Network Working Group D. Wing Internet-Draft Cisco Systems Intended status: Standards Track July 1, 2007 Expires: January 2, 2008 SIP Identity using Media Path draft-wing-sip-identity-media-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on January 2, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract The existing SIP identity mechanism (RFC4474) creates a signature over the SIP body, including the entire SDP. As part of their normal operation, Session Border Controllers (SBCs) and SIP Back-to-Back User Agents (B2BUAs) modify various fields in the SDP, breaking the signature. This document defines a new mechanism to securely identify the originator of a SIP message while also allowing modification of the Wing Expires January 2, 2008 [Page 1] Internet-Draft SIP Identity using Media Path July 2007 SDP by SBCs and B2BUAs. This new mechanism creates a signature over certain SIP headers and certain SDP lines. Proof of identity over the media path using DTLS, TLS, HIP, and an extension to ICE are described. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1. Media Fingerprint Signature . . . . . . . . . . . . . . . 7 4.2. Authentication Service . . . . . . . . . . . . . . . . . . 9 4.3. Validation . . . . . . . . . . . . . . . . . . . . . . . . 9 5. Proof of Identity Techniques . . . . . . . . . . . . . . . . . 9 5.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.2. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.3. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.3.1. ICE Public Key SDP Attribute . . . . . . . . . . . . . 11 5.3.2. New STUN attributes . . . . . . . . . . . . . . . . . 11 5.4. HIP . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6. ABNF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 7.1. Device Disclosure . . . . . . . . . . . . . . . . . . . . 12 8. Operational Differences from RFC4474 . . . . . . . . . . . . . 13 9. Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 13 10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 10.1. DTLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 10.2. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 13. Normative References . . . . . . . . . . . . . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17 Intellectual Property and Copyright Statements . . . . . . . . . . 18 Wing Expires January 2, 2008 [Page 2] Internet-Draft SIP Identity using Media Path July 2007 1. Introduction SIP Identity [RFC4474] defines a mechanism to provide cryptographic identity for SIP requests. It provides this protection by signing certain SIP header fields (Contact, Date, Call-ID, CSeq, To, and From) and the body of the message. RFC4474 also signs the SIP body, which typically contains the SDP, with this explanation: This mechanism also provides a signature over the bodies of SIP requests. The most important reason for doing so is to protect Session Description Protocol (SDP) bodies carried in SIP requests. There is little purpose in establishing the identity of the user that originated a SIP request if this assurance is not coupled with a comparable assurance over the media descriptors. A weakness of RFC4474's approach is that SBCs and B2BUAs typically modify the media transport address and thus destroy the RFC4474 signature. Furthermore, even if such modification were not typical, the transport address by itself does not ensure media communication with the expected endpoint when NATs, Session Border Controllers, and media relays (e.g., TURN [I-D.ietf-behave-turn]) are considered as part of the end-to-end architecture. This is because the transport address could be reused by a malicious party within the replay window. The mechanism described in this document provides cryptographic assurance of the endpoint's identity by signing certain SIP headers, much like RFC4474. However, unlike RFC4474 which signs the entire SDP, the mechanism described in this document signs only certain SDP attributes. The remote endpoint is expected to validate the signature over the SIP headers and to initiate a proof of possession test over the media path, which proves the session has been established with the "From:" party in the SIP header. Mechanisms to perform this proof of possession are shown using DTLS and using a small extension ICE. Readers of this document are expected to be familiar with RFC4474, "Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP)", which defines the Identity and Identity- Info header fields. A future version of this document will have less reliance on RFC4474. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Wing Expires January 2, 2008 [Page 3] Internet-Draft SIP Identity using Media Path July 2007 3. Background SIP signaling has been evolving from direct UA-to-UA signaling to always signaling through proxies. This has been driven primarily by technical reasons, such as NATs or firewalls that prevent direct UA- to-UA signaling. While quite controversial in the IETF, Session Border Controllers are an aspect of SIP's evolution, and are driven by both technical reasons and market reasons. The primary technical driver for SBCs is to police media traffic into a network so that only media that was appropriately signaled via SIP is permitted into the network, ensuring IP packets are sent only to/from the SBC which eases ACL configuration (among other things). The primary market driver for SBCs is to hide business relationships -- this is, removal of Via headers and exposing only the IP address of the SBC to customers. The following diagram shows two service providers (SP1 and SP2), and each has an SBC at the edge of their respective networks. Each of these SBCs would need to rewrite the IP addresses in the SDP. +----[SP1-SBC1]-[SP1-SBC1]---[SP2-SBC1]-[SP2-SBC2]----+ | | [Enterprise-A] [Enterprise-B] Figure 1: Two Service Providers with SBCs Between Two Enterprises Enterprise-A can populate the "From:" address in its SIP requests using E.164 telephone number URLs (e.g., 'sip:+17005551008@example.com;user=phone') or using a mailto URL (e.g., 'sip:john.doe@example.com'). The characteristics of each choice, as the message traverses the SBCs operated by another administrative domain (service providers) are described below: E.164 telephone numbers: SP1 would validate the RFC4474 signature and modify the SDP. This breaks the RFC4474 signature created by the enterprise. So that a new RFC4474 signature can be created using its own public/private key pair, SP1 needs to modify the From: field. SP1 would substitute its own domain on the right-hand side, and signs the message with its own private key. SP2 would receive the SIP request from SP1, validate the signature, and perform a similar SDP modification, substitution, and resigning operation. Enterprise-B would receive the SIP request from SP2, validate its signature, and process the SIP request. Wing Expires January 2, 2008 [Page 4] Internet-Draft SIP Identity using Media Path July 2007 mailto URLs: SP1 would validate the RFC4474 signature modify the SDP. This breaks the RFC4474 signature created by the enterprise. So that a new RFC4474 signature can be created using its own public/private key pair, SP1 needs to modify the From: field. Unlike with E.164 numbers which are globally unique, the SP1 isn't able to substitute its domain name for the enterprise's domain name due to name collisions (that is, dwing@cisco.com cannot simply be rewritten as dwing@pacbell.net). One unappealing technique is to resurrect the percent hack from email: SP1 would rewrite the address to be 'sip:john.doe%example.com@sp1.net', sign it, and send it to SP2. SP2 would validate the signature, modify the SDP, and rewrite the address to be 'sip:john.doe%example.com@sp2.net', sign it, and send it to Enterprise-B. Enterprise-B would receive the SIP request from SP2, validate its signature, and process the SIP request. Both of these approaches share several weaknesses: 1. They create a natural incentive for the service providers to use transitive trust between themselves, rather than RFC4474, due to the computational expense of the per-call public key operations on each SIP request. For similar reasons, there is a natural incentive for the service providers to not even validate an enterprise's RFC4474 signature but rather to rely on a contract or rely on TLS to ensure the SIP signaling originated from the enterprise. 2. Because the original signature is destroyed by the first SBC, no other network (SP2 nor Enterprise-B) can validate the original signature. This means all downstream entities (in the example above, SP2 and Enterprise-B) are relying wholly on SP1 to validate the signature. This creates transitive trust which is undesirable - a single bad actor or compromised system anywhere along the path can compromise the entire identity system. 3. If an enterprise is connected to different service providers, one call from the same identity at Enterprise-A might appear to be from +14085551212@sp2.net and the next call from the same identity, routed through a different service provider, would appear to come from +14085551212@sp3.net. The terminating system would need to treat both From: addresses as identical for purposes of call routing, whitelists, reputation systems, and so on. This adds further complexity to system administration. Thus, we need a mechanism that allows Enterprise-B to cryptographically validate the identity of the remote party at Enterprise-A, even though intermediate SBCs have rewritten the media Wing Expires January 2, 2008 [Page 5] Internet-Draft SIP Identity using Media Path July 2007 transport address and forced the media to pass through their networks. 4. Operation The operation is very similar to RFC4474 and uses authentication service proxies exactly like RFC4474. Rather than reproducing the text of RFC4474, this section describes only the differences from RFC4474. The differences are: o A new header is created containing certain SDP attributes o This new header is signed in addition to the same set of SIP headers signed by RFC4474 (detailed in ) o Unlike RFC4474, the body of the SIP message (containing the SDP) is not signed The following figure shows how the Authentication Service and the media validation is performed. The figure assumes the endpoints themselves perform the media validation. In practice, if the : Service : Enterprise-A : Provider(s): Enterprise-B : : Auth. : B2BUA or : Auth. Endpoint-A Service : SBC : Service Endpoint-B | | : | : | | 1. |--Invite->| : | : | | 2. | sign : | : | | 3. | |-Invite-->|-Invite-->| | 4. | | : | : validate | 5. | | : | : |-------->| 6. |<=========tls, dtls, ice, or hip=========>| 7. | | : | : | validated 8. | | : | : | ring phone | | : | : | | : : Figure 2: Message Flow Step 1: Originating endpoint prepares to send an Invite and chooses the identity-challenge technique it supports, and indicates that in the SDP it generates. Described in this document are identity challenges for TLS, DTLS, ICE, and HIP. It then sends the Invite to its local SIP proxy. Wing Expires January 2, 2008 [Page 6] Internet-Draft SIP Identity using Media Path July 2007 Step 2: Originating endpoint's authentication service creates a new header, Identity-Fingerprints, containing certain lines of the SDP (e.g., a=fingerprint, a=ice-pub-key). The authentication service then creates a signature over certain SIP headers (e.g., From, To, Call-Id) and this new Identity- Fingerprints header. The resulting signature is inserted into the new Identity-Media header. The invite is forwarded to the next administrative domain. Step 3: The next administrative domain has an SBC (or B2BUA). The SBC modifies or rewrites certain SDP fields. Most typically an SBC will modify the "m" and "c" lines. These modifications do not break the signature. Step 4: The terminating endpoint's authentication service receives the Invite. It validates the Identity-Media signature is valid and was validates it was generated by the originating domain in step 2. Step 5: If the validation was successful, the terminating endpoint's authentication service forwards the Invite to the endpoint. Step 6: The terminating endpoint chooses a compatible identity- challenge technique from the Invite, and performs that challenge. Described in this document are identity challenges for TLS, DTLS, ICE, and HIP. Step 7: TLS, DTLS, and HIP cause the exchange of a certificate or public key. The terminating endpoint validates the certificate or public key has a fingerprint matching the Identity-Fingerprint header (originally created in step 2). If it does, the terminating endpoint completes the identity challenge exchange. After completion, the originating endpoint has proven (to the terminating endpoint) that it knows the private key associated with the certificate (or public key) signed in step 2. The terminating endpoint has now validated the identity of the originating endpoint. Step 8: You can reliably and honestly indicate calling party information ("caller-id") to the terminating endpoint, and ring their phone. 4.1. Media Fingerprint Signature In RFC4474, a signature is formed over some SIP headers and over the entire body (which most typically contains SDP). In this specification, some SIP headers are signed but only specific SDP attributes that provide cryptographic identity are signed (e.g., Wing Expires January 2, 2008 [Page 7] Internet-Draft SIP Identity using Media Path July 2007 "fingerprint"). The specific SDP attribute that are signed depends on which cryptographic identity technique(s) is used; see section Section 5. The SIP headers that are signed are signed the same as done by RFC4474, with the additional signing of the Media-Identity header; the body is not signed. They signed headers are: o The AoR of the UA sending the message, or addr-spec of the From header field (referred to occasionally here as the 'identity field'). o The addr-spec component of the To header field, which is the AoR to which the request is being sent. o The callid from Call-Id header field. o The digit (1*DIGIT) and method (method) portions from CSeq header field, separated by a single space (ABNF SP, or %x20). Note that the CSeq header field allows linear whitespace (LWS) rather than SP to separate the digit and method portions, and thus the CSeq header field may need to be transformed in order to be canonicalized. The authentication service MUST strip leading zeros from the 'digit' portion of the Cseq before generating the digest-string. o The Date header field, with exactly one space each for each SP and the weekday and month items case set as shown in BNF in RFC 3261. RFC 3261 specifies that the BNF for weekday and month is a choice amongst a set of tokens. The RFC 2234 rules for the BNF specify that tokens are case sensitive. However, when used to construct the canonical string defined here, the first letter of each week and month MUST be capitalized, and the remaining two letters must be lowercase. This matches the capitalization provided in the definition of each token. All requests that use the Identity mechanism MUST contain a Date header. o The addr-spec component of the Contact header field value. If the request does not contain a Contact header, this field MUST be empty (i.e., there will be no whitespace between the fourth and fifth "|" characters in the canonical string). o The fingerprints component of the Identity-Media header field value. In this specification, the Identity-Media header is signed instead of the message body. The Identity-Media contains only certain SDP lines from the SDP body. Wing Expires January 2, 2008 [Page 8] Internet-Draft SIP Identity using Media Path July 2007 4.2. Authentication Service The authentication service examines the SIP message body for the application/sdp Content-Type. For all such content-types found, the authentication service retrieves the cryptographic attributes described in Section 5, concatenates them together, and inserts a new SIP header field called Media-Fingerprints containing a comma- separated list of those signed attributes. This new header, along with all the headers and portions of headers signed by RFC4474 (From, Call-ID, etc.), are all signed by the authentication service. The resulting signature is placed on the new Identity-Fingerprints header. 4.3. Validation The validation service can be performed by the remote endpoint itself or by an SBC acting on behalf of the endpoint. The validation service first checks the Identity-Fingerprints signature. If this is valid, the endpoint (or its validation service operating on its behalf) then initiates a DTLS, TLS, ICE, or HIP identity proof (Section 5). This causes the originating endpoint to prove possession of its private key that corresponds to the certificate (or public key) that was signed by the remote domain's authentication service. 5. Proof of Identity Techniques Four techniques are described below, TLS, DTLS, ICE, and HIP. Each provides a means to cryptographically prove the identity signed by the authentication service in SIP is the same as the identity on the media path. Each of these techniques work similarly -- a fingerprint of the certificate (or, with ICE, the public key itself) is included in the SDP. The authentication service creates a new Identity-Fingerprints header and places into that header those certificate fingerprints (or, with ICE, the fingerprint of the public key). The authentication service then creates a signature over specific SIP headers (see Section 4.1), and places that signature into the new Identity-Media header. The SIP request is then sent outside of the originating domain. The receiving domain validates the Identity-Media signature. If successful, the SIP request is forwarded to the end system (or an SBC operating on its behalf). The end system initiates a TLS, DTLS, ICE, or HIP session and validates the certificate fingerprint presented in Wing Expires January 2, 2008 [Page 9] Internet-Draft SIP Identity using Media Path July 2007 SIP signaling matches the certificate presented in the TLS, DTLS, ICE, or HIP exchange. If they match, and the TLS, DTLS, ICE, or HIP exchange completes successfully, the local endpoint has validated the identity of the remote endpoint. Note: Due to SIP forking, the calling party may receive many identity challenges, each incurring a public key operation to prove identity. Mechanisms to deal with this are for future study. 5.1. TLS TLS uses the "fingerprint" attribute to provide a hash of the certificate in the SDP. The fingerprint attribute is defined by [RFC4572] for TLS. 5.2. DTLS DTLS uses the same "fingerprint" attribute originally described for TLS. The syntax is described in [I-D.fischl-sipping-media-dtls]. Note: DTLS is only necessary to prove identity with DTLS; SRTP [RFC3711] does not need to be used afterwards. Obviously, using SRTP provides significant benefits over continuing to use RTP, because an attacker can inject bogus RTP after a successful validation of identity which is quite undesirable. The SDP for doing RTP after a DTLS exchange might be signaled in SDP by using "RTP/AVP" rather than "RTP/SAVP" (lines folded for readability): v=0 o=- 25678 753849 IN IP4 192.0.2.1 s= c=IN IP4 192.0.2.1 t=0 0 m=audio 3456 RTP/AVP 0 18 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=setup:passive a=connection:new Of course, it would be desirable to more clearly indicate this somehow in SDP, as there are existing "best-effort" media encryption mechanisms which overload the meaning of a=crypto and a=key-mgmt to mean RTP/SAVP, and other implementations may also overload a=fingerprint in a similar, undesirable, way. Wing Expires January 2, 2008 [Page 10] Internet-Draft SIP Identity using Media Path July 2007 5.3. ICE ICE doesn't have inherent support for public/private keys. If public keys were sent with other ICE attributes, there can be a real risk of an ICE connectivity check exceeding the MTU. ICE lacks a mechanism to fragment such large messages. It is also bandwidth inefficient to send multiple ICE connectivity checks containing public keys, either as retransmissions or with multiple candidates. Thus, for ICE, the public key is sent in SDP and the public key's fingerprint is exchanged on the media path -- opposite of TLS, DTLS, and HIP. 5.3.1. ICE Public Key SDP Attribute The offerer includes its public key, which it will use for the subsequent PK-CHALLANGE and PK-RESPONSE, in its SDP. The syntax is a BASE64-encoded version of the endpoint's public key. The new attribute is called "ice-pub-key", which may appear on the session level, media level, or both. 5.3.2. New STUN attributes Two new STUN attributes are defined to carry the plaintext challenge and the encrypted response. 5.3.2.1. PK-CHALLANGE This is sent in a STUN Binding Request, and contains the bits to be encrypted by the private key. Up to 256 bits can be included in the challenge. When a STUN Binding Request is received containing this attribute, the contents of the PK-CHALLENGE are encrypted using the private key, and the result is included in the PK-RESPONSE attribute of the Binding Response. The PK-CHALLENGE MUST be the same for each candidate address that is being tested for connectivity. If this requirement is not followed, the peer will incur a public key operation for every ICE connectivity check, which is not reasonable or necessary. 5.3.2.2. PK-RESPONSE This is sent in a STUN Binding Response from the offerer to the answerer, and contains the encrypted result of the PK-CHALLENGE. 5.4. HIP In [I-D.tschofenig-hiprg-host-identities], a new attribute "key- mgmt:host-identity-tag" is defined which contains the hash of the Wing Expires January 2, 2008 [Page 11] Internet-Draft SIP Identity using Media Path July 2007 public key used in the subsequent HIP exchange. This can be utilized and signed exactly like the "fingerprint" attribute for TLS or DTLS. 6. ABNF The following figure shows the syntax of the new SIP header fields using ABNF [RFC4234] media-fingerprint = "Identity-Fingerprints" HCOLON fingerprints fingerprints = fingerprint *(COMMA fingerprint) fingerprint = 2UHEX *(":" 2UHEX) ; Each byte in upper-case hex, separated ; by colons. UHEX = DIGIT / %x41-46 ; A-F uppercase identity-media = "Identity-Media" HCOLON signature signature = LDQUOT 32LHEX RDQUOT Figure 4: ABNF for new SIP headers The following figure shows the syntax of the new SDP attribute containing the ICE public key: ice-pub-key = token ; BASE64 encoded public key Figure 5: ABNF for new SDP attribute 7. Security Considerations [[some of RFC4474's security considerations also apply.]] 7.1. Device Disclosure Although the mechanism described in this paper allows SBCs to be used with a cryptographic identity scheme, it does expose the identity of the user's certificate. If a unique certificate is installed on each user's device, the remote party will be able to discern which device is terminating the call. This problem is more pronounced when SIP retargeting occurs in conjunction with Connected Identity [RFC4916]. If this isn't desired, there are two solutions: Wing Expires January 2, 2008 [Page 12] Internet-Draft SIP Identity using Media Path July 2007 o all devices under the control of the user will need to have the same certificate (and associated private key) installed on them, or o the device to manufacture a new self-signed certificate (or public key) for each call, and populate the a=fingerprint or a=ice-pub- key attributes, as appropriate. This is possible because the identity service described in this paper does not require the same certificate or public key to be used on every call. 8. Operational Differences from RFC4474 RFC4474 imposes one public key operation for the authentication service and one for validation. In addition to that, the mechanism described in this paper also requires an additional public key operation for the authentication service and an additional public key operation for validation. If Connected Identity [RFC4916] is used, only one additional public key operation is necessary for the header signature validation; the expense of the DTLS, TLS, or ICE public key operation has already been incurred by both parties and is not repeated. The mechanism described in this document has the following advantages over RFC4474: o Only the edge network needs to create signatures on SIP requests -- not every intervening SBC, o The original cryptographically-provable identity is preserved across any number of SBCs. 9. Limitations For the identity procedure described in this document to function, every device -- including Session Border Controllers -- on the path MUST permit DTLS, TLS, ICE, or HIP messages to be exchanged in the media path. Further, those devices MUST NOT interfere with the SDP attributes or two new SIP headers necessary for Identity Media to operate. 10. Examples Wing Expires January 2, 2008 [Page 13] Internet-Draft SIP Identity using Media Path July 2007 10.1. DTLS This example shows how two a=fingerprint lines in SDP would populate a the Media-Fingerprints SIP header field. The following is an example of an Invite created by the endpoint. (lines folded for readability) INVITE sip:bob@biloxi.example.org SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8 To: Bob From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Max-Forwards: 70 Date: Thu, 21 Feb 2002 13:02:03 GMT Contact: Content-Type: application/sdp Content-Length: 147 v=0 o=- 6418913922105372816 2105372818 IN IP4 192.0.2.1 s=example2 c=IN IP4 192.0.2.1 t=0 0 m=audio 54113 RTP/SAVP 0 a=setup:active a=connection:new a=fingerprint:SHA-1 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB Figure 6: Example with DTLS The SIP proxy performing the Media Identity authentication service would then insert the following two SIP headers into the message. The Media-Fingerprints header contains all of the fingerprint lines and the Identity-Signature header contains the signature of all of the relevant SIP headers and of the Media-Fingerprints header. Lines are folded for readability: Identity-Fingerprints: 4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB Identity-Media: "ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U=" Figure 7: SIP Headers Inserted by Authentication Service Wing Expires January 2, 2008 [Page 14] Internet-Draft SIP Identity using Media Path July 2007 10.2. ICE With ICE, the public key is exchanged in the signaling path (in SDP) rather than in the media path (as is done with TLS, DTLS, and HIP). The Media-Fingerprints header only needs to contain the fingerprint of the ICE public key that is in the SDP. This is the INVITE as it left the SIP user agent (lines folded for readability): INVITE sip:bob@biloxi.example.org SIP/2.0 Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8 To: Bob From: Alice ;tag=1928301774 Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Max-Forwards: 70 Date: Thu, 21 Feb 2002 13:02:03 GMT Contact: Content-Type: application/sdp Content-Length: 147 v=0 o=- 6418913922105372816 2105372818 IN IP4 192.0.2.1 s=example2 c=IN IP4 192.0.2.1 t=0 0 a=ice-pwd:asd88fgpdd777uzjYhagZg a=ice-ufrag:8hhY a=pub-key:ejfiwj289ceucuezeceEJFjefkcjeiquiefekureickejfeefe uirujejfecejejejkfeJJCEIUQQIEFJCQUCJCEQUURIE09dnjkeefjek m=audio 54113 RTP/AVP 0 a=candidate:1 1 UDP 2130706431 192.0.2.1 54113 typ host Figure 8: Example with ICE The SIP proxy performing the Media Identity authentication service would then insert the following two SIP headers into the message. The Media-Fingerprints header contains the fingerprint of the ICE public key (A3:EA:B3:...), and the Identity-Signature header contains the signature of all of the relevant SIP headers and of the Media- Fingerprints header (lines are folded for readability): Identity-Fingerprints: A3:EA:B3:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB:08 Identity-Identity: "jjsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI+p6q5TOQXHMmz6uEo3svJsSH49th8qc efQBbHC00VMZr2k+t6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0Ssjcd Wing Expires January 2, 2008 [Page 15] Internet-Draft SIP Identity using Media Path July 2007 VcunyaZucyRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Jcqe=" Figure 9: Headers Inserted by Authentication Service 11. Acknowledgements The mechanism described in this paper is derived from Jon Peterson and Cullen Jennings' [RFC4474], which was formerly a document of the SIP working group. 12. IANA Considerations This document will add new IANA registrations for new STUN attributes. [[This section will be completed in a later version of this document.]] 13. Normative References [I-D.ietf-behave-turn] Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN)", draft-ietf-behave-turn-03 (work in progress), March 2007. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4474] Peterson, J. and C. Jennings, "Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP)", RFC 4474, August 2006. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [I-D.fischl-sipping-media-dtls] Fischl, J., "Datagram Transport Layer Security (DTLS) Protocol for Protection of Media Traffic Established with the Session Initiation Protocol", draft-fischl-sipping-media-dtls-02 (work in progress), March 2007. [RFC4572] Lennox, J., "Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Wing Expires January 2, 2008 [Page 16] Internet-Draft SIP Identity using Media Path July 2007 Description Protocol (SDP)", RFC 4572, July 2006. [RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [RFC4916] Elwell, J., "Connected Identity in the Session Initiation Protocol (SIP)", RFC 4916, June 2007. [I-D.tschofenig-hiprg-host-identities] Tschofenig, H., "Interaction between SIP and HIP", draft-tschofenig-hiprg-host-identities-05 (work in progress), June 2007. Author's Address Dan Wing Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA Email: dwing@cisco.com Wing Expires January 2, 2008 [Page 17] Internet-Draft SIP Identity using Media Path July 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Wing Expires January 2, 2008 [Page 18]