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. 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.¶

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.¶

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

This document defines Rich Call Data (RCD) which is a PASSporT extension [RFC8225] that defines an extensible claim for asserting information about the call beyond the telephone number. This includes information such as more detailed information about the calling party or calling number being presented or the purpose of the call. There are many use-cases that will be described in this document around the entities responsible for the signing and integrity of this information, whether it is the entity that originates a call, a service provider acting on behalf of a caller or use-cases where third-party services may be authoritative over the rich call data on behalf of the caller. In general, PASSporT [RFC8225] has been defined to be a communications protocol independent technology, but it's initial usage as detailed in [RFC8224] is with the SIP protocol [RFC3261]. There are many SIP specific references and definitions in this document, but future specifications may extend the usage of RCD PASSporTs and claims to other protocol specific usage and definitions.¶

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-Identity [RFC3325], or an icon associated with the caller. 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 a string 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 are policy and restrictions around what data elements are 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 enforces the policies 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 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 [W3C-SubresourceIntegrity]. 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 modes 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

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. "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 one of the following objects referenced by the JSON pointer key,¶

• content inline to the referenced object¶
• the content of a resource referenced by an inline URI object¶
• the content of a resource specified by a URI that is in embedded in content specified by an inline URI object(e.g., jcl)¶

This is 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 [RFC6234] 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 [RFC1321] or SHA-1 [RFC3174]. Future specifications may update the list of algorithms and how they are referenced based on future 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.¶

"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": "sha256-7kdCBZqH0nqMSPsmABvsKlHPhZEStgjojhdSJGRr3rk",
  "/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": "sha256-7kdCBZqH0nqMSPsmABvsKlHPhZEStgjojhdSJGRr3rk",
  "/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. PASSporT "crn" claim - Call Reason Definition and Usage

This document defines a new JSON Web Token claim for "crn", Call Reason, the value of which is a single string that can contain 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"}

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" key and value, is mandatory.¶

After the header and claims PASSporT objects have been constructed, their signature is computed 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",
     "icn":"https://example.com/photos/quartermaster-256x256.png",
     "nam":"Her Majesty's Secret Service" } }

{  "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" } }

{
  "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"
  }
}

9. Compact form of "rcd" PASSporT

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.¶

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

11. Levels of Assurance

As "rcd" can be provided by either first party providers that are directly authorized to sign PASSporTs in the STIR eco-system or third party providers that are indirectly or delegated authority to sign PASSporTs. Relying parties could benefit from an additional claim that indicates the identification, in the form of a uniquely identifiable name, of the attesting party to the caller. Even in first party cases, 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 be determined 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.¶

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.¶

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

13. Using "rcd", "rcdi", "crn" 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.¶

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"}
}

14. 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 16.3. 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.¶

15. 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.¶

16. IANA Considerations

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 dereferencing and download of any RCD URI linked resources as part of verification either in-network or on device could provide some level of information about calling patterns, so this should be considered when making these resources available.¶

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.¶

As general guidance, the use of URLs and URIs that reference potentially dangerous or intentionally harmful content should be considered in implimenting this specification. [RFC3986] Section 7 contains good additional guidance to consider when communicating or dereferencing URLs and URIs.¶

Authors' Addresses