RFC 8224
Authenticated Identity Management in the Session Initiation Protocol (SIP)
6. Signature Generation and Validation
SIP entities that instantiate the authentication service and verification service roles will, respectively, generate and validate the Identity header and the signature it contains.6.1. Authentication Service Behavior
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 Section 7.1). The authentication service role can be instantiated, for example, by an intermediary such as a proxy server or by a UA. 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 the 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 Section 8.1. The information in that section will lead to either the telephone number canonicalization procedures in Section 8.3 for telephone numbers or the URI normalization procedures described in Section 8.5 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 the following: o 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). o If the authentication service is instantiated by a SIP UA, a UA may authenticate its own user through any system-specific means, perhaps simply by virtue of having physical access to the UA. 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 Section 7.1 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 Section 12.6. 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. 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 Section 12.1 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 either the full or compact form of PASSporT. For the baseline PASSporT header (headers containing no "ppt" parameter), this follows the procedures in Section 4; 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 Section 9). After the Identity header field has been added to the request, the authentication service MUST also add an "info" parameter to the Identity header field. The "info" parameter contains a URI from which the authentication service's credential can be acquired; see Section 7.3 for more on credential acquisition. An authentication service MAY use the full form of the PASSporT in the Identity header field. The presence of the full form is OPTIONAL because the information carried in the baseline PASSporT headers and claims is usually redundant with information already carried elsewhere in the SIP request. Using the compact form can significantly reduce SIP message size, especially when the PASSporT payload contains media keys. The syntax of the compact form is given in [RFC8225], Section 7; essentially, it contains only the signature component of the PASSporT. Note that per the behavior specified in [RFC8225], use of the full form is mandatory when optional extensions are included. See Section 9.6.1.1. Handling Repairable Errors
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 ("Invalid Identity Header") response indicating a verification failure. If the authentication
service did not originally send the full form of the PASSporT object in the Identity header field, it SHOULD retry the request with the full form after receiving a 438 response; however, implementations SHOULD NOT retry the request more than once. Authentication services implemented at proxy servers would retry such a request as a sequential fork, by reprocessing the destination as a new target and handling it serially as described in Section 16.6 of [RFC3261]. The information in the full form 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 Section 12.1 for more information) that can be resolved by the inclusion of the full form of PASSporT. Finally, the authentication service forwards the message normally.6.2. Verifier Behavior
This document specifies a logical role for SIP entities; this role is 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 a verification service cannot use any Identity header in a request, due to the absence of Identity headers or unsupported "ppt" parameters, 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 Section 6.2.2. 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 specified below. Note that when an Identity header field contains a full-form PASSporT object, the verifier MUST follow the additional procedures in Section 6.2.4. Step 1: Check for an Unsupported "ppt" The verifier MUST inspect any optional "ppt" parameter appearing in the Identity header. If no "ppt" parameter is present, then the verifier proceeds normally with Steps 2 through 5. 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 Section 8.1. The information in that section will lead to either the telephone number canonicalization procedures in Section 8.3 for telephone numbers or the URI normalization procedures described in Section 8.5 for domain names. Step 3: Identify Credential for Validation The verifier must ensure that it has access to the proper keying material to validate the signature in the Identity header field; this usually involves dereferencing a URI in the "info" parameter of the Identity header field. See Section 7.2 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 Section 12.1. If the full form of the PASSporT is present, the verifier SHOULD compare the "iat" value in the PASSporT to the Date header field value in the request. If the two are different, and the "iat" value differs from the Date header field value but remains 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 as described in Section 4. If a "ppt" parameter is present (and, per Step 1, is supported), the verifier follows the procedures for that "ppt" (see Section 9). If a verifier determines that the signature
in the Identity header field does not correspond to the reconstructed signed-identity-digest, then the Identity header field should be considered invalid.6.2.1. Authorization of Requests
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 UAs or proxy servers to follow based on the presence of a valid Identity header field, the presence of an invalid Identity header field, the absence of an Identity header field, or the presence of a stale Date header field value. However, 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 UA implementations. The presence of multiple Identity header fields within a message raises the prospect that a verification service could receive a message containing both valid and invalid Identity header fields. As a guideline, this specification recommends that only if a verifier determines that all Identity header fields within a message are invalid should the request be considered to have an invalid identity. If at least one Identity header field value is valid and from a trusted source, then relying parties can use that header for authorization decisions regardless of whether other untrusted or invalid Identity headers appear in a request.6.2.2. Failure Response Codes Sent by a Verification Service
[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 a 403 "Forbidden" response when a stale Date header field value is received; see below. A 428 response will be sent (per Section 6.2) 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 a human-readable reason phrase like "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 dereference 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" response (previously "Unsupported Certificate"; see Section 13.2) 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 certification authority (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. Sending a full-form PASSporT can repair some of these conditions (see Section 6.2.4), so the recommended way to attempt to repair this failure is to retry the request with the full form of PASSporT if it had originally been sent with the compact form. The alternative reason phrase "Invalid PASSporT" can be used when an extended full-form PASSporT lacks required headers or claims, or when an extended full-form PASSporT signaled with the "ppt" parameter lacks required claims for that extension. Sending a string along these lines will help humans debugging the sending system.
Finally, a 403 response 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 full form of a PASSporT has a value older than the local policy for freshness permits. The reason phrase "Stale Date" can be sent to help humans debug the failure. Future specifications may explore ways, including Reason codes or Warning headers, to communicate further information that could be used to disambiguate the source of errors in cases with multiple Identity headers in a single request or to provide similar detailed feedback for debugging purposes.6.2.3. Handling Retried Requests
If a verification service sends a failure response in the backwards direction, the authentication service may retry the request as described in Section 6.1.1. If the authentication service is instantiated at a proxy server, then it will retry the request as a sequential fork. Verification services implemented at a proxy server will recognize this request as a spiral rather than a loop due to the proxy behavior fix documented in [RFC5393], Section 4.2. However, if the verification service is implemented in an endpoint, the endpoint will need to override the default UAS behavior (in particular, the SHOULD in [RFC3261], Section 8.2.2.2) to accept this request as a spiral rather than a loop.6.2.4. Handling the Full Form of PASSporT
If the full form of PASSporT is present in an Identity header, this permits the use of optional extensions as described in [RFC8225], Section 8.3. Furthermore, the verification service can extract from the "orig" and "dest" elements of the PASSporT full form the canonical telephone numbers created by the authentication service, as well as an "iat" claim corresponding to the Date header field that the authentication service used. These values may be used to debug canonicalization problems or to avoid unnecessary signature breakage caused by intermediaries that alter certain SIP header field values in transit. However, the verification service MUST NOT treat the value in the "orig" of a full-form PASSporT as the originating identity of the call: the originating identity of the call is always derived from the SIP signaling, and it is that value, per the procedures above in Section 6.2 Step 2, that is used to recompute the signature at the verification service. That value, rather than the value inside the PASSporT object, is rendered to an end user in ordinary SIP operations, and if a verification service were to simply trust that
the value in the "orig" corresponded to the call that it received without comparing it to the call signaling, this would enable various cut-and-paste attacks. As an optimization, when the full form is present, the verification service MAY delay performing that cryptographic operation and first compute its own canonicalization of an originating telephone number to compare it to the values in the "orig" element of PASSporT. This would allow the verification service to ascertain whether or not the two ends agree on the canonical number form; if they do not, then surely the signature validation would fail.7. Credentials
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 UA 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, UAs 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 UAs (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.7.1. Credential Use by the Authentication Service
In order to act as an authentication service, a SIP entity must possess 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 the 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 by the Third Generation Partnership Project (3GPP) [TS-3GPP.23.228], 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 [RFC3323]), 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.7.2. Credential Use by the Verification Service
In order to act as a verification service, a SIP entity must have a way to acquire 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 in Section 7.3) 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 dereference 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 by the URI given in the "info" parameter value. Further consideration of how to cache credentials is deferred to the credential mechanism specifications.7.3. "info" Parameter URIs
An "info" parameter MUST contain a URI that 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 UA 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.
7.4. Credential System Requirements
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 [RFC5280]) and the domain name system itself (e.g., DNS-Based Authentication of Named Entities (DANE); see [RFC6698]). 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 [RFC2585]). The "info" parameter might use the DNS URL scheme (see [RFC4501]) 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 [RFC8226], but this specification can work with other credential systems; for example, using the DNS was proposed in [CIDER]. In order for a credential system to work with this mechanism, its specification must detail: o which URI schemes the credential will use in the "info" parameter, and any special procedures required to dereference the URIs, o how the verifier can learn the scope of the credential, o any special procedures required to extract keying material from the resources designated by the URI, o any algorithms required to validate the credentials (e.g., for certificates, any algorithms used by certificate authorities to sign certificates themselves), and o how the associated credentials will support the mandatory signing algorithm(s) required by PASSporT [RFC8225]. 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).8. Identity Types
The STIR problem statement [RFC7340] 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 confirm 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 AoR 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 full form of PASSporT by authentication services is RECOMMENDED, but because the "orig" claim of PASSporT itself could then divulge information about users or networks, implementers should be mindful of the guidelines in Section 11.8.1. Differentiating Telephone Numbers from URIs
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 that contains a numeric user part. Note that the same procedures are followed for creating the canonical form of a URI found in the From header field as the procedures used for a URI found in the To header field or the P-Asserted-Identity header field. First, implementations will ascertain if 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 SIP URIs with the "user=phone" parameter conforms to the syntax of the tel URI scheme [RFC3966]. It is also possible for a tel URI to appear in SIP header fields outside the context of a SIP or Session Initiation Protocol Secure (SIPS) URI (e.g., "tel:+17005551008"). Thus, in standards-compliant environments, numbers will be explicitly labeled by the use of tel URIs or the "user=phone" parameter. Alternatively, implementations in environments that do not conform to those standards MAY follow local policies for identifying telephone numbers. For example, implementations could infer that the user part is a telephone number due to the presence of the "+" indicator at the start of the user portion. Absent even that indication, if there are numbers present in the user portion, implementations might conceivably 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 E.164 number [E.164], a nationally specific number, or even a private branch exchange number. Implementations could also rely on external hints: for example, a verification service implementation could infer from the type of credential that signed a request that the signature must be over a telephone number. Regardless of how the implementation detects telephone numbers, once a telephone number has been detected, implementations SHOULD follow the procedures in Section 8.3. 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 Section 8.5. These URI normalization procedures are invoked to canonicalize the URI before it is included in a PASSporT object in, for example, a "uri" claim. See Section 8.5 for that behavior.8.2. Authority for Telephone Numbers
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 Section 7.4, enrollment procedures and credential management are outside the scope of this document; approaches to credential management for telephone numbers are discussed in [RFC8226].
8.3. Telephone Number Canonicalization Procedures
Once an implementation has identified a telephone number, it must construct a number string. That requires performing the following steps: o Implementations MUST drop any "+"s, internal dashes, parentheses, or other non-numeric characters, except for the "#" 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. o 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 [RFC3966]), 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, calls to those numbers are routed in a very strict fashion, which ordinarily prevents them from reaching entities that don't understand the numbers. o Some domains may need to take unique steps to convert their numbers into a global format, and such transformations during canonicalization can also 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. o 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:
tn-spec = 1*tn-char
tn-char = "#" / "*" / DIGIT
The resulting number string is used in the construction of the
telephone number field(s) in a PASSporT object.
8.4. Authority for Domain Names
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 [RFC5922], Section 7.2. While [RFC5922] 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 [RFC5922], Section 7.8). Secondly, some SIP
services may delegate SIP functions to a subordinate domain and
utilize the procedures in [RFC3263] 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 might 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 [RFC3263] 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.8.5. URI Normalization
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 that captures the AoR 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: canon-uri = ( "sip" / "sips" ) ":" user "@" host Finally, the URI will be subject to the syntax-based URI normalization procedures of [RFC3986], 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 [RFC3986], Section 2.3). However, note that normalization procedures face known challenges in some internationalized environments (see [IRI-COMPARISON]) 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 could eliminate the scheme component. A more exact definition is left to future specifications.9. Extensibility
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 [RFC8225], Section 8. Note that per the guidance in that section, "ppt" is used only to enforce a mandatory extension: optional claims may be added to any PASSporT object without requiring the use of "ppt", but the compact form of PASSporT MUST NOT be used when optional claims are present in the PASSporT payload. The potential for extensions is one of 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 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 reasons of future compatibility. 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.
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