PASSporT Extension for Rich Call Data

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Table of Contents

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, so baseline STIR provides no features for caller name. This specification documents an optional mechanism for PASSporT and the associated STIR procedures which extend PASSporT objects to protect additional elements conveying richer information: information that is intended to be rendered to assist a called party in determining whether to accept or trust incoming communications. This includes the name of the person or entity on one side of a communications session, 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.¶

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 derive a name from the calling party number by consulting a local address book or an external database. SIP 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. However, both are unsecured fields that really cannot be trusted in most interconnected SIP deployments, and therefore is a good starting point for a framework that utilizes STIR techniques and procedures for protecting call related information including but not limited to calling name.¶

As such, the baseline use-case for this document extends PASSporT to provide cryptographic protection for the "display-name" field of SIP requests 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. This document furthermore specifies a third-party profile that would allow external authorities to convey rich information associated with a calling number via a new type of PASSporT. 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.¶

2. Terminology

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Overview of the use of the Rich Call Data PASSporT extension

The main intended use of the signing of Rich Call Data (RCD) using STIR within SIP [RFC8224] or more generally as a PASSporT extension [RFC8225] is for the entity that originates a call, either directly the caller themselves, if they are authoritative, or a service provider or third-party service that may be authoritative over the rich call data on behalf of the caller.¶

The RCD associated with the identity of the calling party described in this document is of two main categories. The first data is a more traditional set of info about a caller associated with "display-name" in SIP [RFC3261], typically a textual description of the caller, or alternate presentation numbers often used in From Header field [RFC3261] or P-Asserted-ID [RFC3325]. The second category is a set of RCD that is defined as part of the jCard definitions or extensions to that data. [I-D.ietf-sipcore-callinfo-rcd] describes the optional use of jCard in Call-Info header field as RCD with the "jcard" Call-Info purpose token. Either or both of these two types of data can be incorporated into an "rcd" claim defined in this document.¶

Additionally, in relation to the description of the specific communications event itself (versus the identity description in previous paragraph), [I-D.ietf-sipcore-callinfo-rcd] also describes a "call-reason" parameter intended for description of the intent or reason for a particular call. A new PASSporT claim "crn", or call reason, can contain the string or object that describes the intent of the call. This claim is intentionally kept separate from the "rcd" claim because it is envisioned that call reason is not the same as information associated with the caller and may change on a more frequent, per call, type of basis.¶

4. Overview of Rich Call Data Integrity

When incorporating call data that represents a user, even in traditional calling name services today, often there is 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 sign 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 by the RCD passport is pass-by-value or passed-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 RCD 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 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 external URIs   |      Includes URI refs         |
+----------+---------------------+--------------------------------+
|   Auth   | 1: No integrity req | 2: RDC 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 external reference URIs 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 to have forward control at the time of the creation of the certificate of the allowed 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 [I-D.ietf-stir-cert-delegation] although other cases may apply. As with the first and second modes, the third and fourth modes differ with the absence or inclusion of externally referenced content using URIs.¶

5. PASSporT Claim "rcd" Definition and Usage

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

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. If this claim is present it MUST be included only once with the corresponding single "rcd" claim. 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 external URI referenced 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 external URI references that include JSON objects themselves, for the specific case of the use of "jcl". JSON pointer syntax is the key value that specifies 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 minus character, "-". 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.¶

"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"
}

"rcdi": {
  "/jcd": "sha256-tbxXX9mRY2dtss3vNdNkNkt9hrV9N1LqGST2hDlw97I",
  "/jcd/1/3/3": "sha256-RojgWwU6xUtI4q82+kHPyHm1JKbm7+663bMvzymhkl4",
  "/jcd/1/4/3": "sha256-jL4f47fF82LuwcrOrSyckA4SWrlElfARHkW6kYo1JdI",
  "/jcd/1/5/3": "sha256-GKNxxqlLRarbyBNh7hc/4lbZAdK6B0kMRf1AMRWPkSo"
  }
}

"rcd": {
  "jcl": "https://example.com/qbranch.json",
  "nam": "Q Branch Spy Gadgets"
},
"rcdi": {
  "/jcl": "sha256-Gb0lOkj7Z9+plqbOkN32H+YX0Yav3fbioSk7DxQdGZU",
  "/jcl/1/3/3": "sha256-RojgWwU6xUtI4q82+kHPyHm1JKbm7+663bMvzymhkl4",
  "/jcl/1/4/3": "sha256-jL4f47fF82LuwcrOrSyckA4SWrlElfARHkW6kYo1JdI",
  "/jcl/1/5/3": "sha256-GKNxxqlLRarbyBNh7hc/4lbZAdK6B0kMRf1AMRWPkSo"
}

["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"]
  ]
]

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

The integrity overview Section 4 of this document describes a fourth mode where both "rcdi" and JWT Claim Constraints is used. The use of this mode implies the signing of an "rcdi" claim is required to be protected by the authoritative certificate creator using JWT Claims Constraints in the certificate. The objective of the use of both of these mechanisms is to constrain the signer to construct the "rcd" and "rcdi" claims with the "rcd" jCard object including reference external content via URI. Once both the contents of the "rcd" claim and any linked content is certified by the party that is authoritative for the certificate being created and the construction of the "rcdi" claim is complete, the "rcdi" claim is linked to the STIR certificate associated with the signature in the PASSporT via JWT Claim Constraints extension as defined in [RFC8226] Section 8. 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 [I-D.ietf-stir-cert-delegation] 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 case that there should always be both "rcd" and "rcdi" values included in the "rcd" 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. PASSporT "crn" claim - Call Reason Definition and Usage

This specification defines a new JSON Web Token claim for "crn", Call Reason, the value of which is a single string or object that can contains information as defined in [I-D.ietf-sipcore-callinfo-rcd] corresponding to the "call-reason" parameter for the Call-Info header. This claim is optional.¶

Example "crn" claim with "rcd":

"crn" : "For your ears only",
"rcd": { "nam": "James Bond",
         "jcl": "https://example.org/james_bond.json"}

As also noted in [I-D.ietf-sipcore-callinfo-rcd] this claim is included as corresponding to "call-reason" Call-Info parameter, but there is an alternative suggested way to include call-reason which is to use the "cif" claim with a "call-reason" key value, as defined below in this document.¶

9. Rich Call Data Claims Usage Rules

Either or both the "rcd" or "crn" claims may appear in any PASSporT claims object as optional elements. The creator of a PASSporT MAY also add a "ppt" value of "rcd" to the header of a PASSporT as well, in which case the PASSporT claims MUST contain either an "rcd" or "crn" claim, and any entities verifying the PASSporT object are required to understand the "ppt" extension in order to process the PASSporT in question. An example PASSporT header with the "ppt" 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 generated normally per the guidance in [RFC8225].¶

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

{  "orig":{"tn":"12025551000"},
   "dest":{"tn":["12155551001"]},
   "iat":1443208345,
   "rcd":{
     "apn":"12025559990",
     "nam":"Her Majesty's Secret Service" } }

{
  "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"
  }
}

["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"]
  ]
]

{
  "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"
  }
}

{
  "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. Compact form of "rcd" PASSporT

11. Further Information Associated with Callers

Beyond naming information and the information that can be contained in a jCard [RFC7095] object, there may be additional human-readable information about the calling party that should be rendered to the end user in order to help the called party decide whether or not to pick up the phone. This is not limited to information about the caller, but includes information about the call itself, which may derive from analytics that determine based on call patterns or similar data if the call is likely to be one the called party wants to receive. Such data could include:¶

• information related to the location of the caller, or¶
• any organizations or institutions that the caller is associated with, or even categories of institutions (is this a government agency, or a bank, or what have you), or¶
• hyperlinks to images, such as logos or pictures of faces, or to similar external profile information, or¶
• information processed by an application before rendering it to a user, like social networking data that shows that an unknown caller is a friend-of-a-friend, or reputation scores derived from crowdsourcing, or confidence scores based on broader analytics about the caller and callee.¶

All of these data elements would benefit from the secure attestations provided by the STIR and PASSporT frameworks. A new IANA registry has been defined to hold potential values of the "rcd" array; see Section 17.3. Specific extensions to the "rcd" PASSporT claim are left for future specification.¶

There is a few ways RCD can be extended in the future, jCard is an extensible object and the key/values in the RCD claim object can also be extended. General guidance for future extensibility that were followed by the authors is that jCard generally should refer to data that references the caller as an individual or entity, where other claims, such as "crn" refer to data regarding the specific call. There may be other considerations discovered in the future, but this logical grouping of data to the extent possible should be followed for future extensibility.¶

12. 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 SIP 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 [I-D.ietf-stir-oob] 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: 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 "rcd" 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 "rcd" PASSporT object matches 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.¶

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

13. Levels of Assurance

As "rcd" can be provided by either first or third parties, relying parties could benefit from an additional claim that indicates the relationship of the attesting party to the caller. Even in first party cases, this admits of some complexity: the Communications Service Provider (CSP) to which a number was assigned might in turn delegate the number to a reseller, who would then sell the number to an enterprise, in which case the CSP might have little insight into the caller's name. In third party cases, a caller's name could derive from any number of data sources, on a spectrum between public data scraped from web searches to a direct business relationship to the caller. As multiple PASSporTs can be associated with the same call, potentially a verification service could receive attestations of the caller name from multiple sources, which have different levels of granularity or accuracy. Therefore, third-party PASSporTs that carry "rcd" data MUST also carry an indication of the relationship of the generator of the PASSporT to the caller in the form of the "iss" claim. As stated in the previous section, the use of "iss" MUST reflect the subject field of the certificate used to sign a third-party PASSporT to represent that relationship.¶

14. Using "rcd" in SIP

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

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

15. 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 cooresponding to multiple PASSporT extensions, the authentication service can alternatively directly add the "rcd" claim to a PASSporT that authenticates the calling identity.¶

It is critically important for the user of this specification to recognize that the certificates used must include the necessary JWT Claims Constraints and permitted values for proper integrity and security of the values in the "rcd" claim incorporated into PASSporTs that are not "rcd". The verifier of "rcd" claims MUST recognize if the signing certificate contains no claim constraints for direct values or referenced content that they should have some sort of "trust" relationship with the signer of the PASSPorT that can vouch for or have an understanding of how the rich call data was properly vetted. An example scenario that demonstrates this might be when an "rcd" PASSporT with information that was properly vetted, integrity protected and constrained and is transferred by a middle party to another PASSporT and signed by that party without that integrity protection and constraints provided in the certificate. These scenarios lose the end-to-end trust and integrity required by this specification. However, it is recognized that some UNI or provider to device scenarios where there is an authenticated "trust" relationship MAY warrant the technique described in this section.¶

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"}
}

16. Acknowledgements

We would like to thank David Hancock, Robert Sparks, Russ Housley, Eric Burger, Alec Fenichel, Ben Campbell, Jack Rickard, Jordan Simpson for helpful suggestions, review, and comments.¶

17. IANA Considerations

18. Security Considerations

Whether its identities, alternate identities, images, logos, physical addresses, all of the information contained in a RCD PASSporT must follow some form of vetting in which the authoritative entity or user of the information being signed MUST follow an applicable policy of the eco-system using RCD. This can be of many forms, depending on the setup and constraints of the eco-system so 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 over in a using protocol like SIP is all information that SIP carries 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 availbility in the deployment of the PKI system, may present a privacy issue. The use of "rcdi" claim and digests for representing JWT claim contents may be a recommended way of preventing 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.¶

Authors' Addresses