-256",
"digest": "4A:AD:B9:B1:3F:...:E5:7C:AB" },
{ "algorithm": "sha-1",
"digest": "74:E9:76:C8:19:...:F4:45:6B" }
]
}
The fingerprint value is an array of objects. Each object in the array contains algorithm and digest values, which correspond directly to the algorithm and digest values in the fingerprint attribute of the SDP [RFC8122].
This object is encoded in a JSON string for passing to the IdP. The identity assertion returned by the IdP, which is encoded in the identity attribute, is a JSON object that is encoded as described in Section 7.4.1.
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.
Once an IdP has generated an assertion (see Section 7.6), it is attached to the SDP offer/answer message. This is done by adding a new 'identity' attribute to the SDP. The sole contents of this value is the identity assertion. The identity assertion produced by the IdP is encoded into a UTF-8 JSON text, then Base64-encoded to produce this string. For example:
v=0
o=- 1181923068 1181923196 IN IP4 ua1.example.com
s=example1
c=IN IP4 ua1.example.com
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:\
eyJpZHAiOnsiZG9tYWluIjoiZXhhbXBsZS5vcmciLCJwcm90b2NvbCI6ImJvZ3Vz\
In0sImFzc2VydGlvbiI6IntcImlkZW50aXR5XCI6XCJib2JAZXhhbXBsZS5vcmdc\
IixcImNvbnRlbnRzXCI6XCJhYmNkZWZnaGlqa2xtbm9wcXJzdHV2d3l6XCIsXCJz\
aWduYXR1cmVcIjpcIjAxMDIwMzA0MDUwNlwifSJ9
a=...
t=0 0
m=audio 6056 RTP/SAVP 0
a=sendrecv
...
Note that long lines in the example are folded to meet the column
width constraints of this document; the backslash ("\") at the end of
a line, the carriage return that follows, and whitespace shall be ignored.
The 'identity' attribute attests to all fingerprint attributes in the session description. It is therefore a session-level attribute.
Multiple fingerprint values can be used to offer alternative certificates for a peer. The identity attribute MUST include all fingerprint values that are included in fingerprint attributes of the session description.
The RP browser MUST verify that the in-use certificate for a DTLS connection is in the set of fingerprints returned from the IdP when verifying an assertion.
In order to ensure that the IdP is under control of the domain owner rather than someone who merely has an account on the domain owner's server (e.g., in shared hosting scenarios), the IdP JavaScript is hosted at a deterministic location based on the IdP's domain name. Each IdP proxy instance is associated with two values:
Each IdP MUST serve its initial entry page (i.e., the one loaded by the IdP proxy) from a well-known URI. The well-known URI for an IdP proxy is formed from the following URI components:
For example, for the IdP "identity.example.com" and the protocol "example", the URL would be:
https://identity.example.com/.well-known/idp-proxy/example
The IdP MAY redirect requests to this URL, but they MUST retain the "https" scheme. This changes the effective origin of the IdP, but not the domain of the identities that the IdP is permitted to assert and validate. I.e., the IdP is still regarded as authoritative for the original domain.
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:
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 in the idp field of the JSON-encoded object (see Section 7.6), 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 8.1.
The input to identity assertion is the JSON-encoded object described in Section 7.4 that contains the set of certificate fingerprints the browser intends to use. This string is treated as opaque from the perspective of the IdP.
The browser also identifies the origin that the PeerConnection is run in, which allows the IdP to make decisions based on who is requesting the assertion.
An application can optionally provide a user identifier hint when specifying an IdP. This value is a hint that the IdP can use to select amongst multiple identities, or to avoid providing assertions for unwanted identities. The username is a string that has no meaning to any entity other than the IdP, it can contain any data the IdP needs in order to correctly generate an assertion.
An identity assertion that is successfully provided by the IdP consists of the following information:
Figure 5 shows an example assertion formatted as JSON. 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.
{
"idp":{
"domain": "example.org",
"protocol": "bogus"
},
"assertion": "{\"identity\":\"bob@example.org\",
\"contents\":\"abcdefghijklmnopqrstuvwyz\",
\"signature\":\"010203040506\"}"
}
Figure 5: Example assertion
For use in signaling, the assertion is serialized into JSON, Base64-encoded, and used as the value of the identity attribute. IdPs SHOULD ensure that any assertions they generate cannot be interpreted in a different context. E.g., they should use a distinct format or have separate cryptographic keys for assertion generation and other purposes. Line breaks are inserted solely for readability.
In order to generate an identity assertion, the IdP needs proof of the user's identity. It is common practice to authenticate users (using passwords or multi-factor authentication), then use Cookies or HTTP authentication for subsequent exchanges.
The IdP proxy is able to access cookies, HTTP authentication or other persistent session data because it operates in the security context of the IdP origin. Therefore, if a user is logged in, the IdP could have all the information needed to generate an assertion.
An IdP proxy is unable to generate an assertion if the user is not logged in, or the IdP wants to interact with the user to acquire more information before generating the assertion. If the IdP wants to interact with the user before generating an assertion, the IdP proxy can fail to generate an assertion and instead indicate a URL where login should proceed.
The application can then load the provided URL to enable the user to enter credentials. The communication between the application and the IdP is described in [webrtc-api].
The input to identity validation is the assertion string taken from a decoded 'identity' attribute.
The IdP proxy verifies the assertion. Depending on the identity protocol, the proxy might contact the IdP server or other servers. For instance, an OAuth-based protocol will likely require using the IdP as an oracle, whereas with a signature-based scheme might be able to verify the assertion without contacting the IdP, provided that it has cached the relevant public key.
Regardless of the mechanism, if verification succeeds, a successful response from the IdP proxy consists of the following information:
Figure 6 shows an example response, which is JSON-formatted.
{
"identity": "bob@example.org",
"contents": "{\"fingerprint\":[ ... ]}"
}
Figure 6: Example verification result
The identity provided from the IdP to the RP browser MUST consist of a string representing the user's identity. This string is in the form "<user>@<domain>", where user consists of any character, and domain is aninternationalized domain name [RFC5890] encoded as a sequence of U-labels.
The PeerConnection API MUST check this string as follows:
Any "@" or "%" characters in the "user" portion of the identity MUST be escaped according to the "Percent-Encoding" rules defined in Section 2.1 of [RFC3986]. Characters other than "@" and "%" MUST NOT be percent-encoded. For example, with a "user" of "user@133" and a "domain" of "identity.example.com", the resulting string will be encoded as "user%40133@identity.example.com".
Implementations are cautioned to take care when displaying user identities containing escaped "@" characters. If such characters are unescaped prior to display, implementations MUST distinguish between the domain of the IdP proxy and any domain that might be implied by the portion of the "<user>" portion that appears after the escaped "@" sign.
Much of the security analysis of this problem is contained in [I-D.ietf-rtcweb-security] or in the discussion of the particular issues above. In order to avoid repetition, this section focuses on (a) residual threats that are not addressed by this document and (b) threats produced by failure/misbehavior of one of the components in the system.
IF HTTPS is not used to secure communications to the signaling server, and the identity mechanism used in Section 7 is not used, then any on-path attacker can replace the DTLS-SRTP fingerprints in the handshake and thus substitute its own identity for that of either endpoint.
Even if HTTPS is used, the signaling server can potentially mount a man-in-the-middle attack unless implementations have some mechanism for independently verifying keys. The UI requirements in Section 6.5 are designed to provide such a mechanism for motivated/security conscious users, but are not suitable for general use. The identity service mechanisms in Section 7 are more suitable for general use. Note, however, that a malicious signaling service can strip off any such identity assertions, though it cannot forge new ones. Note that all of the third-party security mechanisms 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 the user's connection to the Web site itself. Users who wish to assure themselves of security against a malicious identity provider can only do so by verifying peer credentials directly, e.g., by checking the peer's fingerprint against a value delivered out of band.
In order to protect against malicious content JavaScript, that JavaScript MUST NOT be allowed to have direct access to---or perform computations with---DTLS keys. For instance, if content JS were able to compute digital signatures, then it would be possible for content JS to get an identity assertion for a browser's generated key and then use that assertion plus a signature by the key to authenticate a call protected under an ephemeral Diffie-Hellman (DH) key controlled by the content JS, thus violating the security guarantees otherwise provided by the IdP mechanism. Note that it is not sufficient merely to deny the content JS direct access to the keys, as some have suggested doing with the WebCrypto API [webcrypto]. The JS must also not be allowed to perform operations that would be valid for a DTLS endpoint. By far the safest approach is simply to deny the ability to perform any operations that depend on secret information associated with the key. Operations that depend on public information, such as exporting the public key are of course safe.
The requirements in this document are intended to allow:
However, these privacy protections come at a performance cost in terms of using TURN relays and, in the latter case, delaying ICE. Sites SHOULD make users aware of these tradeoffs.
Note that the protections provided here assume a non-malicious calling service. As the calling service always knows the users status and (absent the use of a technology like Tor) their IP address, they can violate the users privacy at will. Users who wish privacy against the calling sites they are using must use separate privacy enhancing technologies such as Tor. Combined WebRTC/Tor implementations SHOULD arrange to route the media as well as the signaling through Tor. Currently this will produce very suboptimal performance.
Additionally, any identifier which persists across multiple calls is potentially a problem for privacy, especially for anonymous calling services. Such services SHOULD instruct the browser to use separate DTLS keys for each call and also to use TURN throughout the call. Otherwise, the other side will learn linkable information that would allow them to correlate the browser across multiple calls. Additionally, browsers SHOULD implement the privacy-preserving CNAME generation mode of [RFC7022].
The consent mechanisms described in this document are intended to mitigate denial of service attacks in which an attacker uses clients to send large amounts of traffic to a victim without the consent of the victim. While these mechanisms are sufficient to protect victims who have not implemented WebRTC at all, WebRTC implementations need to be more careful.
Consider the case of a call center which accepts calls via WebRTC. An attacker proxies the call center's front-end and arranges for multiple clients to initiate calls to the call center. Note that this requires user consent in many cases but because the data channel does not need consent, he can use that directly. Since ICE will complete, browsers can then be induced to send large amounts of data to the victim call center if it supports the data channel at all. Preventing this attack requires that automated WebRTC implementations implement sensible flow control and have the ability to triage out (i.e., stop responding to ICE probes on) calls which are behaving badly, and especially to be prepared to remotely throttle the data channel in the absence of plausible audio and video (which the attacker cannot control).
Another related attack is for the signaling service to swap the ICE candidates for the audio and video streams, thus forcing a browser to send video to the sink that the other victim expects will contain audio (perhaps it is only expecting audio!) potentially causing overload. Muxing multiple media flows over a single transport makes it harder to individually suppress a single flow by denying ICE keepalives. Either media-level (RTCP) mechanisms must be used or the implementation must deny responses entirely, thus terminating the call.
Yet another attack, suggested by Magnus Westerlund, is for the 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 browsers to get them to attempt a call to someone. It then translates their offers into apparent answers to the victim, which looks like large-scale parallel forking. The victim still responds to ICE responses and now the browsers all try to send media to the victim. Implementations can defend themselves from this attack by only responding to ICE Binding Requests for a limited number of remote ufrags (this is the reason for the requirement that the JS not be able to control the ufrag and password).
[I-D.ietf-rtcweb-rtp-usage] Section 13 documents a number of potential RTCP-based DoS attacks and countermeasures.
Note that attacks based on confusing one end or the other about consent are possible even in the face of the third-party identity mechanism as long as major parts of the signaling messages are not signed. On the other hand, signing the entire message severely restricts the capabilities of the calling application, so there are difficult tradeoffs here.
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.
Fundamentally, the IdP proxy is just a piece of HTML and JS loaded by the browser, so nothing stops a Web attacker from creating their own IFRAME, loading the IdP proxy HTML/JS, and requesting a signature over his own keys rather than those generated in the browser. However, that proxy would be in the attacker's origin, not the IdP's origin. Only the browser itself can instantiate a context that (a) is in the IdP's origin and (b) exposes the correct API surface. Thus, the IdP proxy on the sender's side MUST ensure that it is running in the IdP's origin prior to issuing assertions.
Note that this check only asserts that the browser (or some other entity with access to the user's authentication data) attests to the request and hence to the fingerprint. It does not demonstrate that the browser has access to the associated private key, and therefore an attacker can attach their own identity to another party's keying material, thus making a call which comes from Alice appear to come from the attacker. See [I-D.ietf-mmusic-sdp-uks] for defenses against this form of attack.
As described in Section 7.5 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.
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 "domain" field of the assertion 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.
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.
Because a broad range of characters are permitted in identity strings, it may be possible for attackers to craft identities which are confusable with other identities (see [RFC6943] for more on this topic). This is a problem with any identifier space of this type (e.g., e-mail addresses). Those minting identifers should avoid mixed scripts and similar confusable characters. Those presenting these identifiers to a user should consider highlighting cases of mixed script usage (see [RFC5890], section 4.4). Other best practices are still in development.
A number of optional Web security features have the potential to cause issues for this mechanism, as discussed below.
When popup blocking is in use, the IdP proxy is unable to generate popup windows, dialogs or any other form of user interactions. This prevents the IdP proxy from being used to circumvent user interaction. The "LOGINNEEDED" message allows the IdP proxy to inform the calling site of a need for user login, providing the information necessary to satisfy this requirement without resorting to direct user interaction from the IdP proxy itself.
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.
This specification defines the identity SDP attribute per the procedures of Section 8.2.4 of [RFC4566]. The required information for the registration is included here:
This section reqisters the idp-proxy well-known URI from [RFC5785].
Bernard Aboba, Harald Alvestrand, Richard Barnes, Dan Druta, Cullen Jennings, Hadriel Kaplan, Matthew Kaufman, Jim McEachern, Martin Thomson, Magnus Westerland. Matthew Kaufman provided the UI material in Section 6.5. Christer Holmberg provided the initial version of Section 5.1.
[RFC Editor: Please remove this section prior to publication.]
Rewrite the Identity section in more conventional offer/answer format.
Clarify rules on changing identities.
Update discussion of IdP security model
Replace "domain name" with RFC 3986 Authority
Clean up discussion of how to generate IdP URI.
Remove obsolete text about null cipher suites.
Remove obsolete appendixes about older IdP systems
Require support for ECDSA, PFS, and AEAD
Update cipher suite profiles.
Rework IdP interaction based on implementation experience in Firefox.
Replaced RTCWEB and RTC-Web with WebRTC, except when referring to the IETF WG
Forbade use in mixed content as discussed in Orlando.
Added a requirement to surface NULL ciphers to the top-level.
Tried to clarify SRTP versus DTLS-SRTP.
Added a section on screen sharing permissions.
Assorted editorial work.
The following changes have been made since the -05 draft.
Version -04 was a version control mistake. Please ignore.
The following changes have been made since the -04 draft.
The following changes have been made since the -02 draft.
| [RFC3261] | Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002. |
| [RFC5705] | Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, March 2010. |
| [RFC6120] | Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120, March 2011. |
| [RFC6265] | Barth, A., "HTTP State Management Mechanism", RFC 6265, DOI 10.17487/RFC6265, April 2011. |
| [RFC6455] | Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, DOI 10.17487/RFC6455, December 2011. |
| [RFC6943] | Thaler, D., "Issues in Identifier Comparison for Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May 2013. |
| [RFC7617] | Reschke, J., "The 'Basic' HTTP Authentication Scheme", RFC 7617, DOI 10.17487/RFC7617, September 2015. |
| [XmlHttpRequest] | van Kesteren, A., "XMLHttpRequest Level 2", January 2012. |