SIP Identity using Media Path
draft-wing-sip-identity-media-01

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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 which breaks that signature.

This document defines a new identity mechanism which operates with SBCs and B2BUAs. This new identity mechanism creates a signature over certain SIP headers and certain SDP lines, and uses both SIP signaling and the media path to perform its identity function.

Table of Contents

1.  Introduction
2.  Notational Conventions
3.  Analysis of SIP-Identity with SBCs
    3.1.  SIP-Identity with SBCs
        3.1.1.  Validate the Signature
        3.1.2.  Destroy Existing Signature
        3.1.3.  Create New Identity
        3.1.4.  Sign the New Identity
    3.2.  Transport Address as Identifier
4.  Operation
    4.1.  Identity Media Signature
    4.2.  Authentication Service
    4.3.  Validation
5.  Proof of Identity Techniques
    5.1.  TLS
    5.2.  DTLS
        5.2.1.  SRTP after DTLS optional
    5.3.  ICE
        5.3.1.  ICE Public Key SDP Attribute
        5.3.2.  New STUN attributes
    5.4.  HIP
6.  ABNF
7.  Security Considerations
    7.1.  Device Disclosure
8.  Operational Differences from RFC4474
9.  Limitations
10.  Examples
    10.1.  DTLS
    10.2.  ICE
    10.3.  Request without SDP
11.  Acknowledgements
12.  IANA Considerations
13.  References
    13.1.  Normative References
    13.2.  Informational References
Appendix A.  ToDo List
Appendix B.  Changes From Previous Versions
    B.1.  Changes from 00 to 01
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

SIP Identity (Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” August 2006.) [RFC4474] provides 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 SIP message body. The SIP message body typically contains the SDP.

In cases where the originating, authenticating domain directly connect to the terminating, validating domain, RFC4474 has questionable value. The reason is that in such cases TLS can simply be used for the communication between the edge proxies of each domain, which not only provides additional security properties (e.g., encryption), but is also more efficient and protects all signaling messages. The real value of RFC4474 lies in cases where SIP signaling crosses multiple domains, belonging to different organizations. Unfortunately, in the presence of SBCs or B2BUAs that are in a different trust domain than the calling party or called party, SIP Identity provides the same security properties as using P-Asserted-Identity (Jennings, C., Peterson, J., and M. Watson, “Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity within Trusted Networks,” November 2002.) [RFC3325] between those same trust domains, if it succeeds at all. In most cases it would probably fail, and force the UAC to re-send its request without any Identity mechanism if it wanted to establish communication.

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, and not all the same SIP headers. The remote endpoint is expected to validate the signature over the SIP headers and specified SDP attributes, 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 to ICE (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.) [I‑D.ietf‑mmusic‑ice]. This mechanism is also extensible, in order to be usable by future mechanisms which need signed SDP attributes

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.  Notational Conventions

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Analysis of SIP-Identity with SBCs

This section examines how SIP Identity (Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” August 2006.) [RFC4474] operates with SBCs(Section 3.1 (SIP-Identity with SBCs)) or certain back-to-back user-agents (B2BUAs), and how SIP Identity uses transport addresses as identifiers (Section 3.2 (Transport Address as Identifier)).

3.1.  SIP-Identity with SBCs

After an authorization agent signs a SIP request, the SIP request traverses SBC(s) operated by other trust domains and finally arrives at the destination domain. If all of those intervening SBCs support SIP Identity, those SBCs will validate the request's signature, destroy the existing signature, create a new identity for the request, and sign the request's new identity. These necessary SBC actions (described in detail below) provide the same identity assurance as using P-Asserted-Identity between trust domains.

The functions of SBCs, and why they do what they do, is mainly detailed in [I‑D.ietf‑sipping‑sbc‑funcs] (Hautakorpi, J., Camarillo, G., Penfield, B., Hawrylyshen, A., and M. Bhatia, “Requirements from Session Initiation Protocol (SIP) Session Border Control (SBC) Deployments,” February 2010.). Regardless of the architectural implications of their actions, the fact is their functions are often performed as a SIP request/response traverses across SIP domains. Other B2BUAs in the path sometimes also perform functions which invalidate an RFC4474 signature. With regard to RFC4474, the SBC functions which impact the signature are:

• Replacing IP addresses in SDP body parts
• Replacing the Contact URI
• Modifying the Call-ID
• Modifying the CSeq

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 rewrite the IP addresses in the SDP. There may be an SBC in the Enterprise as well, however that SBC would own the appropriate Enterprise domain certificate and re-sign the request, and thus logically appear as the Enterprise User Agent

+---------+ +---------+  +---------+  +---------+
|SP1-SBC-1|-|SP1-SBC-2|--|SP2-SBC-1|--|SP2-SBC-2|
+---+-----+ +---------+  +---------+  +-+-------+
    |                                   |
    |                                   |
+--------------+                 +------+-------+
|SIP User Agent|                 |SIP User Agent|
| Enterprise-A |                 | Enterprise-B |
+--------------+                 +--------------+

Enterprise-A can populate the "From:" address in its SIP requests using E.164 telephone numbers (e.g., 'sip:+17005551008@example.com;user=phone') or using a SIP URI (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:

3.1.1.  Validate the Signature

Per RFC4474, the SBC would validate the incoming SIP request. This requires a public key operation to be performed against the SIP request.

[[computationally expensive. Will use TLS or IPSEC instead (doesn't require public key operation for every SIP request), or will rely on ACLs or a dedicated link or dedicated network.]]

This creates 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 or IPSEC to ensure the SIP signaling originated from the enterprise. Since service providers do not allow Enterprises to be transitive domains, they only allow From-URI domains of the enterprise, and thus do not need a per-request 4474-style signature from the Enterprise at the first ingress hop.

3.1.2.  Destroy Existing Signature

SBCs and B2BUAs typically modify the media transport addresses and thus invalidate the RFC4474 signature. This prevents downstream systems from validating original signature.

Because the original signature is invalidated 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. Note this does not require malicious intent within the trust domain - a weak or mis-configured entry point into the trust chain of service providers compromises the entire trust chain.

3.1.3.  Create New Identity

In order to generate a SIP Identity signature (in the next step), the SBC requires the private key associated with the domain of that From: address. Because the SBC and the initiator of the SIP request are different entities, it is unlikely the SBC will possess the initiating domain's private key. Thus, the SBC will have to create a new identity -- using its domain -- for the request, if it wants to provide RFC4474.

The new identity creates difficulties with downstream whitelists, call routing, and reputation systems. For example, downstream systems may sometimes see the identity +14085551212@example.com when the request is routed through certain SBCs, and may see a different identity, +14085551212@example.net, when the request is routed through different SBCs. Such different routing might be done due to network outages or for cost savings (e.g., evening rates). Due to these different identities, the domain name no longer has much meaning -- for E.164 telephone numbers, the user-part becomes the entire identity. In some sense that's appropriate; if the user-part is truly a global-scope E.164 number, then the domain portion is essentially meaningless. It might as well have been a tel-URI, except that making it a SIP-URI is more common and allows rfc4474 to be used.

A SBC might receive an identity containing an E.164 phone number or containing a SIP URI. When forming a new identity, the SBC performs different steps for each of those cases:

E.164 telephone numbers:

The SBC would remove the original domain name and substitute its own domain on the right-hand side.

SIP URIs:

Unlike E.164 telephone numbers, the SBC is not able to simply substitute its domain name for the enterprise's domain name due to user-part name collisions. There is only one unappealing solution: use the so-called escape-hack from email. For example, the SBC could rewrite sip:john.doe@example.com to sip:john.doe%example.com@sp1.example.net.

3.1.4.  Sign the New Identity

Finally, the SBC will sign the new identity, using the private key associated with the new identity. This private key is known to the SBC (because it is the SBC's domain name). This function incurs a public key operation for that SIP request.

3.2.  Transport Address as Identifier

SIP Identity signs the SDP so that the IP address (contained in the SDP) provides identity. From [RFC4474] (Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” August 2006.):

"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."

RFC4474 ties an identifier (IP address) with the identity (SIP "From:" address), which is counter to ongoing efforts in the IETF to split identifiers from identity (e.g., [I‑D.ietf‑hip‑base] (Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson, “Host Identity Protocol,” October 2007.), [I‑D.farinacci‑lisp] (Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, “Locator/ID Separation Protocol (LISP),” March 2009.), [I‑D.carpenter‑idloc‑map‑cons] (Carpenter, B., “General Identifier-Locator Mapping Considerations,” June 2007.), [I‑D.whittle‑ivip‑arch] (Whittle, R., “Ivip (Internet Vastly Improved Plumbing) Architecture,” March 2010.)).

The presence of media relays (e.g., TURN (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” July 2009.) [I‑D.ietf‑behave‑turn]), which may be used by an attacker, means that relying exclusively on such identifiers is risky. For example, if an attacker were able to cause re-use of the (signed) transport address within the replay window, the attacker can successfully impersonate the identity.

Additionally, RFC4474's tying of identities to IP address causes a failure in certain NAT scenarios when the source transport address of arriving media does not agree with the SDP. While not written down in a standard at this time, if both endpoints are using ICE (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.) [I‑D.ietf‑mmusic‑ice], the ICE username and password (sent in SDP and signed by RFC4474) can be reliably used to establish identity. However, if either endpoint does not support ICE, the arriving media will be considered fraudulent because the arriving media does not match with the RFC4474-signed SDP.

4.  Operation

The operation of SIP-Identity-Media is similar to RFC4474 and uses authentication service proxies much like RFC4474. The basic steps are:

• A new header, Identity-Media, is created containing the names of certain SDP attributes from SDP bodyparts, and containing a hash of non-SDP bodyparts.
• Several SIP headers and the Identity-Media header are all signed (as detailed in Section 4.1 (Identity Media Signature)), and the result is placed in Identity-Media-Signature.
• The receiving domain validates the signature, and if the request is an invitation to establish a media channel, performs a proof of identity validation using DTLS, TLS, ICE, or HIP over the media path.

The following figure shows how the Authentication Service and the media validation is performed. The figure assumes the endpoints themselves perform the media validation.

                        :  Service   :
       Enterprise-A     : Provider-1 :    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
        |          |    :     |      :   |         |
                        :            :

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.

Step 2:

Originating endpoint's authentication service creates a new header, Identity-Media, containing certain attribute names from 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) and this new Identity-Media header. The resulting signature is inserted into the new Identity-Media-Signature header. An Identity-Info header is added, pointing to this domain's certificate. The INVITE, with these additional headers, is forwarded to the next administrative domain. [NOTE: alternatively, we could allow the UAC to create the Identity- Media header with the attributes it wants signed, then have the auth server sign them and insert the signature header - this would be more flexible]

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, so long as the SBC doesn't remove the headers Identity-Media, Identity-Media-Signature, or Identity-Info, and do not remove or alter the signed attributes from the SDP.

Step 4:

The terminating endpoint's authentication service receives the INVITE. It validates that the signature contained in the Identity-Media-Signature header, and validates that the signing certificate is owned by the originating domain from step 2. This validation is done by using the certificate pointed to in the Identity-Info header, which MUST match the domain in the From: address.

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 (TLS, DTLS, ICE, or HIP), and performs that challenge. Described in this document are identity challenges for TLS, DTLS, ICE, and HIP.

Step 7:

All of the identity challenges (TLS, DTLS, ICE, and HIP) cause the exchange of either a certificate or a public key in the media path. The terminating endpoint compares the certificate or public key with the fingerprint in the (signed) Identity-Media header (originally created in step 2). If they match, the terminating endpoint completes the identity challenge exchange. After completion, the originating endpoint has proven (to the terminating endpoint) that the originating endpoint 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:

The terminating endpoint can reliably and honestly indicate calling party information ("caller-id") and ring the phone.

4.1.  Identity Media 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., "a=fingerprint" and its value). The specific SDP attributes that are signed depends on which cryptographic identity technique(s) is used; see section Section 5 (Proof of Identity Techniques).

The SIP headers that are signed, for the signature placed into the Identity-Media-Signature header are:

• The AoR of the UA sending the message, or addr-spec of the From header field (referred to occasionally here as the 'identity field').
• The addr-spec component of the To header field, which is the AoR to which the request is being sent.
• The SIP method.
• [NOTE: Contact, CSeq and Call-Id not included]
• The Date header field, with exactly one space each for each SP and the weekday and month items case set as shown in the BNF in RFC3261. RFC3261 specifies that the BNF for weekday and month is a choice amongst a set of tokens. The RFC2234 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-Media mechanism MUST contain a Date header.
• The Identity-Media header field value.

The hash is formed of these elements:

   digest-string = addr-spec "|" addr-spec "|"
                   Method "|" SIP-date "|"
                   attrib-bodyhash-list

The first addr-spec MUST be taken from the From header field value, the second addr-spec MUST be taken from the To header field value.

The Identity-Info header points to where the authentication service's certificate can be retrieved from.

4.2.  Authentication Service

The authentication service examines the SIP message body to build the Identity-Media header. For each message body found, in the order found:

• if the body part is application/sdp, the authentication service retrieves only the cryptographic attributes from the SDP (as described in Section 5 (Proof of Identity Techniques)), and appends that information to the Identity-Media header.
• otherwise, for all other body parts, the body part is hashed using SHA-1, and the first 96 bytes are appended to the Identity-Media header using "BPH=".

For example, A SIP request with three bodyparts: text/plain, application/sdp, and image/jpg, the Identity-Media attribute would contain a bodypart hash of the text/plain part, certain SDP attribute lines from the application/sdp bodypart (a=fingerprint in this example), and a bodypart hash of the image/jpg bodypart:

  Identity-Media: BPH="e32je3j23cjek3dz","a=fingerprint",
    BPH="8fj289r3i892381c"

This Identity-Media header, along with the headers and portions of headers described in Section 4.1 (Identity Media Signature) are all signed by the authentication service. The resulting signature is placed on the new Identity-Media-Signature header.

4.3.  Validation

The validation service can be performed by the remote endpoint itself or by a device acting on behalf of the endpoint. The validation service first checks the signature in the Identity-Media-Signature field. 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 (Proof of Identity Techniques)). 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-Media header and places into that header those SDP attribute names associated with that technique. The authentication service then creates a signature over specific SIP headers (see Section 4.1 (Identity Media Signature)), and places that signature into the new Identity-Media-Signature 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. The end system initiates a TLS, DTLS, ICE, or HIP session and validates that the (signed) certificate fingerprint presented in the 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] (Lennox, J., “Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP),” July 2006.) for TLS.

5.2.  DTLS

DTLS uses the same "fingerprint" attribute originally described for TLS. The syntax is described in [I‑D.ietf‑sip‑dtls‑srtp‑framework] (Fischl, J., Tschofenig, H., and E. Rescorla, “Framework for Establishing an SRTP Security Context using DTLS,” March 2009.).

5.2.1.  SRTP after DTLS optional

[[Discussion Point: Is there interest in having identity without SRTP??]]

DTLS is only necessary to prove identity with DTLS; SRTP (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [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

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-CHALLENGE 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-CHALLENGE

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] (Tschofenig, H., Ott, J., Schulzrinne, H., Henderson, T., and G. Camarillo, “Interaction between SIP and HIP,” February 2008.), a new attribute "key-mgmt:host-identity-tag" is defined which contains the hash of the 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 (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” October 2005.) [RFC4234]

   identity-media        = "Identity-Media" HCOLON
                           attrib-bodyhash-list
   attrib-bodyhash-list  = attrib-bodyhash *(COMMA attrib-bodyhash)
   attrib-bodyhash       = quoted-attrib | quoted-bodyparthash
   quoted-attribute      = DQUOTE attribute DQUOTE  ; SDP "a=" line
   quoted-bodyhash       = "BPH" EQUAL DQUOTE bodyparthash DQUOTE
   bodyparthash          = 32HEXDIG

   identity-media-sig    = "Identity-Media-Signature" HCOLON
                           signature
   signature             = DQUOT 32HEXDIG DQUOT

   Identity-Info = "Identity-Info" HCOLON ident-info
                    *( SEMI ident-info-params )
   ident-info = LAQUOT absoluteURI RAQUOT
   ident-info-params = ident-info-alg / ident-info-extension
   ident-info-alg = "alg" EQUAL token
   ident-info-extension = generic-param

The following figure shows the syntax of the new SDP attribute containing the ICE public key. This is used only by endpoints implementing the ICE proof of identity technique (Section 5.3 (ICE)).

  ice-pub-key        = token    ; BASE64 encoded public key

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 (Elwell, J., “Connected Identity in the Session Initiation Protocol (SIP),” June 2007.) [RFC4916].

If this isn't desired, there are two solutions:

• All devices under the control of the user will need to have the same certificate (and associated private key) installed on them.
• The device needs to manufacture a new self-signed certificate (or public key) for each call, and populate the appropriate SDP attributes with that certificate (or public key). 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. If Connected Identity (Elwell, J., “Connected Identity in the Session Initiation Protocol (SIP),” June 2007.) [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.

RFC4474 includes the Contact URI in the signed headers. That is not required by this mechanism because it adds no security property, and will fail validation when crossing SBCs and B2BUA's. It is of dubious security value because Via/Record-Route can be inserted for response interception regardless, and some requests don't contain a Contact anyway (e.g., MESSAGE). It does not provide any replay/copy-paste protection either, for the same reasons.

RFC4474 includes the CSeq in the signed headers. That is not required by this mechanism because it adds little security, and will fail validation when crossing SBCs and B2BUA's in some cases. It is of little security value because it provides no protection from cut-paste attack for different targets, and although it would prevent replay attack within the same session, since the media key-related SDP portions are signed anyway, replaying the request will not do anything useful.

RFC4474 includes the Call-Id in the signed headers. That is not required by this mechanism because it adds little security, and will fail validation when crossing SBCs and B2BUA's in some cases. It is of little security value because it provides no protection from cut-paste attack for different targets, and although it would prevent replay attack for the same target, since the media key-related SDP portions are signed anyway, replaying the request will not do anything useful.

The mechanism described in this document has the following advantages over RFC4474:

• Only the edge network needs to create signatures on SIP requests -- not every intervening SBC,
• The original cryptographically-provable identity is preserved across any number of SBCs, B2BUA's, etc.
• SBCs, B2BUA's, and other "middle-boxes" in intermediate domains do not need to be upgraded or changed in order for the originating and terminating domains to use this new mechanism.

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 signed SDP attributes or the new SIP headers necessary for Identity Media to operate.

For the technique described in this document to function, all on-path SIP elements -- SBCs, B2BUAs, and SIP proxies -- MUST NOT interfere with the signed headers. The identity mechanism described in this document is not harmed if on-path SIP elements alter the SDP (e.g., by deleting non-signed attributes, connection addresses, etc.).

10.  Examples

10.1.  DTLS

This example shows how two a=fingerprint lines in SDP would populate the Identity-Media 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 <sip:bob@biloxi.example.org>
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Max-Forwards: 70
   Date: Thu, 21 Feb 2002 13:02:03 GMT
   Contact: <sip:alice@pc33.atlanta.example.com>
   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=fingerprint:SHA-1
     4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
   m=video 54115 RTP/SAVP 0
   a=fingerprint:SHA-1
     4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB

The SIP proxy performing the Media Identity authentication service would then insert the following three SIP headers into the message. The Identity-Media header contains all of the SDP attribute lines that are signed and the Identity-Media header contains the signature of all of the relevant SIP headers and of the Identity-Media header. Lines are folded for readability:

  Identity-Info: <https://atlanta.example.com/atlanta.cer>
     ;alg=rsa-sha1
  Identity-Media: "a=fingerprint","a=fingerprint"
  Identity-Media-Signature:
   "ZYNBbHC00VMZr2kZt6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0SsSAa
    ifsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI6p6q5TOQXHMmz6uEo3svJsSH49thyGn
    FVcnyaZ++yRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Yc6U="

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).

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 <sip:bob@biloxi.example.org>
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710
   CSeq: 314159 INVITE
   Max-Forwards: 70
   Date: Thu, 21 Feb 2002 13:02:03 GMT
   Contact: <sip:alice@pc33.atlanta.example.com>
   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

The SIP proxy performing the Media Identity authentication service would then insert the following three SIP headers into the message. The Identity-Media header contains the ICE public key attribute and the Identity-Media header contains the signature of all of the relevant SIP headers and of the Identity-Media header (lines are folded for readability):

  Identity-Info: <https://atlanta.example.com/atlanta.cer>
    ;alg=rsa-sha1
  Identity-Media: "a=pub-key"
  Identity-Media-Signature:
   "jjsRdiOPoQZYOy2wrVghuhcsMbHWUSFxI+p6q5TOQXHMmz6uEo3svJsSH49th8qc
    efQBbHC00VMZr2k+t6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB0Ssjcd
    VcunyaZucyRlBYYQTLqWzJ+KVhPKbfU/pryhVn9Jcqe="

10.3.  Request without SDP

This example shows how a SIP request without SDP is signed.

Message as sent by the UAC (lines folded for readability)

   MESSAGE sip:user2@example.com SIP/2.0
   Via: SIP/2.0/TCP user1pc.example.com;branch=z9hG4bK776sgdkse
   Max-Forwards: 70
   From: sip:user1@example.com;tag=49583
   To: sip:user2@example.com
   Call-ID: asd88asd77a@1.2.3.4
   CSeq: 1 MESSAGE
   Content-Type: text/plain
   Content-Length: 18

   Watson, come here.

The authentication service would add the following headers to the above message:

  Identity-Info: <https://atlanta.example.com/atlanta.cer>
    ;alg=rsa-sha1
  Identity-Media:
    BPH="MZr2k+t6VmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxA"
  Identity-Media-Signature:
   "diOPoQZYOy2wrVghuhcsMbHWUSFxI+p6q5TOQXHMmz6uEo3svJsSH49th8qcjjsR
    bHC00VMZr2k+t6efQBVmCvPonWJMGvQTBDqghoWeLxJfzB2a1pxAr3VgrB09JcVc
    unyaZucyRlBYYQTLqWzJ+KVhPKbfU/pryhVnqeSsjcd="

11.  Acknowledgements

The mechanism described in this paper is derived from Jon Peterson and Cullen Jennings' [RFC4474] (Peterson, J. and C. Jennings, “Enhancements for Authenticated Identity Management in the Session Initiation Protocol (SIP),” August 2006.), which was formerly a document of the SIP working group.

Thanks to Hans Persson for his suggestions which improved this document.

12.  IANA Considerations

This document will add new IANA registrations for its new STUN attributes.

[[This section will be completed in a later version of this document.]]

13.  References

13.1. Normative References

13.2. Informational References

Appendix A.  ToDo List

• Add Table-2 of RFC3261
• re-use RFC4474 response code for failures, or invent new ones?
• describe what occurs if both SIP-Identity-Media and SIP-Identity are both used?

Appendix B.  Changes From Previous Versions

B.1.  Changes from 00 to 01

• Removed "Contact" header from signature. SBCs need to change it.
• Removed "Call ID" header from signature. This header often contains an IP address, so many SBCs change it.
• Removed "CSeq" header from signature. This header is sometimes changed by SBCs and B2BUA's.
• include SDP attribute names in Identity-Media signature. This allows any attribute to be signed.
• Old "Identity-Fingerprints" header renamed to "Identity-Media", and only attribute names are listed in it, not attribute values.
• Old "Identity-Media" header renamed to "Identity-Media-Signature".
• Described how to sign SIP requests without an SDP body part, and with a mix of SDP and non-SDP bodyparts.

Authors' Addresses

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