Subject Identifier

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Full Title or Meme

A Subject Identifier is an abstract definition of an Identifier that only identifies an entity that has private data or access to private resources on the web.



  • While the Subject ID (sub) is often taken to be a persistent Identifier for a real-world entity, there are several examples of its use as and Ephemeral ID.


  • Users are often asked to use their email address or cell phone number as a local user name since the email address and phone number with country code (+1 in North America) are known to be a URI and hence unique in that context. Reuse of email and phone numbers could be an issue. Note that these Identifiers are also subject to change at the user's discretion and can be diverted by a determined attacker.
  • This wiki uses Subject ID primarily as identifying the subject of a claim about a real-world entity. It is subject to revocation by the user or the claim issuer at any time.

Other Definitions

  • From JWT[1]
    The "sub" (subject) claim identifies the principal that is the subject of the JWT. The Claims in a JWT are normally statements about the subject. The subject value MUST either be scoped to be locally unique in the context of the issuer or be globally unique. The processing of this claim is generally application specific. The "sub" value is a case-sensitive string containing a StringOrURI value. Use of this claim is OPTIONAL.
  • From Security Event Token
    A Subject Identifier Type is a light-weight schema that describes a set of claims that identifies a subject. Every Subject Identifier Type MUST have a unique name registered in the IANA "Security Event Subject Identifier Types" registry established by Section 6.1 of Security Event Token. A Subject Identifier Type MAY describe more claims than are strictly necessary to identify a subject, and MAY describe conditions under which those claims are required, optional, or prohibited.


  1. Synonyms for a Subject ID include User Name, display name, gamertag, nom de guerre, Pseudonym, URI or (on Facebook) Name subject to arbitrary termination.
  2. Anonym is not used in the context of identity as it does not provide one. It may be used as the condition (Anonymous) of a user prior to accepting (1) a cookie, (2) a fixed IP address, (3) an HTTPS connection or (4) a request for an Identifier.
    1. M. Jones J. Bradley N. Sakimura, JSON Web Token (JWT). (2016-02) IETF