Securing an API with JSON Web Tokens (What is a JWT)?

JSON Web Token, prounced “jot”, is an open standard that lets you securely transmit information as a JSON object. The token can be signed with a secret using HMAC or a public/private key using RSA. This makes JWTs a great option for authentication. Each JWT is comprised of three parts separated by dots, which are the header, payload, and signature.

Here is an example of a JWT.

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.
eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9.
TJVA95OrM7E2cBab30RMHrHDcEfxjoYZgeFONFh7HgQ

Header

The header defines what kind of token it is, and the type of hashing algorithm that was used to create the token.

{
“alg”: “HS256”,
“typ”: “JWT”
}

The header is Base64URL encoded, which then becomes the first third of the JWT.

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9

Payload

The second part of the token is the payload, and contains claims and any other metadata about the user. In the example below, claims are included in the payload and then encrypted to create the payload.

{
“sub”: “1234567890”,
“name”: “John Doe”,
“admin”: true
}

eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiYWRtaW4iOnRydWV9

The claims used in the payload are categorized into three groups.

Registered Claims are predefined claims that are defined in RFC 7519. An application should define which claims it uses, and when they are required or optional. The names of the claims are short, three letters, to keep the size of the token small.

  • “iss” : Issuer Claim – identifies the principal that issued the JWT.
  • “sub” : Subject Claim – identifies the principal that is the subject of the JWT.
  • “aud” : Audience Claim – identifies the recipients that the JWT is intended for.
  • “exp” : Expiration Time – identifies the expiration time on or after which the JWT must not be accepted.
  • “jti” : JWT ID Claim – provides a unique identifier for the JWT. The value can be used to prevent the JWT from being replayed.

Public Claims are defined by who ever is creating the token. The concern here is to choose a name that is not likely to collide with another claim name. Ideally, they should be registered in the IANA “JSON Web Token Claims” registry.

Private Claims are agreed to be used by the who ever produced the token, and those who consume the token. The names are not registered, and are likely to be subject to collisions.

Signature

The purpose of the signature is to verify that the sender is who they say they are, and to ensure that the token was not altered in transmission. The signature is created by combining the encoded header, the encoded payload, and a secret. The information is then encrypted.

How Are JWTs Used To Authenticate Users

Traditionally when a user logs into a server, the service will create a session in the server, and return a cookie. JWTs work a bit differently, because when the service authenticates a consumer, it will create the JWT and return it to the consumer, where it is stored locally. On subsequent calls to the service, the consumer includes the JWT in the Authorization header using the Bearer schema.

Authorization: Bearer

There are multiple benefits to this method. The authentication method is stateless, because there is no need to maintain a session on the server. Furthermore, the JWTs are self-contained, and don’t require additional calls to a database to validate the credentials. If you have a service that makes calls to other APIs, you can simple pass the JWT along. There is no restriction on what domain is serving the API, so CORS, Cross-Origin Resource Sharing, is not an issue.

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