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Subject Identifiers for Security Event Tokens
draft-ietf-secevent-subject-identifiers-12

Document Type Active Internet-Draft (secevent WG)
Authors Annabelle Backman , Marius Scurtescu , Prachi Jain
Last updated 2022-08-05 (Latest revision 2022-07-24)
Replaces draft-backman-secevent-subject-identifiers
Stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Formats
Stream WG state Submitted to IESG for Publication
Document shepherd Yaron Sheffer
Shepherd write-up Show Last changed 2022-08-05
IESG IESG state Publication Requested
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Responsible AD Roman Danyliw
Send notices to yaronf.ietf@gmail.com
draft-ietf-secevent-subject-identifiers-12
Security Events Working Group                            A. Backman, Ed.
Internet-Draft                                                    Amazon
Intended status: Standards Track                            M. Scurtescu
Expires: 25 January 2023                                        Coinbase
                                                                 P. Jain
                                                                  Fastly
                                                            24 July 2022

             Subject Identifiers for Security Event Tokens
               draft-ietf-secevent-subject-identifiers-12

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 describe a subject, and named formats
   that define the syntax and semantics for encoding subject identifiers
   as JSON objects.  It also defines a registry for defining and
   allocating names for such formats, as well as the sub_id JSON Web
   Token (JWT) claim.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25 January 2023.

Copyright Notice

   Copyright (c) 2022 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.

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   5
     2.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Subject Identifiers . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Identifier Formats versus Principal Types . . . . . . . .   6
     3.2.  Identifier Format Definitions . . . . . . . . . . . . . .   6
       3.2.1.  Account Identifier Format . . . . . . . . . . . . . .   6
       3.2.2.  Email Identifier Format . . . . . . . . . . . . . . .   7
       3.2.3.  Issuer and Subject Identifier Format  . . . . . . . .   8
       3.2.4.  Opaque Identifier Format  . . . . . . . . . . . . . .   8
       3.2.5.  Phone Number Identifier Format  . . . . . . . . . . .   9
       3.2.6.  Decentralized Identifier (DID) Format . . . . . . . .   9
       3.2.7.  Uniform Resource Identifier (URI) Format  . . . . . .  10
       3.2.8.  Aliases Identifier Format . . . . . . . . . . . . . .  10
   4.  Subject Identifiers in JWTs . . . . . . . . . . . . . . . . .  11
     4.1.  sub_id Claim  . . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  sub_id and iss_sub Subject Identifiers  . . . . . . . . .  13
   5.  Considerations for Specifications that Define Identifier
           Formats . . . . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  14
     6.1.  Identifier Correlation  . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  Confidentiality and Integrity . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Security Event Identifier Formats Registry  . . . . . . .  15
       8.1.1.  Registry Location . . . . . . . . . . . . . . . . . .  16
       8.1.2.  Registration Template . . . . . . . . . . . . . . . .  16
       8.1.3.  Initial Registry Contents . . . . . . . . . . . . . .  16
       8.1.4.  Guidance for Expert Reviewers . . . . . . . . . . . .  18
     8.2.  JSON Web Token Claims Registration  . . . . . . . . . . .  19
       8.2.1.  Registry Contents . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  21
   Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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1.  Introduction

   As described in Section 1.2 of SET [RFC8417], 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 host might be identified by an IP or MAC address, while
   a user might be identified by an email address).  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 - JSON [RFC7159] objects containing information
   identifying a subject - and Identifier Formats - 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 sub_id claim (defined in this
   specification) to identify the JWT Subject.

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   {
     "iss": "issuer.example.com",
     "sub_id": {
       "format": "phone_number",
       "phone_number": "+12065550100"
     }
   }

         Figure 2: Example: JWT using a Subject Identifier with the
                               "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

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2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.1.  Definitions

   This specification utilizes terminology defined in [RFC7159] 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 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 [RFC7159] object 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; they 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 by
   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 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.

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3.1.  Identifier Formats versus Principal Types

   Identifier Formats define how to encode identifying information for a
   subject.  They do not define the type or nature of the subject
   itself.  E.g., 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 do not
   remove ambiguity around how to resolve that identifier to 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 by 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].  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 the name account.

   Below is a non-normative example Subject Identifier for the Account
   Identifier Format:

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   {
     "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 SHOULD 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 [RFC5322] email address
   is consistently treated as case-insensitive per [RFC1034], some
   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.
   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.  When receiving an Email Subject
   Identifier, the recipient SHOULD use their implementation's
   canonicalization algorithm to resolve the email address to the same
   string used in their system.

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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, analagous to how subjects are identified
   using the 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": "http://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 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

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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 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 Email
   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 Format identifies a subject using a
   Decentralized Identifier (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"
   }

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        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 members 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.

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   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 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.  sub_id Claim

   The sub JWT Claim is defined in Section 4.1.2 of [RFC7519] as
   containing a string value, and therefore cannot contain a Subject
   Identifier (which is a JSON object) as its value.  This document
   defines the sub_id JWT 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
   sub_id claim MAY be included in a JWT, whether or not the sub claim
   is present.  When both the 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 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

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   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 whose format
   is not recognized by the implementation.

   Below are non-normative examples of JWTs containing the sub_id claim:

   {
     "iss": "issuer.example.com",
     "sub_id": {
       "format": "email",
       "email": "user@example.com"
     }
   }

      Figure 14: Example: JWT containing a "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 both the "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 both the "sub" and "sub_id" claims
        identify the JWT Subject using different values of the same
                              identifier type

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   {
     "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 "sub" and "sub_id" claims
        identify the JWT Subject via different types of identifiers

4.2.  sub_id and iss_sub Subject Identifiers

   The 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, in 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 iss claim and iss
   member within the 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 JWT issuer and JWT Subject issuer are the same

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   {
     "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), as such statements are
   outside the scope of Identifier Formats and Subject Identifiers, and
   expanding that scope for some Identifier Formats but not others would
   harm interoperability, as 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 sub
   and sub_id JWT 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

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   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 specs 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

7.1.  Confidentiality and Integrity

   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 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 is asked to
   create and maintain 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 Expert Review registration policy [BCP26] and shall
   follow the template presented in Section 8.1.2.

   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.  In cases where a registration decision could
   be perceived as creating a conflict of interest for a particular
   Expert, that Expert should defer to the judgment of the other
   Experts.

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8.1.1.  Registry Location

   (This section to be removed by the RFC Editor before publication as
   an RFC.)

   The authors recommend that the Identifier Formats registry be located
   at https://www.iana.org/assignments/secevent/.

8.1.2.  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 lower-case
      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 may also be provided.

   Defining Document(s)
      A reference to the document or documents that define the
      Identifier Format.  The definition 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, prohibited, or the circumstances under
      which the member may be optional, required, or prohibited.  URIs
      that can be used to retrieve copies of each document SHOULD be
      included.

8.1.3.  Initial Registry Contents

8.1.3.1.  Account Identifier Format

   *  Format Name: "account"

   *  Format Description: Subject identifier based on acct URI.

   *  Change Controller: IETF

   *  Defining Document(s): Section 3 of this document.

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8.1.3.2.  Email Identifier Format

   *  Format Name: email

   *  Format Description: Subject identifier based on email address.

   *  Change Controller: IETF

   *  Defining Document(s): Section 3 of this document.

8.1.3.3.  Issuer and Subject Identifier Format

   *  Format Name: "iss_sub"

   *  Format Description: Subject identifier based on an issuer and
      subject.

   *  Change Controller: IETF

   *  Defining Document(s): Section 3 of this document.

8.1.3.4.  Opaque Identifier Format

   *  Format Name: "opaque"

   *  Format Description: Subject identifier based on an opaque string.

   *  Change Controller: IETF

   *  Defining Document(s): Section 3 of this document.

8.1.3.5.  Phone Number Identifier Format

   *  Format Name: "phone_number"

   *  Format Description: Subject identifier based on an phone number.

   *  Change Controller: IETF

   *  Defining Document(s): Section 3 of this document.

8.1.3.6.  Decentralized Identifier Format

   *  Format Name: "did"

   *  Format Description: Subject identifier based on a decentralized
      identifier (DID).

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   *  Change Controller: IETF

   *  Defining Document(s): Section 3 of this document.

8.1.3.7.  Uniform Resource Identifier Format

   *  Format Name: "uri"

   *  Format Description: Subject identifier based on a uniform resource
      identifier (URI).

   *  Change Controller: IETF

   *  Defining Document(s): Section 3 of this document.

8.1.3.8.  Aliases Identifier Format

   *  Format Name: "aliases"

   *  Format Description: Subject identifier that groups together
      multiple different subject identifiers for the same subject.

   *  Change Controller: IETF

   *  Defining Document(s): Section 3 of this document.

8.1.4.  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.  In the case
   where a request is rejected, the Expert Reviewer should provide the
   requesting party with a written statement expressing the reason for
   rejection, and be prepared to cite any sources of information that
   went into that decision.

   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.

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8.2.  JSON Web Token Claims Registration

   This document defines the sub_id JWT Claim, which IANA is asked to
   register in the "JSON Web Token Claims" registry IANA 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: IESG

   *  Specification Document(s): Section 4.1 of this document.

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, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [DID]      World Wide Web Consortium (W3C), "Decentralized
              Identifiers (DIDs) v1.0", 2021,
              <https://www.w3.org/TR/did-core/>.

   [E164]     International Telecommunication Union, "The international
              public telecommunication numbering plan", 2010,
              <http://www.itu.int/rec/T-REC-E.164-201011-I/en>.

   [IANA.JWT.Claims]
              IANA, "JSON Web Token Claims", n.d.,
              <http://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>.

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   [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>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7565]  Saint-Andre, P., "The 'acct' URI Scheme", RFC 7565,
              DOI 10.17487/RFC7565, May 2015,
              <https://www.rfc-editor.org/info/rfc7565>.

   [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", November 2014,
              <http://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>.

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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.

Change Log

   (This section to be removed by the RFC Editor before publication as
   an RFC.)

   Draft 00 - AB - First draft

   Draft 01 - AB:

   *  Added reference to RFC 5322 for format of email claim.

   *  Renamed iss_sub type to iss-sub.

   *  Renamed id_token_claims type to id-token-claims.

   *  Added text specifying the nature of the subjects described by each
      type.

   Draft 02 - AB:

   *  Corrected format of phone numbers in examples.

   *  Updated author info.

   Draft 03 - AB:

   *  Added account type for acct URIs.

   *  Replaced id-token-claims type with aliases type.

   *  Added email canonicalization guidance.

   *  Updated semantics for email, phone, and iss-sub types.

   Draft 04 - AB:

   *  Added sub_id JWT Claim definition, guidance, examples.

   *  Added text prohibiting aliases nesting.

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   *  Added privacy considerations for identifier correlation.

   Draft 05 - AB:

   *  Renamed the phone type to phone-number and its phone claim to
      phone_number.

   Draft 06 - AB:

   *  Replaced usage of the word "claim" to describe members of a
      Subject Identifier with the word "member", in accordance with
      terminology in RFC7159.

   *  Renamed the phone-number type to phone_number and iss-sub to
      iss_sub.

   *  Added normative requirements limiting the use of both sub and
      sub_id claims together when processing a JWT.

   *  Clarified that identifier correlation may be acceptable when it is
      a core part of the use case.

   *  Replaced references to OIDF with IETF in IANA Considerations.

   *  Recommended the appointment of multiple Designated Experts, and a
      location for the Subject Identifier Types registry.

   *  Added "_" to list of allowed characters in the Type Name for
      Subject Identifier Types.

   *  Clarified that Subject Identifiers don't provide confidentiality
      or integrity protection.

   *  Added references to SET, JWT privacy and security considerations.

   *  Added section describing the difference between subject identifier
      type and principal type that hopefully clarifies things and
      doesn't just muddy the water further.

   Draft 07 - AB:

   *  Emphasized that the spec is about identifiers, not the things they
      identify:

      -  Renamed "Subject Identifier Type" to "Identifier Format".

      -  Renamed subject_type to format.

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      -  Renamed "Security Event Subject Identifier Type Registry" to
         "Security Event Identifier Format Registry".

      -  Added new section with guidance for specs defining Identifier
         Formats, with normative prohibition on formats that describe
         the subject itself, rather than the identifier.

   *  Clarified the meaning of "subject":

      -  Defined "subject" as applying generically and "JWT Subject" as
         applying specifically to the subject of a JWT.

      -  Replaced most instances of the word "principal" with "subject".

   *  Added opaque Identifier Format

   Draft 08 - JR, AB:

   *  Added did Identifier Format

   *  Alphabetized identifier format definitions

   *  Replaced "type" with "format" in places that had been missed in
      the -07 change. (mostly IANA Considerations)

   *  Miscellaneous editorial fixes

   Draft 09 - AB:

   *  Miscellaneous editorial fixes

   Draft 10 - PJ:

   *  Added author

   *  Editorial nits

   Draft 11 - PJ:

   *  Miscellaneous editorial fixes

   *  Moved aliases to the last in identifier format definitions

   *  Acknowledged individual reviewers

   Draft 12 - PJ:

   *  Restore the DID format that was removed in -11

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   *  Added a generic "URI" format

   *  Normative advice on choosing the format

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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