Creating a New JWT

One of the primary goals of this library is to create a new JWT (or in short token).

With Default Options

The easiest way to create a token is to use the jwt.New function. It then needs one of the available signing methods, to finally sign and convert the token into a string format (using the SignedString method). In the first example, we are using a symmetric signing method, i.e., HS256. For a symmetric method, both the signing and the verifying key are the same and thus, both must be equally protected (and should definitely NOT be stored in your code).

var (
  key []byte
  t   *jwt.Token
  s   string
)

key = /* Load key from somewhere, for example an environment variable */
t = jwt.New(jwt.SigningMethodHS256) // (1)!
s = t.SignedString(key) // (2)!

This initializes a new jwt.Token struct based on the supplied signing method. In this case a symmetric method is chosen.
This step computes a cryptographic signature based on the supplied key.

Signing using an asymmetric signing method (for example ECDSA) works quite similar. For an asymmetric method, the private key (which must be kept secret) is used to sign and the corresponding public key (which can be freely transmitted) is used to verify the token.

var (
  key *ecdsa.PrivateKey
  t   *jwt.Token
  s   string
)

key = /* Load key from somewhere, for example a file */
t = jwt.New(jwt.SigningMethodES256) // (1)!
s = t.SignedString(key) // (2)!

This initializes a new jwt.Token struct based on the supplied signing method. In this case a asymmetric method is chosen.
This step computes a cryptographic signature based on the supplied private key.

Note, that the chosen signing method and the type of key must match. Please refer to Signing Methods for a complete overview.

With Additional Claims

While the step above using jwt.New creates a valid token, it contains an empty set of claims. Claims are certain pieces of information that are encoded into the token. Since they are encoded and signed by the issuer of the token, one can assume that this information is valid (in the scope of the issuer). Claims can be used to provide the basis for user authentication or authorization, e.g., by encoding a user name or ID or roles into the token. This is also commonly in combination with OAuth 2.0. Furthermore, claims can also contain certain metadata about the token itself, e.g., the time until which the token is regarded as valid and not expired.

RFC 7519 provides a list of so called registered claim names ¹, which each JWT parser needs to understand. Using the jwt.NewWithClaims, a token with different claims can be created.

var (
  key *ecdsa.PrivateKey
  t   *jwt.Token
  s   string
)

key = /* Load key from somewhere, for example a file */
t = jwt.NewWithClaims(jwt.SigningMethodES256, // (1)!
  jwt.MapClaims{ // (2)!
    "iss": "my-auth-server", // (3)!
    "sub": "john", // (4)!
    "foo": 2, // (5)!
  })
s = t.SignedString(key) // (6)!

This initializes a new jwt.Token struct based on the supplied signing method. In this case a asymmetric method is chosen, which is the first parameter.
The second parameter contains the desired claims in form of the jwt.Claims interface. In this case jwt.MapClaims are used, which is a wrapper type around a Go map containing string keys.
The "sub"³ claim is a registered claim name that contains the subject this token identifies, e.g. a user name. More technical, this claim identifies the principal that is the subject of the token.
The "iss"² claim is a registered claim name that contains the issuer of the token. More technical, this claim identifies the principal that issued the token.
The "foo" claim is a custom claim containing a numeric value. Any string value can be chosen as a claim name, as long as it does not interfere with a registered claim name.
This step computes a cryptographic signature based on the supplied private key.

With Options

While we already prepared a jwt.TokenOption type, which can be supplied as a varargs to jwt.New and jwt.NewWithClaims, these are strictly for future compatibility and are currently not used.