What are JWTs?

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.

aaaaa.bbbbb.ccccc

I'll break down every part in the upcoming ideas.

The header is the first part of a JWT token and it usually consists of two parts: the type of the token, and the signing algorithm being used. (see picture above - type is "JWT" and the signing algorithm is HMAC SHA256 ).

This JSON is then Base64Url encoded.

The payload is the second part of a JWT token and it contains the claims - statements about an entity (usually the user) and additional data. There are three types of claims: 

• Registered - recommended predefined claims that provide a set of useful, interoperable claims. Some of them are: iss (issuer), exp (expiration time), sub (subject), aud (audience) etc. Names are short because a goal of JWTs is for the representation to be compact.
• Public - can be defined at will by those using the JWT
• Private - custom claims created to share information between parties

This JSON is then Base64Url encoded.

The signature is the third part of a JWT token. In order to create it we have to take the encoded header, the encoded payload, a secret, the algorithm specified in the header, and sign that (see picture).

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.