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rfc:rfc9493



Internet Engineering Task Force (IETF) A. Backman, Ed. Request for Comments: 9493 Amazon Category: Standards Track M. Scurtescu ISSN: 2070-1721 Coinbase

                                                               P. Jain
                                                                Fastly
                                                         December 2023
           Subject Identifiers for Security Event Tokens

Abstract

 Security events communicated within Security Event Tokens may support
 a variety of identifiers to identify subjects related to the event.
 This specification formalizes the notion of Subject Identifiers as
 structured information that describes a subject and named formats
 that define the syntax and semantics for encoding Subject Identifiers
 as JSON objects.  It also establishes a registry for defining and
 allocating names for such formats as well as the JSON Web Token (JWT)
 "sub_id" Claim.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9493.

Copyright Notice

 Copyright (c) 2023 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Revised BSD License text as described in Section 4.e of the
 Trust Legal Provisions and are provided without warranty as described
 in the Revised BSD License.

Table of Contents

 1.  Introduction
 2.  Notational Conventions
   2.1.  Definitions
 3.  Subject Identifiers
   3.1.  Identifier Formats versus Principal Types
   3.2.  Identifier Format Definitions
     3.2.1.  Account Identifier Format
     3.2.2.  Email Identifier Format
     3.2.3.  Issuer and Subject Identifier Format
     3.2.4.  Opaque Identifier Format
     3.2.5.  Phone Number Identifier Format
     3.2.6.  Decentralized Identifier (DID) Format
     3.2.7.  Uniform Resource Identifier (URI) Format
     3.2.8.  Aliases Identifier Format
 4.  Subject Identifiers in JWTs
   4.1.  JWT "sub_id" Claim
   4.2.  JWT "sub_id" Claim and "iss_sub" Subject Identifier
 5.  Considerations for Specifications that Define Identifier
         Formats
 6.  Privacy Considerations
   6.1.  Identifier Correlation
 7.  Security Considerations
 8.  IANA Considerations
   8.1.  Security Event Identifier Formats Registry
     8.1.1.  Registration Template
     8.1.2.  Initial Registry Contents
     8.1.3.  Guidance for Expert Reviewers
   8.2.  JSON Web Token Claims Registration
     8.2.1.  Registry Contents
 9.  References
   9.1.  Normative References
   9.2.  Informative References
 Acknowledgements
 Authors' Addresses

1. Introduction

 As described in Section 1.2 of [RFC8417] ("Security Event Token
 (SET)"), subjects related to security events may take a variety of
 forms, including but not limited to a JWT [RFC7519] principal, an IP
 address, a URL, etc.  Different types of subjects may need to be
 identified in different ways (e.g., a user might be identified by an
 email address, a phone number, or an account number).  Furthermore,
 even in the case where the type of the subject is known, there may be
 multiple ways by which a given subject may be identified.  For
 example, an account may be identified by an opaque identifier, an
 email address, a phone number, a JWT "iss" Claim and "sub" Claim,
 etc., depending on the nature and needs of the transmitter and
 receiver.  Even within the context of a given transmitter and
 receiver relationship, it may be appropriate to identify different
 accounts in different ways, for example, if some accounts only have
 email addresses associated with them while others only have phone
 numbers.  Therefore, it can be necessary to indicate within a SET the
 mechanism by which a subject is being identified.
 To address this problem, this specification defines Subject
 Identifiers as JSON [RFC8259] objects containing information
 identifying a subject and defines Identifier Formats as named sets of
 rules describing how to encode different kinds of subject-identifying
 information (e.g., an email address or an issuer and subject pair) as
 a Subject Identifier.
 Below is a non-normative example of a Subject Identifier that
 identifies a subject by email address, using the Email Identifier
 Format.
 {
   "format": "email",
   "email": "user@example.com"
 }
    Figure 1: Example: Subject Identifier Using the Email Identifier
                                 Format
 Subject Identifiers are intended to be a general-purpose mechanism
 for identifying subjects within JSON objects, and their usage need
 not be limited to SETs.  Below is a non-normative example of a JWT
 that uses a Subject Identifier in the JWT "sub_id" Claim (defined in
 this specification) to identify the JWT Subject.
 {
   "iss": "issuer.example.com",
   "sub_id": {
     "format": "phone_number",
     "phone_number": "+12065550100"
   }
 }
     Figure 2: Example: JWT Using a Subject Identifier with the JWT
                             "sub_id" Claim
 Usage of Subject Identifiers also need not be limited to identifying
 JWT Subjects.  They are intended as a general-purpose means of
 expressing identifying information in an unambiguous manner.  Below
 is a non-normative example of a SET containing a hypothetical
 security event describing the interception of a message, using
 Subject Identifiers to identify the sender, intended recipient, and
 interceptor.
 {
   "iss": "issuer.example.com",
   "iat": 1508184845,
   "aud": "aud.example.com",
   "events": {
     "https://secevent.example.com/events/message-interception": {
       "from": {
         "format": "email",
         "email": "alice@example.com"
       },
       "to": {
         "format": "email",
         "email": "bob@example.com"
       },
       "interceptor": {
         "format": "email",
         "email": "eve@example.com"
       }
     }
   }
 }
    Figure 3: Example: SET with an Event Payload Containing Multiple
                          Subject Identifiers

2. Notational Conventions

 The key words "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.

2.1. Definitions

 This specification utilizes terminology defined in [RFC8259] and
 [RFC8417].
 Within this specification, the terms "Subject" and "subject" refer
 generically to anything being identified via one or more pieces of
 information.  The term "JWT Subject" refers specifically to the
 subject of a JWT (i.e., the subject that the JWT asserts claims
 about).

3. Subject Identifiers

 A Subject Identifier is a JSON object [RFC8259] whose contents may be
 used to identify a subject within some context.  An Identifier Format
 is a named definition of a set of information that may be used to
 identify a subject and the rules for encoding that information as a
 Subject Identifier; these rules define the syntax and semantics of
 Subject Identifiers.  A Subject Identifier MUST conform to a specific
 Identifier Format and MUST contain a "format" member whose value is
 the name of that Identifier Format.
 Every Identifier Format MUST have a unique name registered in the
 IANA "Security Event Identifier Formats" registry established in
 Section 8.1 or a Collision-Resistant Name as defined in [RFC7519].
 Identifier Formats that are expected to be used broadly by a variety
 of parties SHOULD be registered in the "Security Event Identifier
 Formats" registry.
 An Identifier Format MAY describe more members than are strictly
 necessary to identify a subject and MAY describe conditions under
 which those members are required, optional, or prohibited.  The
 "format" member is reserved for use as described in this
 specification; Identifier Formats MUST NOT declare any rules
 regarding the "format" member.
 Every member within a Subject Identifier MUST match the rules
 specified for that member by this specification or by a Subject
 Identifier's Identifier Format.  A Subject Identifier MUST NOT
 contain any members prohibited or not described by its Identifier
 Format and MUST contain all members required by its Identifier
 Format.

3.1. Identifier Formats versus Principal Types

 Identifier Formats define how to encode identifying information for a
 subject.  Unlike Principal Types, they do not define the type or
 nature of the subject itself.  For example, while the Email
 Identifier Format declares that the value of the "email" member is an
 email address, a subject in a security event that is identified by an
 Email Subject Identifier could be an end user who controls that email
 address, the mailbox itself, or anything else that the transmitter
 and receiver both understand to be associated with that email
 address.  Consequently, Subject Identifiers remove ambiguity around
 how a subject is being identified and how to parse an identifying
 structure, but they do not remove ambiguity around how to resolve
 that identifier for a subject.  For example, consider a directory
 management API that allows callers to identify users and groups
 through both opaque unique identifiers and email addresses.  Such an
 API could use Subject Identifiers to disambiguate between which of
 these two types of identifiers is in use.  However, the API would
 have to determine whether the subject is a user or group via some
 other means, such as by querying a database, interpreting other
 parameters in the request, or inferring the type from the API
 contract.

3.2. Identifier Format Definitions

 The following Identifier Formats are registered in the IANA "Security
 Event Identifier Formats" registry established in Section 8.1.
 Since the Subject Identifier Format conveys semantic information,
 applications SHOULD choose the most specific possible format for the
 identifier in question.  For example, an email address can be
 conveyed using a "mailto:" URI and the URI Identifier Format, but
 since the value is known to be an email address, the application
 should prefer to use the Email Identifier Format instead.

3.2.1. Account Identifier Format

 The Account Identifier Format identifies a subject using an account
 at a service provider, identified with an "acct" URI as defined in
 [RFC7565].  An account is an arrangement or agreement through which a
 user gets access to a service and gets a unique identity with the
 service provider.  Subject Identifiers in this format MUST contain a
 "uri" member whose value is the "acct" URI for the subject.  The
 "uri" member is REQUIRED and MUST NOT be null or empty.  The Account
 Identifier Format is identified by a value of "account" in the
 "format" member.
 Below is a non-normative example Subject Identifier for the Account
 Identifier Format:
 {
   "format": "account",
   "uri": "acct:example.user@service.example.com"
 }
    Figure 4: Example: Subject Identifier for the Account Identifier
                                 Format

3.2.2. Email Identifier Format

 The Email Identifier Format identifies a subject using an email
 address.  Subject Identifiers in this format MUST contain an "email"
 member whose value is a string containing the email address of the
 subject, formatted as an "addr-spec" as defined in Section 3.4.1 of
 [RFC5322].  The "email" member is REQUIRED and MUST NOT be null or
 empty.  The value of the "email" member MUST identify a mailbox to
 which email may be delivered, in accordance with [RFC5321].  The
 Email Identifier Format is identified by the name "email".
 Below is a non-normative example Subject Identifier in the Email
 Identifier Format:
 {
   "format": "email",
   "email": "user@example.com"
 }
  Figure 5: Example: Subject Identifier in the Email Identifier Format

3.2.2.1. Email Canonicalization

 Many email providers will treat multiple email addresses as
 equivalent.  While the domain portion of an email address [RFC5322]
 is consistently treated as case-insensitive per [RFC1034], most
 providers treat the local part of the email address as case-
 insensitive as well and consider "user@example.com",
 "User@example.com", and "USER@example.com" as the same email address.
 Some providers also treat dots (".") as optional; for example,
 "user.name@example.com", "username@example.com",
 "u.s.e.r.name@example.com", and "u.s.e.r.n.a.m.e@example.com" might
 all be treated as equivalent.  This has led users to view these
 strings as equivalent, driving service providers to implement
 proprietary email canonicalization algorithms to ensure that email
 addresses entered by users resolve to the same canonical string.
 Email canonicalization is not standardized, and there is no way for
 the event recipient to determine the mail provider's canonicalization
 method.  Therefore, the recipient SHOULD apply its own
 canonicalization algorithm to incoming events in order to reproduce
 the translation done by the local email system.

3.2.3. Issuer and Subject Identifier Format

 The Issuer and Subject Identifier Format identifies a subject using a
 pair of "iss" and "sub" members, analogous to how subjects are
 identified using the JWT "iss" and "sub" Claims in OpenID Connect
 [OpenID.Core] ID Tokens.  These members MUST follow the formats of
 the "iss" member and "sub" member defined by [RFC7519], respectively.
 Both the "iss" member and the "sub" member are REQUIRED and MUST NOT
 be null or empty.  The Issuer and Subject Identifier Format is
 identified by the name "iss_sub".
 Below is a non-normative example Subject Identifier in the Issuer and
 Subject Identifier Format:
 {
   "format": "iss_sub",
   "iss": "https://issuer.example.com/",
   "sub": "145234573"
 }
    Figure 6: Example: Subject Identifier in the Issuer and Subject
                           Identifier Format

3.2.4. Opaque Identifier Format

 The Opaque Identifier Format describes a subject that is identified
 with a string with no semantics asserted beyond its usage as an
 identifier for the subject, such as a Universally Unique Identifier
 (UUID) or hash used as a surrogate identifier for a record in a
 database.  Subject Identifiers in this format MUST contain an "id"
 member whose value is a JSON string containing the opaque string
 identifier for the subject.  The "id" member is REQUIRED and MUST NOT
 be null or empty.  The Opaque Identifier Format is identified by the
 name "opaque".
 Below is a non-normative example Subject Identifier in the Opaque
 Identifier Format:
 {
   "format": "opaque",
   "id": "11112222333344445555"
 }
 Figure 7: Example: Subject Identifier in the Opaque Identifier Format

3.2.5. Phone Number Identifier Format

 The Phone Number Identifier Format identifies a subject using a
 telephone number.  Subject Identifiers in this format MUST contain a
 "phone_number" member whose value is a string containing the full
 telephone number of the subject, including an international dialing
 prefix, formatted according to E.164 [E164].  The "phone_number"
 member is REQUIRED and MUST NOT be null or empty.  The Phone Number
 Identifier Format is identified by the name "phone_number".
 Below is a non-normative example Subject Identifier in the Phone
 Number Identifier Format:
 {
   "format": "phone_number",
   "phone_number": "+12065550100"
 }
       Figure 8: Example: Subject Identifier in the Phone Number
                           Identifier Format

3.2.6. Decentralized Identifier (DID) Format

 The Decentralized Identifier (DID) Format identifies a subject using
 a DID URL as defined in [DID].  Subject Identifiers in this format
 MUST contain a "url" member whose value is a DID URL for the DID
 Subject being identified.  The value of the "url" member MUST be a
 valid DID URL and MAY be a bare DID.  The "url" member is REQUIRED
 and MUST NOT be null or empty.  The Decentralized Identifier Format
 is identified by the name "did".
 Below are non-normative example Subject Identifiers for the
 Decentralized Identifier Format:
 {
   "format": "did",
   "url": "did:example:123456"
 }
      Figure 9: Example: Subject Identifier for the Decentralized
        Identifier Format, Identifying a Subject with a Bare DID
 {
   "format": "did",
   "url": "did:example:123456/did/url/path?versionId=1"
 }
      Figure 10: Example: Subject Identifier for the Decentralized
   Identifier Format, Identifying a Subject with a DID URL with Non-
                    empty Path and Query Components

3.2.7. Uniform Resource Identifier (URI) Format

 The Uniform Resource Identifier (URI) Format identifies a subject
 using a URI as defined in [RFC3986].  This Identifier Format makes no
 assumptions or guarantees with regard to the content, scheme, or
 reachability of the URI within the field.  Subject Identifiers in
 this format MUST contain a "uri" member whose value is a URI for the
 subject being identified.  The "uri" member is REQUIRED and MUST NOT
 be null or empty.  The URI Format is identified by the name "uri".
 Below are non-normative example Subject Identifiers for the URI
 Format:
 {
   "format": "uri",
   "uri": "https://user.example.com/"
 }
       Figure 11: Example: Subject Identifier for the URI Format,
                Identifying a Subject with a Website URI
 {
   "format": "uri",
   "uri": "urn:uuid:4e851e98-83c4-4743-a5da-150ecb53042f"
 }
       Figure 12: Example: Subject Identifier for the URI Format,
                Identifying a Subject with a Random URN

3.2.8. Aliases Identifier Format

 The Aliases Identifier Format describes a subject that is identified
 with a list of different Subject Identifiers.  It is intended for use
 when a variety of identifiers have been shared with the party that
 will be interpreting the Subject Identifier, and it is unknown which
 of those identifiers they will recognize or support.  Subject
 Identifiers in this format MUST contain an "identifiers" member whose
 value is a JSON array containing one or more Subject Identifiers.
 Each Subject Identifier in the array MUST identify the same entity.
 The "identifiers" member is REQUIRED and MUST NOT be null or empty.
 It MAY contain multiple instances of the same Identifier Format
 (e.g., multiple Email Subject Identifiers) but SHOULD NOT contain
 exact duplicates.  This format is identified by the name "aliases".
 "aliases" Subject Identifiers MUST NOT be nested, i.e., the
 "identifiers" member of an "aliases" Subject Identifier MUST NOT
 contain a Subject Identifier in the Aliases Identifier Format.
 Below is a non-normative example Subject Identifier in the Aliases
 Identifier Format:
 {
   "format": "aliases",
   "identifiers": [
     {
       "format": "email",
       "email": "user@example.com"
     },
     {
       "format": "phone_number",
       "phone_number": "+12065550100"
     },
     {
       "format": "email",
       "email": "user+qualifier@example.com"
     }
   ]
 }
    Figure 13: Example: Subject Identifier in the Aliases Identifier
                                 Format

4. Subject Identifiers in JWTs

4.1. JWT "sub_id" Claim

 The JWT "sub" Claim is defined in Section 4.1.2 of [RFC7519] as
 containing a string value; therefore, it cannot contain a Subject
 Identifier (which is a JSON object) as its value.  This document
 defines the JWT "sub_id" Claim, in accordance with Section 4.2 of
 [RFC7519], as a common claim that identifies the JWT Subject using a
 Subject Identifier.  When present, the value of this claim MUST be a
 Subject Identifier that identifies the subject of the JWT.  The JWT
 "sub_id" Claim MAY be included in a JWT, whether or not the JWT "sub"
 Claim is present.  When both the JWT "sub" and "sub_id" Claims are
 present in a JWT, they MUST identify the same subject, as a JWT has
 one and only one JWT Subject.
 When processing a JWT with both JWT "sub" and "sub_id" Claims,
 implementations MUST NOT rely on both claims to determine the JWT
 Subject.  An implementation MAY attempt to determine the JWT Subject
 from one claim and fall back to using the other if it determines it
 does not understand the format of the first claim.  For example, an
 implementation may attempt to use "sub_id" and fall back to using
 "sub" upon finding that "sub_id" contains a Subject Identifier with a
 format that is not recognized by the implementation.
 Below are non-normative examples of JWTs containing the JWT "sub_id"
 Claim:
 {
   "iss": "issuer.example.com",
   "sub_id": {
     "format": "email",
     "email": "user@example.com"
   }
 }
     Figure 14: Example: JWT Containing a JWT "sub_id" Claim and No
                              "sub" Claim
 {
   "iss": "issuer.example.com",
   "sub": "user@example.com",
   "sub_id": {
     "format": "email",
     "email": "user@example.com"
   }
 }
    Figure 15: Example: JWT Where the JWT "sub" and "sub_id" Claims
           Identify the JWT Subject Using the Same Identifier
 {
   "iss": "issuer.example.com",
   "sub": "liz@example.com",
   "sub_id": {
     "format": "email",
     "email": "elizabeth@example.com"
   }
 }
    Figure 16: Example: JWT Where the JWT "sub" and "sub_id" Claims
      Identify the JWT Subject Using Different Values of the Same
                            Identifier Type
 {
   "iss": "issuer.example.com",
   "sub": "user@example.com",
   "sub_id": {
     "format": "account",
     "uri": "acct:example.user@service.example.com"
   }
 }
    Figure 17: Example: JWT Where the JWT "sub" and "sub_id" Claims
      Identify the JWT Subject via Different Types of Identifiers

4.2. JWT "sub_id" Claim and "iss_sub" Subject Identifier

 The JWT "sub_id" Claim MAY contain an "iss_sub" Subject Identifier.
 In this case, the JWT's "iss" Claim and the Subject Identifier's
 "iss" member MAY be different.  For example, an OpenID Connect
 [OpenID.Core] client may construct such a JWT when sending JWTs back
 to its OpenID Connect Identity Provider in order to identify the JWT
 Subject using an identifier known to be understood by both parties.
 Similarly, the JWT's "sub" Claim and the Subject Identifier's "sub"
 member MAY be different.  For example, this may be used by an OpenID
 Connect client to communicate the JWT Subject's local identifier at
 the client back to its Identity Provider.
 Below are non-normative examples of a JWT where the JWT "iss" Claim
 and "iss" member within the JWT "sub_id" Claim are the same and a JWT
 where they are different.
 {
   "iss": "issuer.example.com",
   "sub_id": {
     "format": "iss_sub",
     "iss": "issuer.example.com",
     "sub": "example_user"
   }
 }
      Figure 18: Example: JWT with an "iss_sub" Subject Identifier
        Where the JWT Issuer and JWT Subject Issuer Are the Same
 {
   "iss": "client.example.com",
   "sub_id": {
     "format": "iss_sub",
     "iss": "issuer.example.com",
     "sub": "example_user"
   }
 }
      Figure 19: Example: JWT with an "iss_sub" Subject Identifier
       Where the JWT Issuer and JWT Subject Issuer Are Different
 {
   "iss": "client.example.com",
   "sub": "client_user",
   "sub_id": {
     "format": "iss_sub",
     "iss": "issuer.example.com",
     "sub": "example_user"
   }
 }
      Figure 20: Example: JWT with an "iss_sub" Subject Identifier
        Where the JWT "iss" and "sub" Claims Differ from the JWT
                   Subject's "iss" and "sub" Members

5. Considerations for Specifications that Define Identifier Formats

 Identifier Format definitions MUST NOT make assertions or
 declarations regarding the subject being identified by the Subject
 Identifier (e.g., an Identifier Format cannot be defined as
 specifically identifying human end users).  Such statements are
 outside the scope of Identifier Formats and Subject Identifiers.
 Expanding that scope for some Identifier Formats but not others would
 harm interoperability because applications that depend on this
 expanded scope to disambiguate the subject type would be unable to
 use Identifier Formats that do not provide such rules.

6. Privacy Considerations

6.1. Identifier Correlation

 The act of presenting two or more identifiers for a single subject
 together (e.g., within an "aliases" Subject Identifier or via the JWT
 "sub" and "sub_id" Claims) may communicate more information about the
 subject than was intended.  For example, the entity to which the
 identifiers are presented now knows that both identifiers relate to
 the same subject and may be able to correlate additional data based
 on that.  When transmitting Subject Identifiers, the transmitter
 SHOULD take care that they are only transmitting multiple identifiers
 together when it is known that the recipient already knows that the
 identifiers are related (e.g., because they were previously sent to
 the recipient as claims in an OpenID Connect ID Token) or when
 correlation is essential to the use case.  Implementers must consider
 such risks, and specifications that use Subject Identifiers must
 provide appropriate privacy considerations of their own.
 The considerations described in Section 6 of [RFC8417] also apply
 when Subject Identifiers are used within SETs.  The considerations
 described in Section 12 of [RFC7519] also apply when Subject
 Identifiers are used within JWTs.

7. Security Considerations

 This specification does not define any mechanism for ensuring the
 confidentiality or integrity of a Subject Identifier.  Where such
 properties are required, implementations MUST use mechanisms provided
 by the containing format (e.g., integrity protecting SETs or JWTs
 using JSON Web Signature (JWS) [RFC7515]) or at the transport layer
 or other layer in the application stack (e.g., using TLS [RFC8446]).
 Further considerations regarding confidentiality and integrity of
 SETs can be found in Section 5.1 of [RFC8417].

8. IANA Considerations

8.1. Security Event Identifier Formats Registry

 This document defines Identifier Formats, for which IANA has created
 and maintains a new registry titled "Security Event Identifier
 Formats".  Initial values for the "Security Event Identifier Formats"
 registry are given in Section 3.  Future assignments are to be made
 through the Specification Required registration policy [BCP26] and
 shall follow the template presented in Section 8.1.1.
 It is suggested that multiple designated experts be appointed who are
 able to represent the perspectives of different applications using
 this specification in order to enable broadly informed review of
 registration decisions.

8.1.1. Registration Template

 Format Name:
    The name of the Identifier Format, as described in Section 3.  The
    name MUST be an ASCII string consisting only of lowercase
    characters ("a" - "z"), digits ("0" - "9"), underscores ("_"), and
    hyphens ("-") and SHOULD NOT exceed 20 characters in length.
 Format Description:
    A brief description of the Identifier Format.
 Change Controller:
    For formats defined in documents published by the IETF or its
    working groups, list "IETF".  For all other formats, list the name
    of the party responsible for the registration.  Contact
    information, such as mailing address, email address, or phone
    number, must also be provided.
 Reference:
    A reference to the document or documents that define the
    Identifier Format.  The reference document(s) MUST specify the
    name, format, and meaning of each member that may occur within a
    Subject Identifier of the defined format as well as whether each
    member is optional, required, or conditional and the circumstances
    under which these optional or conditional fields would be used.
    URIs that can be used to retrieve copies of each document SHOULD
    be included.

8.1.2. Initial Registry Contents

8.1.2.1. Account Identifier Format

 Format Name:  account
 Format Description:  Subject Identifier based on "acct" URI
 Change Controller:  IETF
 Reference:  Section 3 of RFC 9493

8.1.2.2. Email Identifier Format

 Format Name:  email
 Format Description:  Subject Identifier based on an email address
 Change Controller:  IETF
 Reference:  Section 3 of RFC 9493

8.1.2.3. Issuer and Subject Identifier Format

 Format Name:  iss_sub
 Format Description:  Subject Identifier based on an issuer and
    subject
 Change Controller:  IETF
 Reference:  Section 3 of RFC 9493

8.1.2.4. Opaque Identifier Format

 Format Name:  opaque
 Format Description:  Subject Identifier based on an opaque string
 Change Controller:  IETF
 Reference:  Section 3 of RFC 9493

8.1.2.5. Phone Number Identifier Format

 Format Name:  phone_number
 Format Description:  Subject Identifier based on a phone number
 Change Controller:  IETF
 Reference:  Section 3 of RFC 9493

8.1.2.6. Decentralized Identifier Format

 Format Name:  did
 Format Description:  Subject Identifier based on a decentralized
    identifier (DID)
 Change Controller:  IETF
 Reference:  Section 3 of RFC 9493

8.1.2.7. Uniform Resource Identifier Format

 Format Name:  uri
 Format Description:  Subject Identifier based on a Uniform Resource
    Identifier (URI)
 Change Controller:  IETF
 Reference:  Section 3 of RFC 9493

8.1.2.8. Aliases Identifier Format

 Format Name:  aliases
 Format Description:  Subject Identifier that groups together multiple
    different Subject Identifiers for the same subject
 Change Controller:  IETF
 Reference:  Section 3 of RFC 9493

8.1.3. Guidance for Expert Reviewers

 The Expert Reviewer is expected to review the documentation
 referenced in a registration request to verify its completeness.  The
 Expert Reviewer must base their decision to accept or reject the
 request on a fair and impartial assessment of the request.  If the
 Expert Reviewer has a conflict of interest, such as being an author
 of a defining document referenced by the request, they must recuse
 themselves from the approval process for that request.
 Identifier Formats need not be generally applicable and may be highly
 specific to a particular domain; it is expected that formats may be
 registered for niche or industry-specific use cases.  The Expert
 Reviewer should focus on whether the format is thoroughly documented
 and whether its registration will promote or harm interoperability.
 In most cases, the Expert Reviewer should not approve a request if
 the registration would contribute to confusion or amount to a synonym
 for an existing format.

8.2. JSON Web Token Claims Registration

 This document defines the JWT "sub_id" Claim, which IANA has
 registered in the "JSON Web Token Claims" registry [IANA.JWT.Claims]
 established by [RFC7519].

8.2.1. Registry Contents

 Claim Name:  sub_id
 Claim Description:  Subject Identifier
 Change Controller:  IETF
 Reference:  Section 4.1 of RFC 9493

9. References

9.1. Normative References

 [BCP26]    Cotton, M., Leiba, B., and T. Narten, "Guidelines for
            Writing an IANA Considerations Section in RFCs", BCP 26,
            RFC 8126, June 2017.
            <https://www.rfc-editor.org/info/bcp26>
 [DID]      Sporny, M., Ed., Guy, A., Ed., Sabadello, M., Ed., Reed,
            D., Ed., Longley, D., Steele, O., and C. Allen,
            "Decentralized Identifiers (DIDs) v1.0", July 2022,
            <https://www.w3.org/TR/did-core/>.
 [E164]     ITU-T, "E.164: The international public telecommunication
            numbering plan", ITU-T Recommendation E.164, November
            2010, <https://www.itu.int/rec/T-REC-E.164-201011-I/en>.
 [IANA.JWT.Claims]
            IANA, "JSON Web Token Claims",
            <https://www.iana.org/assignments/jwt>.
 [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>.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, DOI 10.17487/RFC3986, January 2005,
            <https://www.rfc-editor.org/info/rfc3986>.
 [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
            DOI 10.17487/RFC5321, October 2008,
            <https://www.rfc-editor.org/info/rfc5321>.
 [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
            DOI 10.17487/RFC5322, October 2008,
            <https://www.rfc-editor.org/info/rfc5322>.
 [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>.
 [RFC7565]  Saint-Andre, P., "The 'acct' URI Scheme", RFC 7565,
            DOI 10.17487/RFC7565, May 2015,
            <https://www.rfc-editor.org/info/rfc7565>.
 [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>.
 [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
            Interchange Format", STD 90, RFC 8259,
            DOI 10.17487/RFC8259, December 2017,
            <https://www.rfc-editor.org/info/rfc8259>.
 [RFC8417]  Hunt, P., Ed., Jones, M., Denniss, W., and M. Ansari,
            "Security Event Token (SET)", RFC 8417,
            DOI 10.17487/RFC8417, July 2018,
            <https://www.rfc-editor.org/info/rfc8417>.

9.2. Informative References

 [OpenID.Core]
            Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
            C. Mortimore, "OpenID Connect Core 1.0 incorporating
            errata set 1", November 2014,
            <https://openid.net/specs/openid-connect-core-1_0.html>.
 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
            <https://www.rfc-editor.org/info/rfc1034>.
 [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
            Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
            2015, <https://www.rfc-editor.org/info/rfc7515>.
 [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
            Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
            <https://www.rfc-editor.org/info/rfc8446>.

Acknowledgements

 The authors would like to thank the members of the IETF Security
 Events Working Group, as well as those of the OpenID Shared Signals
 and Events Working Group, whose work provided the original basis for
 this document.  We would also like to acknowledge Aaron Parecki,
 Denis Pinkas, Justin Richer, Mike Jones, and other members of the
 working group for reviewing this document.

Authors' Addresses

 Annabelle Backman (editor)
 Amazon
 Email: richanna@amazon.com
 Marius Scurtescu
 Coinbase
 Email: marius.scurtescu@coinbase.com
 Prachi Jain
 Fastly
 Email: prachi.jain1288@gmail.com
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