PASSporT Extension for Rich Call Data

Internet-Draft	RCD	November 2022
Wendt & Peterson	Expires 11 May 2023	[Page]

Abstract

This document extends PASSporT, a token for conveying cryptographically-signed call information about personal communications, to include rich meta-data about a call and caller that can be signed and integrity protected, transmitted, and subsequently rendered to the called party. This framework is intended to include and extend caller and call specific information beyond human-readable display name comparable to the "Caller ID" function common on the telephone network and is also enhanced with a integrity mechanism that is designed to protect the authoring and transport of this information for different authoritative use-cases.¶

1. Introduction

PASSporT [RFC8225] is a token format based on JWT [RFC7519] for conveying cryptographically-signed information about the parties involved in personal communications; it is used to convey a signed assertion of the identity of the participants in real-time communications established via a protocol like SIP [RFC8224]. The STIR problem statement [RFC7340] declared securing the display name of callers outside of STIR's initial scope. This document extends the use of JWT and PASSporT in the overall STIR framework by defining a PASSporT extension and the associated STIR procedures to protect additional caller and call related information. This is additional information beyond the calling party originating identity (e.g. telephone number or SIP URI) that is intended to be rendered to assist a called party in determining whether to accept or trust incoming communications. This includes information such as the name of the person or entity on one side of a communications session, for example, the traditional "Caller ID" of the telephone network along with related display information that would be rendered to the called party during alerting or potentially used by an automaton to determine whether and how to alert a called party to a call and whom is calling.¶

Traditional telephone network signaling protocols have long supported delivering a 'calling name' from the originating side, though in practice, the terminating side is often left to determine a name from the calling party number by consulting a local address book or an external database. SIP, for example, similarly can carry this information in a 'display-name' in the From header field value from the originating to terminating side, or alternatively in the Call-Info header field. In this document, we utilize the STIR framework to more generally extend the assertion of an extensible set of identity information not limited to but including calling name.¶

As such, based on some use-cases, this document extends PASSporT to provide cryptographic protection for the "display-name" field of SIP requests, or similar name fields in other protocols, as well as further "rich call data" (RCD) about the caller, which includes the contents of the Call-Info header field or other data structures that can be added to the PASSporT. In addition, Section 12 describes use-cases that enable external third-party authorities to convey rich information associated with a calling number via a "rcd" PASSporT while clearly identifying the third-party as the source of the Rich Call Data information. Finally, this document describes how to preserve the integrity of the RCD in scenarios where there may be non-authoritative users initiating and signing RCD and therefore a constraint on the RCD data that a PASSporT can attest via certificate-level controls.¶

4. Overview of Rich Call Data Integrity

When incorporating call data that represents a user, even in traditional calling name services today, often there are policy and restrictions around what data is allowed to be used. Whether preventing offensive language or icons or enforcing uniqueness, potential trademark or copyright violations or other policy enforcement, there might be the desire to pre-certify or "vet" the specific use of rich call data. This document defines a mechanism that allows for a direct or indirect party that controls the policy to approve or certify the content, create a cryptographic digest that can be used to validate that data and applies a constraint in the certificate to allow the recipient and verifier to validate that the specific content of the RCD is as intended at its creation and approval or certification.¶

There are two mechanisms that are defined to accomplish that for two distinct categories of purposes. The first of the mechanisms include the definition of an integrity claim. The RCD integrity mechanism is a process of generating a sufficiently strong cryptographic digest for each resource referenced by a URI within a claim value (e.g., an image file referenced by "jcd" or a jCard referenced by "jcl"). This mechanism is inspired by and based on the W3C Subresource Integrity specification (http://www.w3.org/TR/SRI/). The second of the mechanisms uses the capability called JWT Claim Constraints, defined in [RFC8226] and extended in [RFC9118]. The JWT Claim Constraints specifically guide the verifier within the certificate used to compute the signature in the PASSporT for the inclusion (or exclusion) of specific claims and their values, so that the content intended by the signer can be verified to be accurate.¶

Both of these mechanisms, integrity digests and JWT Claims Constraints, can be used together or separately depending on the intended purpose. The first category of purpose is whether the rich call data conveyed in the PASSporT claims is pass-by-value or pass-by-reference; i.e., is the information contained in the PASSporT claims and therefore integrity protected by the PASSporT signature, or is the information contained in an external resource referenced by a URI in the PASSporT. The second category of purpose is whether the signer is authoritative or has responsibility for the accuracy of the RCD based on the policies of the eco-system the "rcd" PASSporTs or "rcd" claims are being used.¶

The following table provides an overview of the framework for how integrity should be used with RCD. ("Auth" represents "authoritative" in this table.)¶

+----------+---------------------+--------------------------------+
|   Modes  |  No URI refs        |      Includes URI refs         |
+----------+---------------------+--------------------------------+
|   Auth   | 1: No integrity req | 2: RCD Integrity               |
+----------+---------------------+--------------------------------+
| Non-Auth | 3: JWT Claim Const. | 4: RCD Integ./JWT Claim Const. |
+----------+---------------------+--------------------------------+

The first and simplest mode is exclusively for when all RCD content is directly included as part of the claims (i.e. no URIs referencing external content are included in the content) and when the signer is authoritative over the content. In this mode, integrity protection is not required and the set of claims is simply protected by the signature of the standard PASSporT [RFC8225] and SIP identity header [RFC8224] procedures. The second mode is an extension of the first where the signer is authoritative and an "rcd" claim contents include a URI identifying external resources. In this mode, an RCD Integrity or "rcdi" claim MUST be included. This integrity claim is defined later in this document and provides a digest of the "rcd" claim content so that, particularly for the case where there are URI references in the RCD, the content of that RCD can be comprehensively validated that it was received as intended by the signer of the PASSporT.¶

The third and fourth mode cover cases where there is a different authoritative entity responsible for the content of the RCD, separate from the signer of the PASSporT itself, allowing the ability, in particular when delegating signing authority for PASSporT, to enable a mechanism for allowing agreed or vetted content included in or referenced by the RCD claim contents. The primary framework for allowing the separation of authority and the signing of PASSporTs by non-authorized entities is detailed in [RFC9060] although other cases may apply. As with the first and second modes, the third and fourth modes differ with the absence or inclusion of referenced external content using URIs.¶

5. PASSporT Claim "rcd" Definition and Usage

5.1. PASSporT "rcd" Claim

This document defines a new JSON Web Token claim for "rcd", Rich Call Data, the value of which is a JSON object that can contain one or more key value pairs. This document defines a default set of key values.¶

5.1.1. "nam" key

The "nam" key value is a display name, associated with the originator of personal communications, which may for example match the display-name component of the From header field value of a SIP request or alternatively from the P-Asserted-Identity header field value, or a similar field in other PASSporT using protocols. This key MUST be included once as part of the "rcd" claim value JSON object. If there is no string associated with a display name, the claim value MUST then be an empty string.¶

5.1.2. "apn" key

The "apn" key value is an optional alternate presentation number associated with the originator of personal communications, which may for example match the user component of the From header field value of a SIP request (in cases where a network number is carried in the P-Asserted-Identity [RFC3325]), or alternatively from the Additional-Identity header field value [3GPP TS 24.229 v16.7.0], or a similar field in other PASSporT using protocols. Its intended semantics are to convey a number that the originating user is authorized to show to called parties in lieu of their default number, such as cases where a remote call agent uses the main number of a call center instead of their personal telephone number. The "apn" key value is a canonicalized telephone number per [RFC8224] Section 8.3. If present, this key MUST be included once as part of the "rcd" claim value JSON object.¶

The use of the optional "apn" key is intended for cases where the signer of an "rcd" PASSporT or "rcd" claims authorizes the use of an alternate presentation number by the user. How the signer determines that a user is authorized to present the number in question is a policy decision outside the scope of this document, however, the vetting of the alternate presentation number should follow the same level of vetting as telephone identities or any other information contained in an "rcd" PASSporT or "rcd" claims. This usage is intended as an alternative to conveying the presentation number in the "tel" key value of a jCard, in situations where no other rich jCard data needs to be conveyed with the call. Only one "apn" key may be present. "apn" MUST be used when it is the intent of the caller or signer to display the alternate presentation number even if "jcd" or "jcl" keys are present in a PASSporT with a "tel" key value.¶

5.1.3. "icn" key

The "icn" key value is an optional URI reference to an image that can be used to pictorially represent the originator of personal communications. This icon key value should be used as a base or default method of associating an image with a calling party.¶

When being used for SIP [RFC3261] this claim key value used to protect the Call-Info header field with a purpose parameter value of "icon" as described in Section 20.9 [RFC3261]. Example as follows:¶

Call-Info: <http://wwww.example.com/alice/photo.jpg>;
  purpose=icon

Note that [I-D.ietf-sipcore-callinfo-rcd] extends the specific usage of "icon" in SIP in the context of the larger rich call data framework with specific guidance on referencing images and image types, sizes and formats.¶

It should be also noted that with jCard, as described in the following "jcd" and "jcl" key value sections and in [I-D.ietf-sipcore-callinfo-rcd], there are alternative ways of including photos and logos as URI references. The "icn" key should be then considered a base or default image and jCard usage should be considered for profiles and extensions that provide more direct guidance on the usage of specific defined usage of what each image type represents for the proper rendering to end users.¶

5.1.4. "jcd" key

The "jcd" key value is defined to contain a jCard [RFC7095] JSON object. The jCard is defined in this specification as an extensible object format used to contain RCD information about the call initiator. This object is intended to directly match the Call-Info header field value defined in [I-D.ietf-sipcore-callinfo-rcd] with a type of "jcard" where the format of the jCard and properties used should follow the normative usage and formatting rules and procedures in that document. It is an extensible object where the calling party can provide both the standard types of information defined in jCard or can use the built-in extensibility of the jCard specification to add additional information. The "jcd" key is optional. The "jcd" or "jcl" keys MAY only appear once in the "rcd" claim but MUST be mutually exclusive.¶

The jCard object value for "jcd" MUST be a jCard JSON object that MAY have URI referenced content, but that URI referenced content MAY NOT further reference URIs. Future specifications may extend this capability, but as stated in [I-D.ietf-sipcore-callinfo-rcd] it constrains the security properties of RCD information and the integrity of the content referenced by URIs.¶

Note: even though we refer to [I-D.ietf-sipcore-callinfo-rcd] as the definition of the jcard properties for usage in "rcd" claims, using Call-Info as protocol with the addition of an identity header carrying the PASSPorT is not required. The identity header carrying a PASSporT with "rcd" claim including a "jcd" value can be used as the primary and only transport of the RCD information.¶

5.1.5. "jcl" key

The "jcl" key value is an HTTPS URL that refers to a jCard [RFC7095] JSON object on a web server. The web server MUST use the MIME media type for JSON text as application/json with a default encoding of UTF-8 [RFC7159]. This link may correspond to the Call-Info header field value defined in [I-D.ietf-sipcore-callinfo-rcd] with a type of "jcard". As also defined in [I-D.ietf-sipcore-callinfo-rcd], format of the jCard and properties used should follow the normative usage and formatting rules and procedures. The "jcl" key is optional. The "jcd" or "jcl" keys MAY only appear once in the "rcd" claim but MUST be mutually exclusive.¶

The jCard object referenced by the URI value for "jcl" MUST be a jCard JSON object that MAY have URI referenced content, but that URI referenced content MAY NOT further reference URIs. Future specifications may extend this capability, but as stated in [I-D.ietf-sipcore-callinfo-rcd] it constrains the security properties of RCD information and the integrity of the content referenced by URIs.¶

6. "rcdi" RCD Integrity Claim Definition and Usage

The "rcdi" claim is included for the second and fourth modes described in the integrity overview Section 4 of this document. "rcdi" and "rcd" claims MAY each appear once in a PASSporT, but if "rcdi" is included the "rcd" MUST correspondingly be present also. The value of the "rcdi" claim is a JSON object that is defined as follows.¶

The claim value of "rcdi" claim key is a JSON object with a set of JSON key/value pairs. These objects correspond to each of the elements of the "rcd" claim object that require integrity protection with an associated digest over the content referenced by the key string. The individual digest of different elements of the "rcd" claim data and URI referenced external content is kept specifically separate to allow the ability to verify the integrity of only the elements that are ultimately retrieved or downloaded or rendered to the end-user.¶

The key value references a specific object within the "rcd" claim value using a JSON pointer defined in [RFC6901] with a minor additional rule to support URI references to external content that include JSON objects themselves, for the specific case of the use of "jcl", defined in Section 6.1.4. JSON pointer syntax is the key value that documents exactly the part of JSON that is used to generate the digest which produce the resulting string that makes up the value for the corresponding key. Detailed procedures are provided below, but an example "rcdi" is provided here:¶

"rcdi" : {
  "/jcl": "sha256-7kdCBZqH0nqMSPsmABvsKlHPhZEStgjojhdSJGRr3rk",
  "/jcl/1/2/3": "sha256-jL4f47fF82LuwcrOrSyckA4SWrlElfARHkW6kYo1JdI"
}

The values of each key/value pair consists of a digest across either the direct values or indirectly referenced resources, combined with a string that defines the crypto algorithm used to generate the digest. RCD implementations MUST support the following hash algorithms, "SHA256", "SHA384", and "SHA512". The SHA-256, SHA-384, and SHA-512 are part of the SHA-2 set of cryptographic hash functions defined by the National Institute of Standards and Technologies (NIST). Implementations MAY support additional algorithms, but MUST NOT support known weak algorithms such as MD5 or SHA-1. In the future, the list of algorithms may be re-evaluated based on security best practices. The algorithms are represented in the text by "sha256", "sha384", or "sha512". The character following the algorithm string MUST be a hyphen character, "-", or ASCII 45. The subsequent characters are the base64 encoded [RFC4648] digest of a canonicalized and concatenated string or binary data based on the JSON pointer referenced elements of "rcd" claim or the URI referenced content contained in the claim. The details of the determination of the input string used to determine the digest are defined in the next section.¶

6.1. Creation of the "rcd" element digests

"rcd" claim objects can contain "nam", "apn", "icn", "jcd", or "jcl" keys as part of the "rcd" JSON object claim value. This document defines the use of JSON pointer [RFC6901] as a mechanism to reference specific "rcd" claim elements.¶

In order to facilitate proper verification of the digests and whether the "rcd" elements or content referenced by URIs were modified, the input to the digest must be completely deterministic at three points in the process. First, at the certification point where the content is evaluated to conform to the application policy and the JWT Claim Constraints is applied to the certificate containing the digest. Second, when the call is signed at the Authentication Service, there may be a local policy to verify that the provided "rcd" claim corresponds to each digest. Third, when the "rcd" data is verified at the Verification Service, the verification is performed for each digest by constructing the input digest string for the element being verified and referenced by the JSON pointer string.¶

The procedure for the creation of each "rcd" element digest string corresponding to a JSON pointer string key is as follows.¶

The JSON pointer either refers to a value that is a part or the whole of a JSON object or to a string that is a URI referencing an external resource.¶
For a JSON value, serialize the JSON to remove all white space and line breaks. The procedures of this deterministic JSON serialization are defined in [RFC8225], Section 9. The resulting string is the input for the hash function.¶
For any URI referenced content, the bytes of the body of the HTTP response is the input for the hash function.¶

Note that the digest is computed on the Json representation of the string, which necessarily includes the beginning and ending double-quote characters.¶

6.1.1. "nam" and "apn" elements

In the case of "nam" and "apn", the only allowed value is a string. For both of these key values an "rcdi" JSON pointer or integrity digest is optional because the direct value is protected by the signature and can be constrained directly with JWTClaimConstraints.¶

6.1.2. "icn" elements

In the case of "icn", the only allowed value is a URI value that references an image file. If the URI references externally linked content there would need to be a JSON pointer and digest entry for the content in that linked resource. In order to reference the "icn" value for a digest, the JSON pointer string would be "/icn" and the digest string would be created using the image file data following the rules of JSON pointer. Even though this is probably not the typical case, an "rcdi" JSON pointer or integrity digest is optional if the image value is directly included via a data URI [RFC2397]. However, even though the direct value can be protected by the signature and can be constrained directly with JWTClaimConstraints, since the length of the image data is likely much larger than the integrity digest, the use of the "rcdi" JSON pointer and integrity digest as the constraint value in JWTClaimConstraints over the image data is RECOMMENDED.¶

6.1.3. "jcd" elements

In the case of "jcd", the value associated is a jCard JSON object, which happens to be a JSON array with sub-arrays. JSON pointer notation uses numeric indices into elements of arrays, including when those elements are arrays themselves.¶

As example, for the following "rcd" claim:¶

"rcd": {
  "jcd": ["vcard",
    [ ["version",{},"text","4.0"],
      ["fn",{},"text","Q Branch"],
      ["org",{},"text","MI6;Q Branch Spy Gadgets"],
      ["photo",{},"uri",
        "https://example.com/photos/quartermaster-256x256.png"],
      ["logo",{},"uri",
        "https://example.com/logos/mi6-256x256.jpg"],
      ["logo",{},"uri",
        "https://example.com/logos/mi6-64x64.jpg"]
    ]
  ],
  "nam": "Q Branch Spy Gadgets"
}

In order to use JSON pointer to refer to the URIs, the following example "rcdi" claim includes a digest for the entire "jcd" array string as well as three additional digests for the URIs, where, as defined in [RFC6901] zero-based array indices are used to reference the URI strings.¶

"rcdi": {
  "/jcd": "sha256-7kdCBZqH0nqMSPsmABvsKlHPhZEStgjojhdSJGRr3rk",
  "/jcd/1/3/3": "sha256-RojgWwU6xUtI4q82+kHPyHm1JKbm7+663bMvzymhkl4",
  "/jcd/1/4/3": "sha256-jL4f47fF82LuwcrOrSyckA4SWrlElfARHkW6kYo1JdI",
  "/jcd/1/5/3": "sha256-GKNxxqlLRarbyBNh7hc/4lbZAdK6B0kMRf1AMRWPkSo"
  }
}

The use of a JSON pointer and integrity digest for the "jcd" claim key and value is optional. The "jcd" value is the directly included jCard array and can be protected by the signature and can be constrained directly with JWTClaimConstraints. However, for data length reasons (as with "icn" above) or more importantly for potential privacy and/or security considerations with a publically accessible certificate, the use of the "rcdi" JSON pointer and integrity digest as the constraint value in JWTClaimConstraints over the jCard data is RECOMMENDED.¶

It is important to remember the array indices for JSON Pointer are dependent on the order of the elements in the jCard. The use of digest for the "/jcd" corresponding to the entire jCard array string can be included as a redundant mechanism to avoid any possibility of substitution, insertion attacks, or other potential techniques that may be possible to avoid integrity detection.¶

Each URI referenced in the jCard array string MUST have a corresponding JSON pointer string key and digest value.¶

6.1.4. "jcl" elements

In the case of the use of a "jcl" URI reference to an external jCard, the procedures are similar to "jcd" with the exception and the minor modification to JSON pointer, where "/jcl" is used to refer to the external jCard array string and any following numeric array indices added to the "jcl" (e.g., "/jcl/1/2/3") are treated as if the external content referenced by the jCard was directly part of the overall "rcd" claim JSON object. The following example illustrates a "jcl" version of the above "jcd" example.¶

"rcd": {
  "jcl": "https://example.com/qbranch.json",
  "nam": "Q Branch Spy Gadgets"
},
"rcdi": {
  "/jcl": "sha256-7kdCBZqH0nqMSPsmABvsKlHPhZEStgjojhdSJGRr3rk",
  "/jcl/1/3/3": "sha256-RojgWwU6xUtI4q82+kHPyHm1JKbm7+663bMvzymhkl4",
  "/jcl/1/4/3": "sha256-jL4f47fF82LuwcrOrSyckA4SWrlElfARHkW6kYo1JdI",
  "/jcl/1/5/3": "sha256-GKNxxqlLRarbyBNh7hc/4lbZAdK6B0kMRf1AMRWPkSo"
}

The following is the example contents of resource pointed to by https://example.com/qbranch.json used to calculate the above digest for "/jcl"¶

["vcard",
  [ ["version",{},"text","4.0"],
    ["fn",{},"text","Q Branch"],
    ["org",{},"text","MI6;Q Branch Spy Gadgets"],
    ["photo",{},"uri",
      "https://example.com/photos/quartermaster-256x256.png"],
    ["logo",{},"uri",
      "https://example.com/logos/mi6-256x256.jpg"],
    ["logo",{},"uri",
      "https://example.com/logos/mi6-64x64.jpg"]
  ]
]

6.2. JWT Claim Constraints for "rcd" claims

When using JWT Claim Constraints for "rcd" claims the procedure when creating the signing certificate should follow the following guidelines.¶

The "permittedValues" for the "rcd" claim can contain a single entry or optionally contain multiple entries with the intent of supporting cases where the certificate holder is authorized to use different sets of rich call data corresponding to different call scenarios.¶

Only including "permittedValues" for "rcd", with no "mustInclude", provides the ability for the construction a valid PASSPorT that can either have no "rcd" claim within or only the set of constrained "permittedValues" values for an included "rcd" claim.¶

6.3. JWT Claim Constraints usage for "rcd" and "rcdi" claims

The use of JWT Claim Constraints with an "rcdi" claim is for cases where URI referenced content is to be protected by the authoritative certificate issuer. The objective for the use of JWT Claim Constraints for the combination of both "rcd" and "rcdi" claims is to constrain the signer to only construct the "rcd" and "rcdi" claims inside a PASSporT to contain and reference only a pre-determined set of content. Once both the contents of the "rcd" claim and any referenced content is certified by the party that is authoritative for the certificate being issued to the signer, the "rcdi" claim is constructed and linked to the STIR certificate associated with the signature in the PASSporT via JWT Claim Constraints extension as defined in [RFC8226] Section 8 and extended in [RFC9118]. It should be recognized that the "rcdi" set of digests is intended to be unique for only a specific combination of "rcd" content and URI referenced external content, and therefore provides a robust integrity mechanism for an authentication service being performed by a non-authoritative party. This would often be associated with the use of delegate certificates [RFC9060] for the signing of calls by the calling party directly as an example, even though the "authorized party" is not necessarily the subject of a STIR certificate.¶

For the cases that there should always be both "rcd" and "rcdi" claims included in the PASSporT, the certificate JWT Claims Constraint extension MUST include both of the following:¶

a "mustInclude" for the "rcd" claim, which simply constrains the fact that an "rcd" must be included¶
a "mustInclude" for the "rcdi" claim and a "permittedValues" equal to the created "rcdi" claim value string.¶

Note that optionally the "rcd" claims may be included in the "permittedValues" however it is recognized that this may be redundant with the "rcdi" permittedValues because the "rcdi" digest will imply the content of the "rcd" claims themselves.¶

The "permittedValues" for the "rcdi" claims (or "rcd" claims more generally) may contain multiple entries, to support the case where the certificate holder is authorized to use different sets of rich call data.¶

8. Rich Call Data Claims Usage Rules

The "rcd" or "crn" claims MAY appear in any PASSporT claims object as optional elements. The creator of a PASSporT MAY also add a PASSporT extension ("ppt") value, defined in [RFC8225] Section 8.1, of "rcd" to the header of a PASSporT as well, in which case the PASSporT claims MUST contain at least one or both an "rcd" or "crn" claim. Any entities verifying the PASSporT claims defined in this document are required to understand the PASSporT extension in order to process the PASSporT in question. An example PASSporT header with the PASSporT extension ("ppt") value of "rcd" included is shown as follows:¶

{ "typ":"passport",
  "ppt":"rcd",
  "alg":"ES256",
  "x5u":"https://www.example.com/cert.cer" }

The PASSporT claims object contains the "rcd" key with its corresponding value. The value of "rcd" is an array of JSON objects, of which one, the "nam" object, is mandatory. The key syntax of "nam" follows the display-name ABNF given in [RFC3261].¶

After the header and claims PASSporT objects have been constructed, their signature is computed normally per the guidance in [RFC8225].¶

8.1. "rcd" PASSporT Verification

A verifier that successfully verifies a PASSportT that contains an "rcd" claim MUST ensure the following about the PASSporT:¶

it has a valid signature¶
it abides by all rules set forth in the proper construction of the claims¶
it abides by JWT Claims Constraint rules defined in [RFC8226] Section 8 or extended in [RFC9118] if present in the certificate used to compute the signature in the PASSporT¶

Consistent with the verification rules of PASSporTs more generally [RFC8225], if any of the above criteria is not met, relying parties MUST NOT use any of the claims in the PASSporT.¶

8.2. "rcdi" Integrity Verification

If the "rcdi" claim exists, any party that dereferences a URI (i.e. downloading content for display to users) from the "rcd" claim MUST perform integrity validation of the content against the corresponding digest. Consequently, if URIs with contents covered by integrity digests are passed to another entity, the corresponding integrity digest MUST also be included, for example by passing the PASSporT. Entities that pass on the content without the URI do not have to pass on the corresponding integrity digest. An entity that does not otherwise need to dereference a URI from the "rcd" claim is NOT RECOMMENDED to unnecessarily dereference the URI solely to perform integrity verification.¶

If there is any issue with completing the integrity verification procedures for referenced external content, including HTTP or HTTPS errors, the referenced content MUST be considered not verified. This SHOULD NOT however impact the result of base PASSporT verification for claims content that is directly included in the claims of the PASSporT.¶

8.3. Example "rcd" PASSporTs

An example of a "nam" only PASSporT claims object is shown next (with line breaks for readability only).¶

{  "orig":{"tn":"12025551000"},
   "dest":{"tn":["12025551001"]},
   "iat":1443208345,
   "rcd":{"nam":"James Bond"} }

An example of a "nam", "apn", and "icn" using an https URI PASSporT claims object is shown next (with line breaks for readability only). Note, in this example, there is no integrity protection over the "icn" element in the "rcd" claim.¶

{  "orig":{"tn":"12025551000"},
   "dest":{"tn":["12155551001"]},
   "iat":1443208345,
   "rcd":{
     "apn":"12025559990",
     "icn":"https://example.com/photos/quartermaster-256x256.png",
     "nam":"Her Majesty's Secret Service" } }

An example of a "nam", "apn", and "icn" using data URI PASSporT claims object is shown next (with line breaks for readability only). Note, in this example, the "icn" data is incorporated directly in the "rcd" claim and therefore separate integrity protection is not required.¶

{  "orig":{"tn":"12025551000"},
   "dest":{"tn":["12155551001"]},
   "iat":1443208345,
   "rcd":{
     "apn":"12025559990",
     "icn":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAY
       AAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OH
       wAAAABJRU5ErkJggg==",
     "nam":"Her Majesty's Secret Service" } }

An example of an "rcd" claims object that includes the "jcd" and also contains URI references to content which requires the inclusion of an "rcdi" claim and corresponding digests. Note, in this example, the "rcdi" claim includes integrity protection of the URI referenced content.¶

{
  "crn": "Rendezvous for Little Nellie",
  "orig": { "tn": "12025551000"},
  "dest": { "tn": ["12155551001"]},
  "iat": 1443208345,
  "rcd": {
    "jcd": ["vcard",
    [ ["version",{},"text","4.0"],
      ["fn",{},"text","Q Branch"],
      ["org",{},"text","MI6;Q Branch Spy Gadgets"],
      ["photo",{},"uri","https://example.com/photos/q-256x256.png"],
      ["logo",{},"uri","https://example.com/logos/mi6-256x256.jpg"],
      ["logo",{},"uri","https://example.com/logos/mi6-64x64.jpg"]
    ] ],
    "nam": "Q Branch Spy Gadgets"
  },
  "rcdi": {
    "/jcd/1/3/3": "sha256-RojgWwU6xUtI4q82+kHPyHm1JKbm7+663bMvzymhkl4",
    "/jcd/1/4/3": "sha256-jL4f47fF82LuwcrOrSyckA4SWrlElfARHkW6kYo1JdI",
    "/jcd/1/5/3": "sha256-GKNxxqlLRarbyBNh7hc/4lbZAdK6B0kMRf1AMRWPkSo"
  }
}

In an example PASSporT, where a jCard is linked via HTTPS URL using "jcl", a jCard file served at a particular URL.¶

An example jCard JSON file hosted at the example web address of https://example.com/qbranch.json is shown as follows:¶

["vcard",
  [ ["version",{},"text","4.0"],
    ["fn",{},"text","Q Branch"],
    ["org",{},"text","MI6;Q Branch Spy Gadgets"],
    ["photo",{},"uri","https://example.com/photos/q-256x256.png"],
    ["logo",{},"uri","https://example.com/logos/mi6-256x256.jpg"],
    ["logo",{},"uri","https://example.com/logos/mi6-64x64.jpg"]
  ]
]

For the above referenced jCard, the corresponding PASSporT claims object would be as follows:¶

{
  "crn": "Rendezvous for Little Nellie",
  "orig": {"tn": "12025551000"},
  "dest": {"tn": ["12155551001"]},
  "iat": 1443208345,
  "rcd": {
    "nam": "Q Branch Spy Gadgets",
    "jcl": "https://example.com/qbranch.json"
  },
  "rcdi": {
    "/jcl": "sha256-qCn4pEH6BJu7zXndLFuAP6DwlTv5fRmJ1AFkqftwnCs",
    "/jcl/1/3/3": "sha256-RojgWwU6xUtI4q82+kHPyHm1JKbm7+663bMvzymhkl4",
    "/jcl/1/4/3": "sha256-jL4f47fF82LuwcrOrSyckA4SWrlElfARHkW6kYo1JdI",
    "/jcl/1/5/3": "sha256-GKNxxqlLRarbyBNh7hc/4lbZAdK6B0kMRf1AMRWPkSo"
  }
}

An example "rcd" PASSporT that uses "nam" and "icn" keys with "rcdi" for calling name and referenced icon image content:¶

{
  "crn": "Rendezvous for Little Nellie",
  "orig": {"tn": "12025551000"},
  "dest": {"tn": ["12155551001"]},
  "iat": 1443208345,
  "rcd": {
    "nam": "Q Branch Spy Gadgets",
    "icn": "https://example.com/photos/q-256x256.png"
  },
  "rcdi": {
    "/nam": "sha256-sM275lTgzCte+LHOKHtU4SxG8shlOo6OS4ot8IJQImY",
    "/icn": "sha256-RojgWwU6xUtI4q82+kHPyHm1JKbm7+663bMvzymhkl4"
  }
}

10. Third-Party Uses

While rich data about the call can be provided by an originating authentication service, an intermediary in the call path could also acquire rich call data by querying a third-party service. Such a service effectively acts as a STIR Authentication Service, generating its own PASSporT, and that PASSporT could be attached to a call by either the originating or terminating side. This third-party PASSporT attests information about the calling number, rather than the call or caller itself, and as such its RCD MUST NOT be used when a call lacks a first-party PASSporT that assures verification services that the calling party number is not spoofed. It is intended to be used in cases when the originating side does not supply a display-name for the caller, so instead some entity in the call path invokes a third-party service to provide rich caller data for a call.¶

In telephone operations today, a third-party information service is commonly queried with the calling party's number in order to learn the name of the calling party, and potentially other helpful information could also be passed over that interface. The value of using a PASSporT to convey this information from third parties lies largely in the preservation of the third party's signature over the data, and the potential for the PASSporT to be conveyed from intermediaries to endpoint devices. Effectively, these use cases form a sub-case of out-of-band [RFC8816] use cases. The manner in which third-party services are discovered is outside the scope of this document.¶

An intermediary use case might look as follows using SIP protocol for this example: a SIP INVITE carries a display name in its From header field value and an initial PASSporT object without the "rcd" claim. When a terminating verification service implemented at a SIP proxy server receives this request, and determines that the signature is valid, it might query a third-party service that maps telephone numbers to calling party names. Upon receiving the PASSporT in a response from that third-party service, the terminating side could add a new Identity header field to the request for the PASSporT object provided by the third-party service. It would then forward the INVITE to the terminating user agent. If the display name in the PASSporT object matches, or is string equivelent to, the display name in the INVITE, then the name would presumably be rendered to the end user by the terminating user agent.¶

A very similar flow could be followed by an intermediary closer to the origination of the call. Presumably such a service could be implemented at an originating network in order to decouple the systems that sign for calling party numbers from the systems that provide rich data about calls.¶

In an alternative use case, the terminating user agent might query a third-party service. In this case, no new Identity header field would be generated, though the terminating user agent might receive a PASSporT object in return from the third-party service, and use the "rcd" field in the object as a calling name to render to users while alerting.¶

While in the traditional telephone network, the business relationship between calling customers and their telephone service providers is the ultimate root of information about a calling party's name, some other forms of data like crowdsourced reputation scores might derive from third parties. When those elements are present, they MUST be in a third-party "rcd" PASSporT using "iss" claim described in the next section.¶

10.1. Signing as a Third Party

A third-party PASSporT contains an "iss" element to distinguish its PASSporTs from first-party PASSporTs. Third-party "rcd" PASSporTs are signed with credentials that do not have authority over the identity that appears in the "orig" element of the PASSporT claims. The presence of "iss" signifies that a different category of credential is being used to sign a PASSporT than the [RFC8226] certificates used to sign STIR calls; it is instead a certificate that identifies the source of the "rcd" data. How those credentials are issued and managed is outside the scope of this document; the value of "iss" however MUST reflect the Subject of the certificate used to sign a third-party PASSporT. The explicit mechanism for reflecting the subject field of the certificate is out of scope of this document and left to the certificate governance policies that define how to map the "iss" value in the PASSporT to the subject field in the certificate. Relying parties in STIR have always been left to make their own authorization decisions about whether to trust the signers of PASSporTs, and in the third-party case, where an entity has explicitly queried a service to acquire the PASSporT object, it may be some external trust or business relationship that induces the relying party to trust a PASSporT.¶

An example of a Third Party issued PASSporT claims object is as follows.¶

{  "orig":{"tn":"12025551000"},
   "dest":{"tn":["12025551001"]},
   "iat":1443208345,
   "iss":"Zorin Industries",
   "rcd":{"nam":"James St. John Smythe"} }

10.2. Verification using Third Party RCD

The third-party "rcd" PASSporT cases must be considered in the verification service, as an attacker could attempt to cut-and-paste such a third-party PASSporT into a SIP request in an effort to get the terminating user agent to render the display name or confidence values it contains to a call that should have no such assurance. Following the rules of [RFC8225] and in particular if there is multiple identity headers, for example with the case of the inclusion of an "rcd" and "shaken" PASSporTs from two different signing providers, a verification service MUST determine that the calling party number shown in the "orig" of the "rcd" PASSporT corresponds to the calling party number of the call it has received, and that the "iat" field of the "rcd" PASSporT is within the date interval that the verification service would ordinarily accept for a PASSporT. It is possible that if there is multiple identity headers are present, only the verified identity information should be considered when presenting call information to an end user.¶

Verification services may alter their authorization policies for the credentials accepted to sign PASSporTs when third parties generate PASSporT objects, per Section 10.1. This may include accepting a valid signature over a PASSporT even if it is signed with a credential that does not attest authority over the identity in the "orig" claim of the PASSporT, provided that the verification service has some other reason to trust the signer. No further guidance on verification service authorization policy is given here.¶

12. Use of "rcd" PASSporTs in SIP

This section documents SIP-specific usage for "rcd" PASSporTs and in the SIP Identity header field value. Other using protocols of PASSporT may define their own usages for the "rcd" PASSporTs.¶

12.1. Authentication Service Behavior for SIP protocol

An authentication service creating a PASSporT containing an "rcd" claim MAY include a PASSporT extension ("ppt" value) of "rcd" or not. Third-party authentication services following the behavior in Section 10.1 MUST include a PASSporT extension value of "rcd". If PASSporT extension does contain an "rcd", then any SIP authentication services MUST add a PASSporT extension "ppt" parameter to the Identity header field containing that PASSporT with a value of "rcd". The resulting Identity header field might look as follows:¶

Identity: sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9
       dlxkWzoeU7d7OV8HweTTDobV3itTmgPwCFjaEmMyEI3d7SyN21yNDo2ER/Ovgt
       w0Lu5csIppPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs=;
       info=<https://biloxi.example.org/biloxi.cer>;alg=ES256;
       ppt="rcd"

This document assumes that by default when using the SIP protocol, an authentication service determines the value of "rcd", specifically only for the "nam" key value, from the display-name component of the From header field value of the request, alternatively for some calls this may come from the P-Asserted-ID header. It is however a matter of authentication service policy to decide how it populates the value of "nam" key, which MAY also match or be determined by other fields in the request, from customer profile data, or from access to external services. If the authentication service generates an "rcd" claim containing "nam" with a value that is not string equivalent to the From header field display-name value, it MUST use the full form of the PASSporT object in SIP.¶

In addition, {I-D.ietf-sipcore-callinfo-rcd}} defines a Call-Info header field that MAY be used as a source of RCD information that an authentication services uses to construct the appropriate PASSporT RCD claim types used. Note that the information that is included in a signed PASSporT is RECOMMENDED to be vetted by an entity that is authoritative over determining the accuracy of that information, so how that information is received by the authentication service is important and the use of Call-Info as a source of RCD information on the authentication side is likely NOT RECOMMENDED best practice.¶

12.2. Verification Service Behavior for SIP protocol

[RFC8224] Section 6.2 Step 5 requires that future specifications defining PASSporT extension ("ppt") values describe any additional verifier behavior specific to the SIP protocol. The general verification proceedures defined in Section 8.1 should be followed, but the following paragraphs describe some of the specifics needed to implement a verification service using the SIP protocol.¶

If the PASSporT is in compact form, then the verification service MUST extract the display-name from the From header field value, if any, and MUST use that as the string value for the "nam" key when it recomputes the header and claims of the PASSporT object. Additionally, if there exists a Call-Info header field as defined in [I-D.ietf-sipcore-callinfo-rcd], the "jcard" JSON object value MUST be used to construct the "jcd" key value when it recomputes the header and claims of the PASSporT object. If the signature validates over the recomputed object, then the verification is considered successful.¶

If the PASSporT is in full form with a PASSporT extension value of "rcd", then the verification service MUST extract the value associated with the "rcd" claim "nam" key in the object. If the PASSporT signature is verified successfully then the verification service MUST additionally compare the string value of the "rcd" claim "nam" key value with the From header field value or the preferred value. The preferred value depends on local policy of the SIP network technique that conveys the display name string through a field other than the From header field to interoperate with this specification (e.g. P-Asserted-Identity) as discussed in [RFC8224]. Similarly, "jcd" or "jcl" jcard information, "icn", "apn", or "crn" can be optionally, based on local policy for devices that support it, used to populate a Call-Info header field following the format of [I-D.ietf-sipcore-callinfo-rcd]. If future defined PASSporT RCD claims types are present, they should follow similar defined proceedures and policies.¶

The behavior of a SIP UAS upon receiving an INVITE or other type of session initiation request containing a PASSporT object with an "rcd" claim largely remains a matter of implementation policy. In most cases, implementations would render this calling party name information to the user while alerting. Any user interface additions to express confidence in the veracity of this information are outside the scope of this specification.¶

13. Using "rcd" and "rcdi" as additional claims to other PASSporT extensions

Rich Call Data, including calling name information, as a common example, is often data that is additive to the personal communications information defined in the core PASSporT data required to support the security properties defined in [RFC8225]. For cases where the entity originating the personal communications is supporting the authentication service for the calling identity and is the authority of the Rich Call Data, rather than creating multiple Identity header fields corresponding to multiple PASSporT extensions, the authentication service can alternatively directly add the "rcd" claim to a PASSporT that authenticates the calling identity.¶

Note: There is one very important caveat to this capability, because generally if there is URI referenced content in an "rcd" PASSporT there is often the requirement to use "rcdi" and JWT Claims Constraints. So, it is important for the user of this specification to recognize that the certificates used should include the necessary JWT Claims Constraints for proper integrity and security of the values in the "rcd" claim incorporated into PASSporTs that are not "rcd".¶

13.1. Procedures for applying "rcd" as claims only

For a given PASSporT using some other extension than "rcd", the Authentication Service MAY additionally include the "rcd" claim as defined in this document. This would result in a set of claims that correspond to the original intended extension with the addition of the "rcd" claim.¶

The Verification service that receives the PASSporT, if it supports this specification and chooses to, should interpret the "rcd" claim as simply just an additional claim intended to deliver and/or validate delivered Rich Call Data.¶

13.2. Example for applying "rcd" as claims only

In the case of [RFC8588] which is the PASSporT extension supporting the SHAKEN specification [ATIS-1000074.v002], a common case for an Authentication service to co-exist in a CSP network along with the authority over the calling name used for the call. Rather than require two identity headers, the CSP Authentication Service can apply both the SHAKEN PASSporT claims and extension and simply add the "rcd" required claims defined in this document.¶

For example, the PASSporT claims for the "shaken" PASSporT with "rcd" claims would be as follows:¶

Protected Header
{
   "alg":"ES256",
   "typ":"passport",
   "ppt":"shaken",
   "x5u":"https://cert.example.org/passport.cer"
}
Payload
{
   "attest":"A",
   "dest":{"tn":["12025551001"]},
   "iat":1443208345,
   "orig":{"tn":"12025551000"},
   "origid":"123e4567-e89b-12d3-a456-426655440000",
   "rcd":{"nam":"James Bond"}
}

A Verification Service that supports "rcd" and "shaken" PASSporT extensions is able to receive the above PASSporT and interpret both the "shaken" claims as well as the "rcd" defined claim.¶

If the Verification Service only understands the "shaken" PASSporT extension claims and doesn't support "rcd" PASSporT extension, then the "rcd" claim is used during PASSporT signature validation but is otherwise ignored and disregarded.¶

17. Security Considerations

The process of signing information contained in a "rcd" PASSporT, whether the identities, identifiers, alternate identities or identifiers, images, logos, physical addresses, or otherwise should follow some vetting process in which an authoritative entity should follow an appropriate consistent policy defined and governed by the eco-system using RCD and the STIR framework. This can be of many forms, depending on the setup and constraints of the policy requirements of the eco-system and is therefore out-of-scope of this document. However, the general chain of trust that signers of "rcd" PASSporT are either directly authoritative or have been delegated authority through certificates using JWT Claim Constraints and integrity mechanisms defined in this and related documents is critical to maintain the integrity of the eco-system utilizing this and other STIR related specifications.¶

Revealing information such as the name, location, and affiliation of a person necessarily entails certain privacy risks. Baseline PASSporT has no particular confidentiality requirement, as the information it signs in many current base communications protocols, for example SIP, is information that carried in the clear anyway. Transport-level security can hide those SIP fields from eavesdroppers, and the same confidentiality mechanisms would protect any PASSporT(s) carried in SIP.¶

The use of JWTClaimConstraints, a mechanism defined in [RFC8226] and extended in [RFC9118] to constrain any of the RCD information in the public certificate by including that information in the certificate, depending on the availability in the deployment of the PKI system, may present a privacy issue. The use of "rcdi" claim and digests for representing JWT claim contents is RECOMMENDED for the prevention of the exposure of that information through the certificates which are often publically accessible and available.¶

Since computation of "rcdi" digests for URIs requires the loading of referenced content, it would be best practice to validate that content at the creation of the "rcdi" or corresponding JWT claim constraint value by checking for content that may cause issues for verification services or that doesn't follow the behavior defined in this document, e.g., unreasonably sized data, the inclusion of recursive URI references, etc. Along the same lines, the verification service should also use precautionary best practices to avoid attacks when accessing URI linked content.¶

17.1. The use of JWT Claim Constraints in delegate certificates to exclude unauthorized claims

While this can apply to any PASSporT that is signed with a STIR Delegate Certificates [RFC9060], it is important to note that when constraining PASSporTs to include specific claims or contents of claims, it is also important to consider potential attacks by non-authorized signers that may include other potential PASSporT claims that weren't originally vetted by the authorized entity providing the delegate certificate. The use of JWT claims constraints as defined in [RFC9118] for preventing the ability to include claims beyond the claims defined in this document may need to be considered.¶

18. References

18.1. Normative References

[I-D.ietf-sipcore-callinfo-rcd]: Wendt, C. and J. Peterson, "SIP Call-Info Parameters for Rich Call Data", Work in Progress, Internet-Draft, draft-ietf-sipcore-callinfo-rcd-04, 7 March 2022, <https://www.ietf.org/archive/id/draft-ietf-sipcore-callinfo-rcd-04.txt>.
[RFC2119]: Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC2397]: Masinter, L., "The "data" URL scheme", RFC 2397, DOI 10.17487/RFC2397, August 1998, <https://www.rfc-editor.org/info/rfc2397>.
[RFC3261]: Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, <https://www.rfc-editor.org/info/rfc3261>.
[RFC4648]: Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, <https://www.rfc-editor.org/info/rfc4648>.
[RFC6901]: Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed., "JavaScript Object Notation (JSON) Pointer", RFC 6901, DOI 10.17487/RFC6901, April 2013, <https://www.rfc-editor.org/info/rfc6901>.
[RFC7095]: Kewisch, P., "jCard: The JSON Format for vCard", RFC 7095, DOI 10.17487/RFC7095, January 2014, <https://www.rfc-editor.org/info/rfc7095>.
[RFC7159]: Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014, <https://www.rfc-editor.org/info/rfc7159>.
[RFC7519]: Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015, <https://www.rfc-editor.org/info/rfc7519>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8224]: Peterson, J., Jennings, C., Rescorla, E., and C. Wendt, "Authenticated Identity Management in the Session Initiation Protocol (SIP)", RFC 8224, DOI 10.17487/RFC8224, February 2018, <https://www.rfc-editor.org/info/rfc8224>.
[RFC8225]: Wendt, C. and J. Peterson, "PASSporT: Personal Assertion Token", RFC 8225, DOI 10.17487/RFC8225, February 2018, <https://www.rfc-editor.org/info/rfc8225>.
[RFC8226]: Peterson, J. and S. Turner, "Secure Telephone Identity Credentials: Certificates", RFC 8226, DOI 10.17487/RFC8226, February 2018, <https://www.rfc-editor.org/info/rfc8226>.
[RFC8588]: Wendt, C. and M. Barnes, "Personal Assertion Token (PaSSporT) Extension for Signature-based Handling of Asserted information using toKENs (SHAKEN)", RFC 8588, DOI 10.17487/RFC8588, May 2019, <https://www.rfc-editor.org/info/rfc8588>.
[RFC9060]: Peterson, J., "Secure Telephone Identity Revisited (STIR) Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060, September 2021, <https://www.rfc-editor.org/info/rfc9060>.
[RFC9118]: Housley, R., "Enhanced JSON Web Token (JWT) Claim Constraints for Secure Telephone Identity Revisited (STIR) Certificates", RFC 9118, DOI 10.17487/RFC9118, August 2021, <https://www.rfc-editor.org/info/rfc9118>.

18.2. Informative References

[ATIS-1000074.v002]: ATIS/SIP Forum NNI Task Group, "Signature-based Handling of Asserted information using toKENs (SHAKEN) <https://access.atis.org/apps/group_public/download.php/62391/ATIS-1000074.v002.pdf>", November 2021.
[RFC3325]: Jennings, C., Peterson, J., and M. Watson, "Private Extensions to the Session Initiation Protocol (SIP) for Asserted Identity within Trusted Networks", RFC 3325, DOI 10.17487/RFC3325, November 2002, <https://www.rfc-editor.org/info/rfc3325>.
[RFC7340]: Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure Telephone Identity Problem Statement and Requirements", RFC 7340, DOI 10.17487/RFC7340, September 2014, <https://www.rfc-editor.org/info/rfc7340>.
[RFC8816]: Rescorla, E. and J. Peterson, "Secure Telephone Identity Revisited (STIR) Out-of-Band Architecture and Use Cases", RFC 8816, DOI 10.17487/RFC8816, February 2021, <https://www.rfc-editor.org/info/rfc8816>.