What is a valid JWT token endpoint response?

In its compact form, JSON Web Tokens consist of three parts separated by dots (.), which are:

  • Header
  • Payload
  • Signature

Therefore, a JWT typically looks like the following.

xxxxx.yyyyy.zzzzz

Let's break down the different parts.

Header

The header typically consists of two parts: the type of the token, which is JWT, and the signing algorithm being used, such as HMAC SHA256 or RSA.

For example:

{ "alg": "HS256", "typ": "JWT" }

Then, this JSON is Base64Url encoded to form the first part of the JWT.

Payload

The second part of the token is the payload, which contains the claims. Claims are statements about an entity (typically, the user) and additional data. There are three types of claims: registered, public, and private claims.

  • Registered claims: These are a set of predefined claims which are not mandatory but recommended, to provide a set of useful, interoperable claims. Some of them are: iss (issuer), exp (expiration time), sub (subject), aud(audience), and others.

    Notice that the claim names are only three characters long as JWT is meant to be compact.

  • Public claims: These can be defined at will by those using JWTs. But to avoid collisions they should be defined in the IANA JSON Web Token Registry or be defined as a URI that contains a collision resistant namespace.

  • Private claims: These are the custom claims created to share information between parties that agree on using them and are neither registered or publicclaims.

An example payload could be:

{ "sub": "1234567890", "name": "John Doe", "admin": true }

The payload is then Base64Url encoded to form the second part of the JSON Web Token.

Do note that for signed tokens this information, though protected against tampering, is readable by anyone. Do not put secret information in the payload or header elements of a JWT unless it is encrypted.

Signature

To create the signature part you have to take the encoded header, the encoded payload, a secret, the algorithm specified in the header, and sign that.

For example if you want to use the HMAC SHA256 algorithm, the signature will be created in the following way:

HMACSHA256( base64UrlEncode(header) + "." + base64UrlEncode(payload), secret)

The signature is used to verify the message wasn't changed along the way, and, in the case of tokens signed with a private key, it can also verify that the sender of the JWT is who it says it is.

Putting all together

The output is three Base64-URL strings separated by dots that can be easily passed in HTML and HTTP environments, while being more compact when compared to XML-based standards such as SAML.

The following shows a JWT that has the previous header and payload encoded, and it is signed with a secret. 

 

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IlJBVkkgbUlTSFJBIiwiaWF0IjoxNTE2MjM5MDIyfQ.4q2wT2ZgqkexX3olulYE7ZUPzAVnX6isUzwynz6fvrQ

 

If you want to play with JWT and put these concepts into practice, you can use jwt.io Debugger to decode, verify, and generate JWTs.