4. JWT Claims
The JWT Claims Set represents a JSON object whose members are the claims conveyed by the JWT. The Claim Names within a JWT Claims Set MUST be unique; JWT parsers MUST either reject JWTs with duplicate Claim Names or use a JSON parser that returns only the lexically last duplicate member name, as specified in Section 15.12 ("The JSON Object") of ECMAScript 5.1 [ECMAScript].
The set of claims that a JWT must contain to be considered valid is context dependent and is outside the scope of this specification. Specific applications of JWTs will require implementations to understand and process some claims in particular ways. However, in the absence of such requirements, all claims that are not understood by implementations MUST be ignored.
There are three classes of JWT Claim Names: Registered Claim Names, Public Claim Names, and Private Claim Names.
4.1. Registered Claim Names
The following Claim Names are registered in the IANA "JSON Web Token Claims" registry established by Section 10.1. None of the claims defined below are intended to be mandatory to use or implement in all cases, but rather they provide a starting point for a set of useful, interoperable claims. Applications using JWTs SHOULD specify which specific claims they use and when they are required or optional. All the names are short because a core goal of JWTs is for the representation to be compact.
4.1.1. "iss" (Issuer) Claim
The "iss" (issuer) claim identifies the principal that issued the JWT. The processing of this claim is generally application specific. The "iss" value is a case-sensitive string containing a StringOrURI value. Use of this claim is OPTIONAL.
4.1.2. "sub" (Subject) Claim
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.
4.1.3. "aud" (Audience) Claim
The "aud" (audience) claim identifies the recipients that the JWT is intended for. Each principal intended to process the JWT MUST identify itself with a value in the audience claim. If the principal processing the claim does not identify itself with a value in the "aud" claim when this claim is present, then the JWT MUST be rejected. In the general case, the "aud" value is an array of case-sensitive strings, each containing a StringOrURI value. In the special case when the JWT has one audience, the "aud" value MAY be a single case-sensitive string containing a StringOrURI value. The interpretation of audience values is generally application specific. Use of this claim is OPTIONAL.
4.1.4. "exp" (Expiration Time) Claim
The "exp" (expiration time) claim identifies the expiration time on or after which the JWT MUST NOT be accepted for processing. The processing of the "exp" claim requires that the current date/time MUST be before the expiration date/time listed in the "exp" claim. Implementers MAY provide for some small leeway, usually no more than a few minutes, to account for clock skew. Its value MUST be a number containing a NumericDate value. Use of this claim is OPTIONAL.
4.1.5. "nbf" (Not Before) Claim
The "nbf" (not before) claim identifies the time before which the JWT MUST NOT be accepted for processing. The processing of the "nbf" claim requires that the current date/time MUST be after or equal to the not-before date/time listed in the "nbf" claim. Implementers MAY provide for some small leeway, usually no more than a few minutes, to account for clock skew. Its value MUST be a number containing a NumericDate value. Use of this claim is OPTIONAL.
4.1.6. "iat" (Issued At) Claim
The "iat" (issued at) claim identifies the time at which the JWT was issued. This claim can be used to determine the age of the JWT. Its value MUST be a number containing a NumericDate value. Use of this claim is OPTIONAL.
4.1.7. "jti" (JWT ID) Claim
The "jti" (JWT ID) claim provides a unique identifier for the JWT. The identifier value MUST be assigned in a manner that ensures that there is a negligible probability that the same value will be accidentally assigned to a different data object; if the application uses multiple issuers, collisions MUST be prevented among values produced by different issuers as well. The "jti" claim can be used to prevent the JWT from being replayed. The "jti" value is a case-sensitive string. Use of this claim is OPTIONAL.
4.2. Public Claim Names
Claim Names can be defined at will by those using JWTs. However, in order to prevent collisions, any new Claim Name SHOULD either be registered in the IANA "JSON Web Token Claims" registry established by Section 10.1 or be a Public Name: a value that contains a Collision-Resistant Name. In each case, the definer of the name or value needs to take reasonable precautions to make sure they are in control of the part of the namespace they use to define the Claim Name.
4.3. Private Claim Names
A producer and consumer of a JWT MAY agree to use Claim Names that are Private Names: names that are not Registered Claim Names (Section 4.1) or Public Claim Names (Section 4.2). Unlike Public Claim Names, Private Claim Names are subject to collision and should be used with caution.