Authenticated Identity Management in the Session Initiation Protocol (SIP)

This document provides enhancements to the existing mechanisms for authenticated identity management in the Session Initiation Protocol (SIP, ). An identity, for the purposes of this document, is defined as either a canonical address-of-record (AoR) SIP URI employed to reach a user (such as 'sip:alice@atlanta.example.com'), or a telephone number, which commonly appears in either a TEL URI or as the user portion of a SIP URI. specifies several places within a SIP request where users can express an identity for themselves, most prominently the user-populated From header field. However, the recipient of a SIP request has no way to verify that the From header field has been populated appropriately, in the absence of some sort of cryptographic authentication mechanism. This leaves SIP vulnerable to a category of abuses, including impersonation attacks that facilitate or enable robocalling, voicemail hacking, swatting, and related problems as described in . Ideally, a cryptographic approach to identity can provide a much stronger and less spoofable assurance of identity than the Caller ID services that the telephone network provides today. encourages user agents (UAs) to implement a number of potential authentication mechanisms, including Digest authentication, Transport Layer Security (TLS), and S/MIME (implementations may support other security schemes as well). However, few SIP user agents today support the end-user certificates necessary to authenticate themselves (via S/MIME, for example), and for its part Digest authentication is limited by the fact that the originator and destination must share a prearranged secret. Practically speaking, originating user agents need to be able to securely communicate their users' identity to destinations with which they have no previous association. As an initial attempt to address this gap, specified a means of signing portions of SIP requests in order to provide an identity assurance. However, RFC4474 was in several ways misaligned with deployment realities (see ). Most significantly, RFC4474 did not deal well with telephone numbers as identifiers, despite their enduring use in SIP deployments. RFC4474 also provided a signature over material that intermediaries in existing deployments commonly altered. This specification therefore deprecates the RFC4474 syntax and behavior, reconsidering the problem space in light of the threat model in and aligning the signature format with PASSporT.

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in RFC 2119. In addition, this document uses three terms specific to the mechanism: Identity: An identifier for the user of a communications service; for the purposes of SIP, either a SIP URI or a telephone number. Identities are derived from an "identity field" a SIP request such as the From header field. Authentication Service: A logical role played by a SIP entity that adds Identity headers to SIP requests. Verification Service (or "Verifier"): A logical role played by a SIP entity that validates Identity headers in a SIP request.

The identity architecture for SIP defined in this specification depends on a logical "authentication service" which validates outgoing requests. An authentication service may be implemented either as part of a user agent or as a proxy server; typically, it is a component of a network intermediary like a proxy to which originating user agents send unsigned requests. Once the originator of the message has been authenticated, through means entirely up to the authentication service, the authentication service then creates and adds an Identity header field to the request. This requires computing cryptographic information, including a digital signature over some components of messages, that lets other SIP entities verify that the sending user has been authenticated and its claim of a particular identity has been authorized. These "verification services" validate the signature and enable policy decisions to be made based on the results of the validation. Policy decisions made after validation depend heavily on the verification service's trust for the credentials that the authentication service uses to sign requests. As robocalling, voicemail hacking, and swatting usually involve impersonation of telephone numbers, credentials that will be trusted by relying parties to sign for telephone numbers are a key component of the architecture. Authority over telephone numbers is however, not so easy to establish on the Internet as authority over traditional domain names. This document assumes the existence of credentials for establishing authority over telephone numbers, for cases where the telephone number is the identity of the user, but this document does not mandate or specify a credential system. describes a credential system compatible with this architecture. Although addressing the vulnerabilities in the STIR problem statement and threat model mostly requires dealing with telephone number as identities, SIP must also handle signing for SIP URIs as identities. This is typically easier to deal with, as these identities are issued to users by authorities over Internet domains. When a new user becomes associated with example.com, for example, the administrator of the SIP service for that domain can issue them an identity in that namespace, such as sip:alice@example.com. Alice may then send REGISTER requests to example.com that make her user agents eligible to receive requests for sip:alice@example.com. In other cases, Alice may herself be the owner of her own domain, and may issue herself identities as she chooses. But ultimately, it is the controller of the SIP service at example.com that must be responsible for authorizing the use of names in the example.com domain. Therefore, for the purposes of baseline SIP, the necessary credentials needed to prove a user is authorized to use a particular From header field must ultimately derive from the domain owner: either a user agent gives requests to the domain name owner in order for them to be signed by the domain owner's credentials, or the user agent must possess credentials that prove in some fashion that the domain owner has given the user agent the right to a name. In order to share a cryptographic assurance of end-user SIP identity in an interdomain or intradomain context, an authentication service constructs tokens based on the PASSporT format, a JSON object comprising values derived from certain header field values in the SIP request. The authentication service computes a signature over those JSON elements as PASSporT specifies. That signature is then placed in the SIP Identity header field. In order to assist in the validation of the Identity header field, this specification also describes a parameter of the Identity header field that can be used by the recipient of a request to recover the credentials of the signer. Note that the scope of this document is limited to providing an identity assurance for SIP requests; solving this problem for SIP responses is outside the scope of this work (see ). Future work might specify ways that a SIP implementation could gateway PASSporT objects to other protocols.

The Identity and Identity-Info header fields that were previously defined in RFC4474 are here deprecated. This revised specification collapses the grammar of Identity-Info into the Identity header field via the "info" parameter. Note that unlike the prior specification in RFC4474, the Identity header field is now allowed to appear more than one time in a SIP request. The revised grammar for the Identity header field builds on the ABNF in RFC 3261 Section 25. It is as follows:

In addition to the "info" parameter, and the "alg" parameter previously defined in RFC4474, this specification defines the optional "canon" and "ppt" parameters. The 'absoluteURI' portion of ident-info-uri MUST contain a URI; see for more on choosing how to advertise credentials through this parameter. The signed-identity-digest is the PASSporT signature component of a PASSporT object, a signature which PASSporT generates over the JSON header and payload objects; some header and claim element values will mirror values of the SIP request. In order to generate that signature, an implementation must construct a complete PASSporT object.

For SIP implementations to populate the PASSporT header JSON object with fields from a SIP request, the following elements message MUST be placed as the values corresponding to the designated JSON keys: First, per baseline , the JSON key "typ" key MUST have the value "passport". Second, the JSON key "alg" MUST mirror the value of the optional "alg" parameter in the SIP Identity header field. Note if the "alg" parameter is absent from the Identity header, the default value is "ES256". Third, the JSON key "x5u" MUST have a value equivalent to the quoted URI in the "info" parameter. Fourth, if a PASSporT extension is in use, then the optional JSON key "ppt" MUST be present and have a value equivalent to the quoted value of the "ppt" parameter of the Identity header field. An example of the PASSporT header JSON object without any extension is:

To populate the PASSporT payload JSON object from a SIP request, the following elements MUST be placed as values corresponding to the designated JSON keys: First, the JSON "orig" array MUST be populated. If the originating identity is a telephone number, then the array MUST be populated with a "tn" claim with a value set to the value of the quoted originating identity, a canonicalized telephone number (see ). Otherwise, the array MUST be populated with a "uri" claim, set to the value of the AoR of the UA sending the message as taken from addr-spec of the From header field, per the procedures in . Second, the JSON "dest" array MUST be populated. If the destination identity is a telephone number, then the array MUST be populated with a "tn" claim with a value set to the value of the quoted destination identity, a canonicalized telephone number (see ). Otherwise, the array MUST be populated with a "uri" claim, set to the value of the addr-spec component of the To header field, which is the AoR to which the request is being sent, per the procedures in . Third, the JSON key "iat" MUST appear, set to the value of a quoted encoding of the value of the SIP Date header field as a JSON NumericDate (as UNIX time, per Section 2). Fourth, if the request contains an SDP message body, and if that SDP contains one or more "a=fingerprint" attributes, then the JSON key "mky" MUST appear with the algorithm(s) and value(s) of the fingerprint attributes (if they differ), following the format given in Section 4.2.2. For example:

For information on the security properties of these SIP message elements, and why their inclusion mitigates replay attacks, see . Note that future extensions to the PASSporT object could introduce new claims, and that further SIP procedures could be required to extract information from the SIP request to populate the values of those claims; see . The "orig" and "dest" arrays may contain identifiers of heterogeneous type; for example, the "orig" array might contain a "tn" claim, while the "dest" contains a "uri" claim. Also note that in some cases, the "orig" and "dest" arrays might be populated with more than one value. This could for example occur when multiple "dest" identities are specified in a meshed conference. Defining how a SIP implementation would provision multiple originating or destination identities is left as a subject for future specification. After these two JSON objects, the header and the paylod, have been constructed and base64-encoded, they must each be hashed per Section 5. The signed value of those concatenated hashes then becomes the signed-identity-string of the Identity header field. The hashing and signing algorithm is specified by the 'alg' parameter of the Identity header field and the mirrored "alg" parameter of PASSporT. This specification inherits from the PASSporT specification one value for the 'alg' parameter: 'ES256', as defined in , which connotes an ECDSA P-256 digital signature. All implementations of this specification MUST support the required signing algorithms of PASSporT. The PASSporT signature that serves as the signed-identity-digest for the SIP Identity header field constitutes only the base64 encoded signed hash, omitting the leading '.' of JWS. The complete form of the Identity header field will therefore look like the following example:

;alg=ES256 ]]>

As Appendix F of the JWS specification notes, there are cases where "it is useful to integrity-protect content that is not itself contained in a JWS." Since the fields that make up the majority of the PASSporT header and payload have values replicated in the SIP request, the SIP usage of PASSporT may exclude the base64 encoded version of the header and payload JSON objects from the Identity header field and instead present a detached signature. Only the signature component of the PASSporT is REQUIRED in SIP, as it forms the contents of the signed-identity-digest field. Optionally, as a debugging measure or optimization, the base64-encoded concatenation of the JSON header and payload MAY be included as the value of a "canon" parameter of the Identity header field. Note however that the use of some future extensions could require "canon" (see ). When the "canon" parameter is present, it MUST contain the base64 encoded header and payload of the PASSporT token per ; following JWS, the header and payload are separated by a single '.'. However, no trailing '.' is included in the "canon": the string consists solely of the base64 encoded JSON header object, followed by a '.', followed by the base64 encoded payload JSON object, as follows:

;alg=ES256;canon= \ "eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cH \ M6Ly9jZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJkZXN0Ijp7 \ InVyaSI6WyJzaXA6YWxpY2VAZXhhbXBsZS5jb20iXX0sImlhdCI6IjE0NDM \ yMDgzNDUiLCJvcmlnIjp7InRuIjoiMTIxNTU1NTEyMTIifX0" ]]> Note that the presence of the "canon" parameter adds considerably to the length of the Identity header field value.

This section provides an informative (non-normative) high-level example of the operation of the mechanisms described in this document. Imagine a case where Bob, who has the home proxy of example.com and the address-of-record sip:12155551212@example.com, wants to communicate with Alice at sip:alice@example.org. They have no prior relationship, and Alice implements best practices to prevent impersonation attacks. Bob's user agent generates an INVITE and places his address-of-record in the From header field of the request. He then sends an INVITE to an authentication service proxy for his domain.

| Verif | . . | Svc | . INVITE . | Svc | . . | Proxy | . . | Proxy | . . > +-------+ . . +-------+ \ . . / | . -> \ . . / | . --. \ . . / | . -- . \ . . / | . -- . \ . . / +-------+. -- . \ . . / | |.<- . \ . . / | Cert |. . > . . +-------+ | Store |. . +-------+ . . | | | |. . | | . . | Bob | +-------+. . | Alice | . . | UA | . . | UA | . . | | . . | | . . +-------+ . . +-------+ . . Domain A . . Domain B . ............................ .............................. ]]> The proxy authenticates Bob, and validates that he is authorized to assert the identity that he populated in the From header field. The proxy authentication service then constructs a PASSporT object which contains a JSON representation of values which mirror certain parts of the SIP request, including the identity in the From header field value. As a part of generating the PASSporT object, the authentication service signs a hash of that JSON header and payload with the private key associated with the appropriate credential for the identity (in this example, a certificate with authority to sign for numbers in a range from 12155551000 to 121555519999), and the signature is inserted by the proxy server into the Identity header field value of the request. Optionally, the JSON header and payload themselves may also be included in the object, encoded in the "canon" parameter of the Identity header field. The proxy authentication service, as the holder of a private key with authority over Bob's telephone number, is asserting that the originator of this request has been authenticated and that he is authorized to claim the identity that appears in the From header field. The proxy inserts an "info" parameter into the Identity header field that tells Alice how to acquire keying material necessary to validate its credentials (a public key), in case she doesn't already have it. When Alice's domain receives the request, a proxy verification service validates the signature provided in the Identity header field, and then determines that the authentication service credentials demonstrate authority over the identity in the From header field. This same validation operation might be performed by a verification service in Alice's user agent server. Ultimately, this valid request is rendered to Alice. If the validation were unsuccessful, some other treatment could be applied by the receiving domain or Alice's user agent.

For the following SIP request:

From: Bob ;tag=1928301774> Call-ID: a84b4c76e66710 CSeq: 314159 INVITE Max-Forwards: 70 Date: Fri, 25 Sep 2015 19:12:25 GMT Contact: Content-Type: application/sdp Content-Length: 147 v=0 o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com s=Session SDP c=IN IP4 pc33.atlanta.example.com t=0 0 m=audio 49172 RTP/AVP 0 a=rtpmap:0 PCMU/8000 ]]> An authentication service will create a corresponding PASSporT object. The properly-serialized PASSporT header and payload JSON objects would look as follows. For the header, the values chosen by the authentication service at "example.org" might read:

The serialized payload will derive values from the SIP request (the From, To, and Date header field values) as follows:

The authentication service would then generate the signature over the object following the procedures in Section 5. That signature would look as follows:

An authentication service signing this request would thus generate and add to the request an Identity header field of the following form:

]]>

SIP entities that instantiate the authentication service and verification service roles will, respectively, generate and validate the Identity header and the signature it contains.

Any entity that instantiates the authentication service role MUST possess the private key of one or more credentials that can be used to sign for a domain or a telephone number (see ). The authentication service role can be instantiated, for example, by an intermediary such as a proxy server or by a user agent. Intermediaries that instantiate this role MUST be capable of authenticating one or more SIP users who can register for that identity. Commonly, this role will be instantiated by a proxy server, since proxy servers are more likely to have a static hostname, hold corresponding credentials, and have access to SIP registrar capabilities that allow them to authenticate users. It is also possible that the authentication service role might be instantiated by an entity that acts as a redirect server, but that is left as a topic for future work. An authentication service adds the Identity header field to SIP requests. The procedures below define the steps that must be taken when each Identity header field is added. More than one Identity header field may appear in a single request, and an authentication service may add an Identity header field to a request that already contains one or more Identity header fields. Entities instantiating the authentication service role perform the following steps, in order, to generate an Identity header field for a SIP request: Step 1: Check Authority for the Identity First, the authentication service must determine whether it is authoritative for the identity of the originator of the request. The authentication service extracts the identity from the URI value from the "identity field"; in ordinary operations, that is the addr-spec component of From header field. In order to determine whether the signature for the identity field should be over the entire identity field URI or just a telephone number, the authentication service MUST follow the process described in . That section will either lead to the telephone number canonicalization procedures in for telephone numbers, or to the URI normalization procedures described in for domain names. Whichever the result, if the authentication service is not authoritative for the identity in question, it SHOULD process and forward the request normally unless the local policy is to block such requests. The authentication service MUST NOT add an Identity header field if the authentication service does not have the authority to make the claim it asserts. Step 2: Authenticate the Originator The authentication service MUST then determine whether or not the originator of the request is authorized to claim the identity given in the identity field. In order to do so, the authentication service MUST authenticate the originator of the message. Some possible ways in which this authentication might be performed include: If the authentication service is instantiated by a SIP intermediary (proxy server), it may authenticate the request with the authentication scheme used for registration in its domain (e.g., Digest authentication). If the authentication service is instantiated by a SIP user agent, a user agent may authenticate its own user through any system-specific means, perhaps simply by virtue of having physical access to the user agent. Authorization of the use of a particular username or telephone number in the user part of the From header field is a matter of local policy for the authentication service; see for more information. Note that this check is performed only on the addr-spec in the identity field (e.g., the URI of the originator, like 'sip:alice@atlanta.example.com'); it does not cover the display-name portion of the From header field (e.g., 'Alice Atlanta'). For more information, see . Step 3: Verify Date is Present and Valid An authentication service MUST add a Date header field to SIP requests that do not have one. The authentication service MUST ensure that any preexisting Date header field in the request is accurate. Local policy can dictate precisely how accurate the Date must be; a RECOMMENDED maximum discrepancy of sixty seconds will ensure that the request is unlikely to upset any verifiers. If the Date header field value contains a time different by more than one minute from the current time noted by the authentication service, the authentication service SHOULD reject the request. This behavior is not mandatory because a user agent client (UAC) could only exploit the Date header field in order to cause a request to fail verification; the Identity header field is not intended to provide a perfect record of when messages are processed. Finally, the authentication service MUST verify that both the Date header field and the current time fall within the validity period of its credential. See for information on how the Date header field assists verifiers. Step 4: Populate and Add the Identity Header Subsequently, the authentication service MUST form a PASSporT object and add a corresponding Identity header field to the request containing this signature. For the baseline PASSporT header (headers containing no "ppt" parameter), this follows the procedures in ; if the authentication service is using an alternative "ppt" format, it MUST add an appropriate "ppt" parameter and follow the procedures associated with that extension (see ). After the Identity header field has been added to the request, the authentication service MUST also add a "info" parameter to the Identity header field. The "info" parameter contains a URI from which the authentication service's credential can be acquired; see for more on credential acquisition. Step 5: Add "canon", if Needed An authentication service MAY add a "canon" parameter to the Identity header field. The presence of "canon" is OPTIONAL because the information carried in the baseline PASSporT object's headers and claims is usually redundant with information already carried elsewhere in the SIP request. Omitting "canon" can significantly reduce SIP message size, especially when the PASSporT object contains media keys. The syntax of "canon" is given in ; essentially, it contains a base64 encoding of the JSON header and payload in the PASSporT object. When however an authentication service creates a PASSporT object that uses extension claims beyond the baseline PASSporT object, including "canon" is REQUIRED in order for the verification service to be capable of validating the signature. See . Also, in some cases, a request signed by an authentication service will be rejected by the verification service on the receiving side, and the authentication service will receive a SIP 4xx status code in the backwards direction, such as a 438 indicating a verification failure. If the authentication service did not originally send the Identity header field with the "canon" parameter, it SHOULD retry a request once after receiving a 438 response, this time including the "canon". The information in "canon" is useful on the verification side for debugging errors, and there are some known causes of verification failures (such as the Date header field value changing in transit, see for more information) that can be resolved by the inclusion of "canon". Finally, the authentication service forwards the message normally.

This document specifies a logical role for SIP entities called a verification service, or verifier. When a verifier receives a SIP message containing one or more Identity header fields, it inspects the signature(s) to verify the identity of the originator of the message. The results of a verification are provided as input to an authorization process that is outside the scope of this document. A SIP request may contain zero, one, or more Identity header fields. A verification service performs the steps below on each Identity header field that appears in a request. If the verifier does not support an Identity header field "ppt" parameter which is present, or if no Identity header field is present at all, and the presence of an Identity header field is required by local policy (for example, based on a per-sending-domain policy, or a per-sending-user policy), then a 428 'Use Identity Header' response MUST be sent in the backwards direction. For more on this and other verifier responses, see . In order to verify an Identity header field in a message, an entity acting as a verifier MUST perform the following steps, in the order here specified. Note that when an Identity header field contains the optional "canon" parameter, the verifier MUST follow the additional procedures in . Step 1: Check for an Unsupported "ppt" The verifier MUST inspect any optional "ppt" parameter appearing in the Identity request. If no "ppt" parameter is present, then the verifier proceeds normally below. If a "ppt" parameter value is present, and the verifier does not support it, it MUST ignore the Identity header field. If a supported "ppt" parameter value is present, the verifier proceeds with Step 2, and will ultimately follow the "ppt" variations described in Step 5. Step 2: Determine the Originator's Identity In order to determine whether the signature for the identity field should be over the entire identity field URI or just a telephone number, the verification service MUST follow the process described in . That section will either lead to the telephone number canonicalization procedures in for telephone numbers, or to the URI normalization procedures described in for domain names. Step 3: Identify Credential for Validation The verifier must ensure that it possesses the proper keying material to validate the signature in the Identity header field, which usually involves dereferencing a URI in the "info" parameter of the Identity header field. See for more information on these procedures. If the verifier does not support the credential described in the "info" parameter, then it treats the credential for this header field as unsupported. Step 4: Check the Freshness of Date The verifier furthermore ensures that the value of the Date header field of the request meets local policy for freshness (sixty seconds is RECOMMENDED) and that it falls within the validity period of the credential used to sign the Identity header field. For more on the attacks this prevents, see . If the "canon" parameter is present, the verifier SHOULD compare the "iat" value in the "canon" to the Date header field value in the request. If the two are different, and the "iat" value is later but within verification service policy for freshness, the verification service SHOULD perform the computation required by Step 5 using the "iat" value instead of the Date header field value. Step 5: Validate the Signature The verifier MUST validate the signature in the Identity header field over the PASSporT object. For baseline PASSporT objects (with no Identity header field "ppt" parameter) the verifier MUST follow the procedures for generating the signature over a PASSporT object described in . If a "ppt" parameter is present (and per Step 1, is supported), the verifier follows the procedures for that "ppt" (see ). If a verifier determines that the that the signature in the Identity does not correspond to the reconstructed signed-identity-digest, then the Identity header field should be considered invalid.

The verification of an Identity header field does not entail any particular treatment of the request. The handling of the message after the verification process depends on how the verification service is implemented and on local policy. This specification does not propose any authorization policy for user agents or proxy servers to follow based on the presence of a valid Identity header field, the presence of an invalid Identity header field, or the absence of an Identity header field, or a stale Date header field value, but it is anticipated that local policies could involve making different forwarding decisions in intermediary implementations, or changing how the user is alerted, or how identity is rendered, in user agent implementations. The presence of multiple Identity header fields within a message raises the prospect that a verification services could receive a message containing some valid and some invalid Identity header fields. As a guideline, this specification recommends that only if a verifier determines all Identity header fields within a message are invalid should the request be considered to have an invalid identity.

RFC4474 originally defined four response codes for failure conditions specific to the Identity header field and its original mechanism. These status codes are retained in this specification, with some slight modifications. Also, this specification details responding with 403 when a stale Date header field value is received. A 428 response will be sent (per ) when an Identity header field is required, but no Identity header field without a "ppt" parameter, or with a supported "ppt" value, has been received. In the case where one or more Identity header fields with unsupported "ppt" values have been received, then a verification service may send a 428 with the special reason phrase "Use Supported PASSporT Format". Note however that this specification gives no guidance on how a verification service might decide to require an Identity header field for a particular SIP request. Such authorization policies are outside the scope of this specification. The 436 'Bad Identity Info' response code indicates an inability to acquire the credentials needed by the verification service for validating the signature in an Identity header field. Again, given the potential presence of multiple Identity header fields, this response code should only be sent when the verification service is unable to deference the URIs and/or acquire the credentials associated with all Identity header fields in the request. This failure code could be repairable if the authentication service resends the request with an 'info' parameter pointing to a credential that the verification service can access. The 437 'Unsupported Credential' is sent when a verification service can acquire, or already holds, the credential represented by the 'info' parameter of at least one Identity header field in the request, but does not support said credential(s), for reasons such as failing to trust the issuing CA, or failing to support the algorithm with which the credential was signed. The 438 'Invalid Identity Header' response indicates that of the set of Identity header fields in a request, no header field with a valid and supported PASSporT object has been received. Like the 428 response, this is sent by a verification service when its local policy dictates that a broken signature in an Identity header field is grounds for rejecting a request. Note that in some cases, an Identity header field may be broken for other reasons than that an originator is attempting to spoof an identity: for example, when a transit network alters the Date header field of the request. Relying on the full PASSporT object presented through the "canon" parameter can repair some of these conditions (see ), so the recommended way to attempt to repair this failure is to retry the request with "canon". Finally, a 403 response with the special reason phase 'Stale Date" may be sent when the verification service receives a request with a Date header field value that is older than the local policy for freshness permits. The same response may be used when the "iat" in the "canon" parameter of a request has a value older than the local policy for freshness permits.

If the optional "canon" parameter of the Identity header field is present, it contains a base64 encoding of the header and claim component of the PASSporT object constructed by the authentication service (as detailed in ). The verification service can thus extract from it the canonical telephone number created by the authentication service, as well as an "iat" claim corresponding to the Date header field that the authentication service used. These may be used to debug canonicalization problems, or to avoid unnecessary signature breakage caused by intermediaries that alter the Date header field value in transit. As an optimization, when "canon" is present, the verification service MAY compute its own canonicalization of an originating telephone number and compare it to the values in the "canon" parameter before performing any cryptographic functions in order to ascertain whether or not the two ends agree on the canonical number form.

This section gives general guidance on the use of credential systems by authentication and verification services, as well as requirements that must be met by credential systems that conform with this architecture. It does not mandate any specific credential system. Furthermore, this specification allows either a user agent or a proxy server to provide the authentication service function and/or the verification service function. For the purposes of end-to-end security, it is obviously preferable for end systems to acquire their own credentials; in this case user agents can act as authentication services. However, for some deployments, end-user credentials may be neither practical nor affordable, given the potentially large number of SIP user agents (phones, PCs, laptops, PDAs, gaming devices) that may be employed by a single user. Synchronizing keying material across multiple devices may be prohibitively complex and require quite a good deal of additional endpoint behavior. Managing several credentials for the various devices could also be burdensome. Thus, for reasons of credential management alone, implementing the authentication service at an intermediary may be more practical. This trade-off needs to be understood by implementers of this specification.

In order to act as an authentication service, a SIP entity must have access to the private keying material of one or more credentials that cover domain names or telephone numbers. These credentials may represent authority over one domain (such as example.com) or a set of domains enumerated by the credential. Similarly, a credential may represent authority over a single telephone number or a range of telephone numbers. The way that the scope of a credential's authority is expressed is specific to the credential mechanism. Authorization of the use of a particular username or telephone number in the From header field value is a matter of local policy for the authentication service, one that depends greatly on the manner in which authentication is performed. For non-telephone number user parts, one policy might be as follows: the username given in the 'username' parameter of the Proxy-Authorization header field MUST correspond exactly to the username in the From header field of the SIP message. However, there are many cases in which this is too limiting or inappropriate; a realm might use 'username' parameters in Proxy-Authorization header field that do not correspond to the user-portion of From header fields, or a user might manage multiple accounts in the same administrative domain. In this latter case, a domain might maintain a mapping between the values in the 'username' parameter of the Proxy-Authorization header field and a set of one or more SIP URIs that might legitimately be asserted for that 'username'. For example, the username can correspond to the 'private identity' as defined in Third Generation Partnership Project (3GPP), in which case the From header field can contain any one of the public identities associated with this private identity. In this instance, another policy might be as follows: the URI in the From header field MUST correspond exactly to one of the mapped URIs associated with the 'username' given in the Proxy-Authorization header field. This is a suitable approach for telephone numbers in particular. This specification could also be used with credentials that cover a single name or URI, such as alice@example.com or sip:alice@example.com. This would require a modification to authentication service behavior to operate on a whole URI rather than a domain name. Because this is not believed to be a pressing use case, this is deferred to future work, but implementers should note this as a possible future direction. Exceptions to such authentication service policies arise for cases like anonymity; if the AoR asserted in the From header field uses a form like 'sip:anonymous@example.com' (see ), then the 'example.com' proxy might authenticate only that the user is a valid user in the domain and insert the signature over the From header field as usual.

In order to act as a verification service, a SIP entity must have a way to acquire and retain credentials for authorities over particular domain names, telephone numbers and/or number ranges. Dereferencing the URI found in the "info" parameter of the Identity header field (as described ) MUST be supported by all verification service implementations to create a baseline means of credential acquisition. Provided that the credential used to sign a message is not previously known to the verifier, SIP entities SHOULD discover this credential by dereferencing the "info" parameter, unless they have some implementation-specific way of acquiring the needed keying material, such as an offline store of periodically-updated credentials. The 436 'Bad Identity Info' response exists for cases where the verification service cannot deference the URI in the "info" parameter. This specification does not propose any particular policy for a verification service to determine whether or not the holder of a credential is the appropriate party to sign for a given SIP identity. Guidance on this is deferred to credential mechanism specifications. Verification service implementations supporting this specification may wish to have some means of retaining credentials (in accordance with normal practices for credential lifetimes and revocation) in order to prevent themselves from needlessly downloading the same credential every time a request from the same identity is received. Credentials cached in this manner may be indexed in accordance with local policy: for example, by their scope of authority, or the URI given in the "info" parameter value. Further consideration of how to cache credentials is deferred to the credential mechanism specifications.

An "info" parameter MUST contain a URI which dereferences to a resource that contains the public key components of the credential used by the authentication service to sign a request. It is essential that a URI in the "info" parameter be dereferencable by any entity that could plausibly receive the request. For common cases, this means that the URI SHOULD be dereferencable by any entity on the public Internet. In constrained deployment environments, a service private to the environment MAY be used instead. Beyond providing a means of accessing credentials for an identity, the "info" parameter further serves as a means of differentiating which particular credential was used to sign a request, when there are potentially multiple authorities eligible to sign. For example, imagine a case where a domain implements the authentication service role for a range of telephone numbers and a user agent belonging to Alice has acquired a credential for a single telephone number within that range. Either would be eligible to sign a SIP request for the number in question. Verification services however need a means to differentiate which one performed the signature. The "info" parameter performs that function.

This document makes no recommendation for the use of any specific credential system. Today, there are two primary credential systems in place for proving ownership of domain names: certificates (e.g., X.509 v3, see ) and the domain name system itself (e.g., DANE, see ). It is envisioned that either could be used in the SIP identity context: an "info" parameter could for example give an HTTP URL of the Content-Type 'application/pkix-cert' pointing to a certificate (following the conventions of ). The "info" parameter might use the DNS URL scheme (see ) to designate keys in the DNS. While no comparable public credentials exist for telephone numbers, either approach could be applied to telephone numbers. A credential system based on certificates is given in , but this specification can work with other credential systems; for example, using the DNS was proposed in . In order for a credential system to work with this mechanism, its specification must detail: which URIs schemes the credential will use in the "info" parameter, and any special procedures required to dereference the URIs how the verifier can learn the scope of the credential any special procedures required to extract keying material from the resources designated by the URI any algorithms required to validate the credentials (e.g. for certificates, any algorithms used by certificate authorities to sign certificates themselves), and how the associated credentials will support the mandatory signing algorithm(s) required by PASSporT. SIP entities cannot reliably predict where SIP requests will terminate. When choosing a credential scheme for deployments of this specification, it is therefore essential that the trust anchor(s) for credentials be widely trusted, or that deployments restrict the use of this mechanism to environments where the reliance on particular trust anchors is assured by business arrangements or similar constraints. Note that credential systems must address key lifecycle management concerns: were a domain to change the credential available at the Identity header field "info" parameter URI before a verifier evaluates a request signed by an authentication service, this would cause obvious verifier failures. When a rollover occurs, authentication services SHOULD thus provide new "info" URIs for each new credential, and SHOULD continue to make older key acquisition URIs available for a duration longer than the plausible lifetime of a SIP transaction (a minute would most likely suffice).

The problem statement of STIR focuses primarily on cases where the called and calling parties identified in the To and From header field values use telephone numbers, as this remains the dominant use case in the deployment of SIP. However, the Identity header mechanism also works with SIP URIs without telephone numbers (of the form "sip:user@host"), and potentially other identifiers when SIP interworks with other protocols. Authentication services vet the identity of the originator of a call, which is typically found in the From header field value. The guidance in this specification also applies to extracting the URI containing the originator's identity from the P-Asserted-Identity header field value instead of the From header field value. In some trusted environments, the P-Asserted-Identity header field is used in lieu of the From header field to convey the address-of-record or telephone number of the originator of a request; where it does, local policy might therefore dictate that the canonical identity derives from the P-Asserted-Identity header field rather than the From header field. Ultimately, in any case where local policy canonicalizes the identity into a form different from how it appears in the From header field, the use of the "canon" parameter by authentication services is RECOMMENDED, but because "canon" itself could then divulge information about users or networks, implementers should be mindful of the guidelines in .

It may not be trivial to tell if a given URI contains a telephone number. In order to determine whether or not the user portion of a SIP URI is a telephone number, authentication services and verification services MUST perform the following procedure on any SIP URI they inspect which contains a numeric user part. Note that the same procedures are followed for creating the canonical form of URIs found in the From header field as they are in the To header field or the P-Asserted-Identity header field. First, implementations must look for obvious indications that the user-portion of the URI constitutes a telephone number. Telephone numbers most commonly appear in SIP header field values in the username portion of a SIP URI (e.g., 'sip:+17005551008@chicago.example.com;user=phone'). The user part of that URI conforms to the syntax of the TEL URI scheme (RFC 3966). It is also possible for a TEL URI to appear in the SIP To or From header field outside the context of a SIP or SIPS URI (e.g., 'tel:+17005551008'). Thus, in some environments, numbers will be explicitly labeled by the use of TEL URIs or the 'user=phone' parameter, or implicitly by the presence of the '+' indicator at the start of the user-portion. Absent these indications, if there are numbers present in the user-portion, implementations may also detect that the user-portion of the URI contains a telephone number by determining whether or not those numbers would be dialable or routable in the local environment -- bearing in mind that the telephone number may be a valid number, a nationally-specific number, or even a private branch exchange number. Once a telephone number has been detected, implementations should follow the procedures in . If the URI field does not contain a telephone number, or if the result of the canonicalization of the From header field value does not form a valid E.164 telephone number, the authentication service and/or verification service SHOULD treat the entire URI as a SIP URI, and apply the procedures in . These URI normalization procedures are invoked to canonicalize the URI before it is included in a PASSporT object in, for example, an "uri" claim. See for that behavior.

In order for telephone numbers to be used with the mechanism described in this document, authentication services must receive credentials from an authority for telephone numbers or telephone number ranges, and verification services must trust the authority employed by the authentication service that signs a request. Per , enrollment procedures and credential management are outside the scope of this document; approaches to credential management for telephone numbers are discussed in .

Once an implementation has identified a telephone number, it must construct a number string. That requires performing the following steps: Implementations MUST drop any "+"s, any internal dashes, parentheses or other non-numeric characters, excepting only the leading "#" or "*" keys used in some special service numbers (typically, these will appear only in the To header field value). This MUST result in an ASCII string limited to "#", "*" and digits without whitespace or visual separators. Next, an implementation must assess if the number string is a valid, globally-routable number with a leading country code. If not, implementations SHOULD convert the number into E.164 format, adding a country code if necessary; this may involve transforming the number from a dial string (see ), removing any national or international dialing prefixes or performing similar procedures. It is only in the case that an implementation cannot determine how to convert the number to a globally-routable format that this step may be skipped. This will be the case, for example, for nationally-specific service numbers (e.g. 911, 112); however, the routing procedures associated with those numbers will likely make sure that the verification service understands the context of their use. Other transformations during canonicalization MAY be made in accordance with specific policies used within a local domain. For example, one domain may only use local number formatting and need to convert all To/From header field user portions to E.164 by prepending country-code and region code digits; another domain might have prefixed usernames with trunk-routing codes, in which case the canonicalization will need to remove the prefix. This specification cannot anticipate all of the potential transformations that might be useful. The resulting canonical number string will be used as input to the hash calculation during signing and verifying processes. The ABNF of this number string is:

The resulting number string is used in the construction of the telephone number field(s) in a PASSporT object.

To use a SIP URI as an identity in this mechanism requires authentication and verification systems to support standard mechanisms for proving authority over a domain name: that is, the domain name in the host portion of the SIP URI. A verifier MUST evaluate the correspondence between the user's identity and the signing credential by following the procedures defined in , Section 7.2. While deals with the use of TLS and is specific to certificates, the procedures described are applicable to verifying identity if one substitutes the "hostname of the server" for the domain portion of the user's identity in the From header field of a SIP request with an Identity header field. This process is complicated by two deployment realities. In the first place, credentials have varying ways of describing their subjects, and may indeed have multiple subjects, especially in 'virtual hosting' cases where multiple domains are managed by a single application (see Section 7.8). Secondly, some SIP services may delegate SIP functions to a subordinate domain and utilize the procedures in that allow requests for, say, 'example.com' to be routed to 'sip.example.com'. As a result, a user with the AoR 'sip:alice@example.com' may process requests through a host like 'sip.example.com', and it may be that latter host that acts as an authentication service. To address the second of these problems, a domain that deploys an authentication service on a subordinate host MUST be willing to supply that host with the private keying material associated with a credential whose subject is a domain name that corresponds to the domain portion of the AoRs that the domain distributes to users. Note that this corresponds to the comparable case of routing inbound SIP requests to a domain. When the NAPTR and SRV procedures of RFC 3263 are used to direct requests to a domain name other than the domain in the original Request-URI (e.g., for 'sip:alice@example.com', the corresponding SRV records point to the service 'sip1.example.org'), the client expects that the certificate passed back in any TLS exchange with that host will correspond exactly with the domain of the original Request-URI, not the domain name of the host. Consequently, in order to make inbound routing to such SIP services work, a domain administrator must similarly be willing to share the domain's private key with the service. This design decision was made to compensate for the insecurity of the DNS, and it makes certain potential approaches to DNS-based 'virtual hosting' unsecurable for SIP in environments where domain administrators are unwilling to share keys with hosting services.

Just as telephone numbers may undergo a number of syntactic transformations during transit, the same can happen to SIP and SIPS URIs without telephone numbers as they traverse certain intermediaries. Therefore, when generating a PASSporT object based on a SIP request, any SIP and SIPS URIs must be transformed into a canonical form which captures the address-of-record represented by the URI before they are provisioned in PASSporT claims such as "uri". The URI normalization procedures required are as follows. Following the ABNF of RFC3261, the SIP or SIPS URI in question MUST discard all elements after the "hostport" of the URI, including all uri-parameters and escaped headers, from its syntax. Of the userinfo component of the SIP URI, only the user element will be retained: any password (and any leading ":" before the password) MUST be removed, and since this userinfo necessarily does not contain a telephone-subscriber component, no further parameters can appear in the user portion. The hostport portion of the SIP or SIPS URI MUST similarly be stripped of any trailing port along with the ":" that proceeds the port, leaving only the host. The ABNF of this canonical URI form (following the syntax defined in RFC3261) is:

Finally, the URI will be subject to syntax-based URI normalization procedures of Section 6.2.2. Implementations MUST perform case normalization (rendering the scheme, user, and host all lowercase) and percent-encoding normalization (decoding any percent-encoded octet that corresponds to an unreserved character, per Section 2.3). However, note that normalization procedures face known challenges in some internationalized environments (see ) and that perfect normalization of URIs may not be possible in those environments. For future PASSporT applications, it may be desirable to provide an identifier without an attached protocol scheme. Future specifications that define PASSporT claims for SIP as a using protocol could use these basic procedures, but eliminate the scheme component. A more exact definition is left to future specifications.

As future requirements may warrant increasing the scope of the Identity mechanism, this specification specifies an optional "ppt" parameter of the Identity header field, which mirrors the "ppt" header in PASSporT. The "ppt" parameter value MUST consist of a token containing an extension specification, which denotes an extended set of one or more signed claims per the type extensibility mechanism specified in Section 6. The potential for extensions is one the primary motivations for allowing the presence of multiple Identity header fields in the same SIP request. It is envisioned that future extensions might allow for alternate information to be signed, or to explicitly allow different parties to provide the signatures than the authorities envisioned by baseline STIR. A request might, for example, have one Identity added by an authentication service at the originating administrative domain, and then another Identity header field added by some further intermediary using a PASSporT extension. While this specification does not define any such specific purpose for multiple Identity header fields, implementations MUST support receiving multiple header fields for future compatibility reasons. An authentication service cannot assume that verifiers will understand any given extension. Verifiers that do support an extension may then trigger appropriate application-level behavior in the presence of an extension; authors of extensions should provide appropriate extension-specific guidance to application developers on this point. If any claim in an extension contains a JSON value that does not correspond to a field of the SIP request, and the extension does not otherwise explain how a verification service could derive or acquire that value, then the optional "canon" parameter MUST be used for the Identity header field containing that extension.

This specification introduces several significant changes from the RFC4474 version of the Identity header field. However, due to the problems enumerated in , it is not believed that the original Identity header field has seen any deployment, or even implementation in deployed products. As such, this mechanism contains no provisions for signatures generated with this specification to work with RFC4474-compliant implementations, nor any related backwards-compatibility provisions. Hypothetically, were an RFC4474-compliant implementation to receive messages containing this revised version of the Identity header field, it would likely fail the request due to the absence of an Identity-Info header field with a 436 response code. Implementations of this specification, for debugging purposes, might interpret a 436 with a reason phrase of "Bad Identity-Info" as an indication that the request has failed because it reached a (hypothetical) RFC4474-compliant verification service.

The purpose of this mechanism is to provide a reliable identification of the originator of a SIP request, specifically a cryptographic assurance that an authority asserts the originator can claim the URI the identity stipulated in the request. This URI may contain or imply a variety of personally identifying information, including the name of a human being, their place of work or service provider, and possibly further details. The intrinsic privacy risks associated with that URI are, however, no different from those of baseline SIP. Per the guidance in , implementers should make users aware of the privacy trade-off of providing secure identity. The identity mechanism presented in this document is compatible with the standard SIP practices for privacy described in . A SIP proxy server can act both as a RFC3323 privacy service and as an authentication service. Since a user agent can provide any From header field value that the authentication service is willing to authorize, there is no reason why private SIP URIs that contain legitimate domains (e.g., sip:anonymous@example.com) cannot be signed by an authentication service. The construction of the Identity header field is the same for private URIs as it is for any other sort of URIs. Similar practices could be used to support opportunistic signing of SIP requests for UA-integrated authentication services with self-signed certificates, though that is outside the scope of this specification and is left as a matter for future investigation. Note, however, that even when using anonymous SIP URIs, an authentication service must possess a certificate corresponding to the host portion of the addr-spec of the From header field value of the request; accordingly, using domains like 'anonymous.invalid' will not be usable by privacy services that simultaneously act as authentication services. The assurance offered by the usage of anonymous URIs with a valid domain portion is "this is a known user in my domain that I have authenticated, but I am keeping its identity private". It is worth noting two features of this more anonymous form of identity. One can eliminate any identifying information in a domain through the use of the domain 'anonymous.invalid," but we must then acknowledge that it is difficult for a domain to be both anonymous and authenticated. The use of the "anonymous.invalid" domain entails that no corresponding authority for the domain can exist, and as a consequence, authentication service functions for that domain are meaningless. The second feature is more germane to the threats this document mitigates . None of the relevant attacks, all of which rely on the attacker taking on the identity of a victim or hiding their identity using someone else's identity, are enabled by an anonymous identity. As such, the inability to assert an authority over an anonymous domain is irrelevant to our threat model. defines the "id" priv-value token, which is specific to the P-Asserted-Identity header field. The sort of assertion provided by the P-Asserted-Identity header field is very different from the Identity header field presented in this document. It contains additional information about the originator of a message that may go beyond what appears in the From header field; P-Asserted-Identity holds a definitive identity for the originator that is somehow known to a closed network of intermediaries. Presumably, that network will use this identity for billing or security purposes. The danger of this network-specific information leaking outside of the closed network motivated the "id" priv-value token. The "id" priv-value token has no implications for the Identity header field, and privacy services MUST NOT remove the Identity header field when a priv-value of "id" appears in a Privacy header field. The optional "canon" parameter of the Identity header field specified in this document provides the complete JSON objects used to generate the signed-identity-digest of the Identity header field value, including the canonicalized form of the telephone number of the originator of a call, if the signature is over a telephone number. In some contexts, local policy may require a canonicalization which differs substantially from the original From header field. Depending on those policies, potentially the "canon" parameter might divulge information about the originating network or user that might not appear elsewhere in the SIP request. Were it to be used to reflect the contents of the P-Asserted-Identity header field, for example, then "canon" would need to be removed when the P-Asserted-Identity header is removed to avoid any such leakage outside of a trust domain. Since, in those contexts, the canonical form of the originator's identity could not be reassembled by a verifier, and thus the Identity signature validation process would fail, using P-Asserted-Identity with the Identity "canon" parameter in this fashion is NOT RECOMMENDED outside of environments where SIP requests will never leave the trust domain. As a side note, history shows that closed networks never stay closed and one should design their implementation assuming connectivity to the broader Internet. Finally, note that unlike , the mechanism described in this specification adds no information to SIP requests that has privacy implications - apart from disclosing that an authentication service is willing to sign for an originator.

This document describes a mechanism that provides a signature over the Date header field of SIP requests, parts of the To and From header fields, and when present any media keying material in the message body. In general, the considerations related to the security of these header fields are the same as those given in for including header fields in tunneled 'message/sip' MIME bodies (see Section 23 of RFC3261 in particular). The following section details the individual security properties obtained by including each of these header fields within the signature; collectively, this set of header fields provides the necessary properties to prevent impersonation. It addresses the solution-specific attacks against in-band solutions enumerated in Section 4.1.

The From header field value (in ordinary operations) indicates the identity of the originator of the message. The SIP address-of-record URI, or an embedded telephone number, in the From header field is the identity of a SIP user, for the purposes of this document. Note that in some deployments the identity of the originator may reside in P-Asserted-Id instead. The originator's identity is the key piece of information that this mechanism secures; the remainder of the signed parts of a SIP request are present to provide reference integrity and to prevent certain types of cut-and-paste attacks. The Date header field value protects against cut-and-paste attacks, as described in , Section 23.4.2. That specification recommends that implementations notify the user of a potential security issue if the signed Date header field value is stale by an hour or more. To prevent cut-and-paste of recently-observed messages, this specification instead RECOMMENDS a shorter interval of sixty seconds. Implementations of this specification MUST NOT deem valid a request with an outdated Date header field. Note that per Section 10 behavior, servers can keep state of recently received requests, and thus if an Identity header field is replayed by an attacker within the Date interval, verifiers can detect that it is spoofed because a message with an identical Date from the same source had recently been received. It has been observed in the wild that some networks change the Date header field value of SIP requests in transit, and that alternative behavior might be necessary to accommodate that use case. Verification services that observe a signature validation failure MAY therefore reconstruct the Date header field component of the signature from the "iat" carried in PASSporT via the "canon" parameter: provided that time recorded by "iat" falls within the local policy for freshness that would ordinarily apply to the Date header, the verification service MAY treat the signature as valid, provided it keeps adequate state to detect recent replays. Note that this will require the inclusion of the "canon" parameter by authentication services in networks where such failures are observed. The To header field value provides the identity of the SIP user that this request originally targeted. Covering the identity in the To header field with the Identity signature serves two purposes. First, it prevents cut-and-paste attacks in which an Identity header field from a legitimate request for one user is cut-and-pasted into a request for a different user. Second, it preserves the starting URI scheme of the request, which helps prevent downgrade attacks against the use of SIPS. The To identity offers additional protection against cut-and-paste attacks beyond the Date header field. For example, without a signature over the To identity, an attacker who receives a call from a target could immediately cut-and-paste the Identity and From header field value from that INVITE into a new request to the target's voicemail service within the Date interval, and the voicemail service would have no way knowing that the Identity header field it received had been originally signed for a call intended for a different number. However, note the caveats below in . When signing a request that contains a fingerprint of keying material in SDP for DTLS-SRTP, this mechanism always provides a signature over that fingerprint. This signature prevents certain classes of impersonation attacks in which an attacker forwards or cut-and-pastes a legitimate request. Although the target of the attack may accept the request, the attacker will be unable to exchange media with the target as they will not possess a key corresponding to the fingerprint. For example, there are some baiting attacks, launched with the REFER method or through social engineering, where the attacker receives a request from the target and reoriginates it to a third party. These might not be prevented by only a signature over the From, To and Date, but could be prevented by securing a fingerprint for DTLS-SRTP. While this is a different form of impersonation than is commonly used for robocalling, ultimately 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 contents of the subsequent media communication. This signature also, per , reduces the potential for passive monitoring attacks against the SIP media. In environments where DTLS-SRTP is unsupported, however, no field is signed and no protections are provided.

Armed with the original value of the To header field, the recipient of a request may be tempted compare it to their own identity in order to determine whether or not the identity information in this call might have been replayed. However, any request may be legitimately retargeted as well, and as a result legitimate requests may reach a SIP endpoint whose user is not identified by the URI designated in the To header field value. It is therefore difficult for any verifier to decide whether or not some prior retargeting was "legitimate." Retargeting can also cause confusion when identity information is provided for requests sent in the backwards direction in a dialog, as the dialog identifiers may not match credentials held by the ultimate target of the dialog. For further information on the problems of response identity see . Any means for authentication services or verifiers to anticipate retargeting is outside the scope of this document, and likely to have equal applicability to response identity as it does to requests in the backwards direction within a dialog. Consequently, no special guidance is given for implementers here regarding the 'connected party' problem (see ); authentication service behavior is unchanged if retargeting has occurred for a dialog-forming request. Ultimately, the authentication service provides an Identity header field for requests in the dialog only when the user is authorized to assert the identity given in the From header field, and if they are not, an Identity header field is not provided. And per the threat model of , resolving problems with 'connected' identity has little bearing on detecting robocalling or related impersonation attacks.

RFC4474 originally had protections for the Contact, Call-ID and CSeq. These are removed from RFC4474bis. The absence of these header field values creates some opportunities for determined attackers to impersonate based on cut-and-paste attacks; however, the absence of these header field values does not seem impactful to preventing the simple unauthorized claiming of an identity for the purposes of robocalling, voicemail hacking, or swatting, which is the primary scope of the current document. It might seem attractive to provide a signature over some of the information present in the Via header field value(s). For example, without a signature over the sent-by field of the topmost Via header field, an attacker could remove that Via header field and insert its own in a cut-and-paste attack, which would cause all responses to the request to be routed to a host of the attacker's choosing. However, a signature over the topmost Via header field does not prevent attacks of this nature, since the attacker could leave the topmost Via intact and merely insert a new Via header field directly after it, which would cause responses to be routed to the attacker's host "on their way" to the valid host, which has exactly the same end result. Although it is possible that an intermediary-based authentication service could guarantee that no Via hops are inserted between the sending user agent and the authentication service, it could not prevent an attacker from adding a Via hop after the authentication service, and thereby preempting responses. It is necessary for the proper operation of SIP for subsequent intermediaries to be capable of inserting such Via header fields, and thus it cannot be prevented. As such, though it is desirable, securing Via is not possible through the sort of identity mechanism described in this document; the best known practice for securing Via is the use of SIPS.

In the end analysis, the Identity header field cannot protect itself. Any attacker could remove the header field from a SIP request, and modify the request arbitrarily afterwards. However, this mechanism is not intended to protect requests from men-in-the-middle who interfere with SIP messages; it is intended only to provide a way that the originators of SIP requests can prove that they are who they claim to be. At best, by stripping identity information from a request, a man-in-the-middle could make it impossible to distinguish any illegitimate messages he would like to send from those messages sent by an authorized user. However, it requires a considerably greater amount of energy to mount such an attack than it does to mount trivial impersonations by just copying someone else's From header field. This mechanism provides a way that an authorized user can provide a definitive assurance of his identity that an unauthorized user, an impersonator, cannot.

In the absence of user agent-based authentication services, the assurance provided by this mechanism is strongest when a user agent forms a direct connection, preferably one secured by TLS, to an intermediary-based authentication service. The reasons for this are twofold: If a user does not receive a certificate from the authentication service over the TLS connection that corresponds to the expected domain (especially when the user receives a challenge via a mechanism such as Digest), then it is possible that a rogue server is attempting to pose as an authentication service for a domain that it does not control, possibly in an attempt to collect shared secrets for that domain. A similar practice could be used for telephone numbers, though the application of certificates for telephone numbers to TLS is left as a matter for future study. Without TLS, the various header field values and the body of the request will not have integrity protection when the request arrives at an authentication service. Accordingly, a prior legitimate or illegitimate intermediary could modify the message arbitrarily. Of these two concerns, the first is most material to the intended scope of this mechanism. This mechanism is intended to prevent impersonation attacks, not man-in-the-middle attacks; integrity over parts of the the header and body is provided by this mechanism only to prevent replay attacks. However, it is possible that applications relying on the presence of the Identity header field could leverage this integrity protection for services other than replay protection. Accordingly, direct TLS connections SHOULD be used between the UAC and the authentication service whenever possible. The opportunistic nature of this mechanism, however, makes it very difficult to constrain UAC behavior, and moreover there will be some deployment architectures where a direct connection is simply infeasible and the UAC cannot act as an authentication service itself. Accordingly, when a direct connection and TLS are not possible, a UAC should use the SIPS mechanism, Digest 'auth-int' for body integrity, or both when it can. The ultimate decision to add an Identity header field to a request lies with the authentication service, of course; domain policy must identify those cases where the UAC's security association with the authentication service is too weak.

Ultimately, the worth of an assurance provided by an Identity header field is limited by the security practices of the authentication service that issues the assurance. Relying on an Identity header field generated by a remote administrative domain assumes that the issuing domain uses recommended administrative practices to authenticate its users. However, it is possible that some authentication services will implement policies that effectively make users unaccountable (e.g., ones that accept unauthenticated registrations from arbitrary users). The value of an Identity header field from such authentication services is questionable. While there is no magic way for a verifier to distinguish "good" from "bad" signers by inspecting a SIP request, it is expected that further work in authorization practices could be built on top of this identity solution; without such an identity solution, many promising approaches to authorization policy are impossible. That much said, it is RECOMMENDED that authentication services based on proxy servers employ strong authentication practices. One cannot expect the Identity header field to be supported by every SIP entity overnight. This leaves the verifier in a compromising position; when it receives a request from a given SIP user, how can it know whether or not the originator's domain supports Identity? In the absence of ubiquitous support for identity, some transitional strategies are necessary. A verifier could remember when it receives a request from a domain or telephone number that uses Identity, and in the future, view messages received from that source without an Identity header field with skepticism. A verifier could consult some sort of directory that indicates whether a given caller should have a signed identity. There are a number of potential ways in which this could be implemented. This is left as a subject for future work. In the long term, some sort of identity mechanism, either the one documented in this specification or a successor, must become mandatory-to-use for the SIP protocol; that is the only way to guarantee that this protection can always be expected by verifiers. Finally, it is worth noting that the presence or absence of the Identity header fields cannot be the sole factor in making an authorization decision. Permissions might be granted to a message on the basis of the specific verified Identity or really on any other aspect of a SIP request. Authorization policies are outside the scope of this specification, but this specification advises any future authorization work not to assume that messages with valid Identity header fields are always good.

As a matter of interface design, SIP user agents might render the display-name portion of the From header field of a caller as the identity of the caller; there is a significant precedent in email user interfaces for this practice. Securing the display-name component of the From header field value is outside the scope of this document, but may be the subject of future work, such as through the "ppt" name mechanism. In the absence of signing the display-name, authentication services might check and validate it, and compare it to a list of acceptable display-names that may be used by the originator; if the display-name does not meet policy constraints, the authentication service could return a 403 response code. In this case, the reason phrase should indicate the nature of the problem; for example, "Inappropriate Display Name". However, the display-name is not always present, and in many environments the requisite operational procedures for display-name validation may not exist, so no normative guidance is given here.

This document contains a number of actions for IANA.

The Identity-Info header in the SIP Header Fields registry should be marked as deprecated by [RFCThis].

The Reason phrase for the 436 response default reason phrase should be changed from "Bad Identity-Info" to "Bad Identity Info" in the SIP Response Code registry. The 437 "Unsupported Certificate" default reason phrase should be changed to "Unsupported Credential".

The IANA manages a registry for Identity-Info parameters. The specification asks the IANA to change the name of this registry to "Identity Parameters". This specification defines two new values for the registry: "canon" as defined in this specification in ; and "info" as defined in this specification in .

This IANA manages an Identity-Info Algorithm Parameter Values registry which this specification deprecates. Since the algorithms for signing PASSporT objects are defined in PASSporT rather than in this specification, there is no longer a need for an algorithm parameter registry for the Identity header field.

The authors would like to thank Olle Jacobson, Dave Frankel, Robert Sparks, Dave Crocker, Stephen Kent, Brian Rosen, Alex Bobotek, Paul Kyzviat, Jonathan Lennox, Richard Shockey, Martin Dolly, Andrew Allen, Hadriel Kaplan, Sanjay Mishra, Anton Baskov, Pierce Gorman, David Schwartz, Eric Burger, Alan Ford, Christer Holmberg, Philippe Fouquart, Michael Hamer, Henning Schulzrinne, and Richard Barnes for their comments.

The following are salient changes from the original RFC 4474: Generalized the credential mechanism; credential enrollment, acquisition and trust is now outside the scope of this document Reduced the scope of the Identity signature to remove CSeq, Call-ID, Contact, and the message body; signing of key fingerprints in SDP is now included Deprecated the Identity-Info header field and relocated its components into parameters of the Identity header field (which obsoletes the previous version of the header field) The Identity header field can now appear multiple times in one request Replaced previous signed-identity-digest format with PASSporT (signing algorithms now defined in a separate specification) Revised status code descriptions