JSON Web Token (JWT) is an open standard (RFC 7519) that defines a compact and self-contained way of securely transmitting information between parties as a JSON object. This information can be verified and trusted because it is digitally signed. JWTs can be signed using a secret (with the HMAC algorithm) or a public/private key pair using RSA or ECDSA.
When should you use JSON Web Tokens?
Here are some scenarios where JSON Web Tokens are useful:
Authorization:
Authorization is the most common scenario for using JWT. Once the user is logged in, each subsequent request will include the JWT, allowing the user to access routes, services, and resources that are permitted with that token. Single Sign-On is a feature that widely uses JWT nowadays, because of its small overhead and its ability to be easily used across different domains.
Information Exchange:
JSON Web Tokens are a good way of securely transmitting information between parties. Because JWTs can be signed—for example, using public/private key pairs—you can be sure the senders are who they say they are. Additionally, as the signature is calculated using the header and the payload, you can also verify that the content has not been tampered with.
What is the JSON Web Token structure?
In its compact form, JSON Web Tokens consist of three parts separated by dots (.), which are:
HeaderPayloadSignature
Therefore, a JWT typically looks like the following: xxxxx.yyyyy.zzzzz
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.
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.
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 this way:
HMACSHA256(base64UrlEncode(header) + "." + base64UrlEncode(payload), secret)
The signature is used to verify that the sender of the JWT is who it says it is and to ensure that the message was not changed along the way.