golang-jwt/jwt

Facts and the checks behind them

Nothing below is copied from documentation. The check content/libraries/verify/golang-jwt/run.sh ran golang-jwt/jwt 5.3.1 in the container image golang:1.27.3-trixie on October 20, 2026; the page must match what it printed, and each fact lists those lines.

• JWS algorithms

  HS256HS384HS512RS256RS384RS512PS256PS384PS512ES256ES384ES512EdDSA

  HS, RS, PS and ES at 256/384/512 bits and EdDSA signed and verified; ES256K, Ed25519 and Ed448 have no registered signing method.

  Recorded by the check

  fact jws-algorithms = HS256, HS384, HS512, RS256, RS384, RS512, PS256, PS384, PS512, ES256, ES384, ES512, EdDSA
  evidence jws-algorithms: ES256K: no signing method registered
  evidence jws-algorithms: Ed25519: no signing method registered
  evidence jws-algorithms: Ed448: no signing method registered

• ES256 signature encoding

  raw

  ES256 signatures are 64 bytes of raw r‖s.

  Recorded by the check

  fact jws-ecdsa-encoding = raw

• DER signature inside a JWS

  rejected

  A DER-encoded ES256 signature fails with token signature is invalid: crypto/ecdsa: verification error.

  Recorded by the check

  fact jws-der-signature = rejected
  evidence jws-der-signature: token signature is invalid: crypto/ecdsa: verification error

• Algorithm allow-list

  from-key

  Without WithValidMethods, the key's Go type decides: an RS256 token against a keyfunc returning an EC key failed with key is of invalid type: RSA verify expects *rsa.PublicKey.

  Recorded by the check

  fact jwt-algorithm-allowlist = from-key
  evidence jwt-algorithm-allowlist: RS256 token, keyfunc returning the EC public key, no WithValidMethods: token signature is invalid: key is of invalid type: RSA verify expects *rsa.PublicKey

• Unsigned tokens (alg none)

  opt-in

  An unsigned token fails with 'none' signature type is not allowed unless the keyfunc returns jwt.UnsafeAllowNoneSignatureType.

  Recorded by the check

  fact jwt-alg-none = opt-in
  evidence jwt-alg-none: with the EC key: token signature is invalid: token is unverifiable: 'none' signature type is not allowed
  evidence jwt-alg-none: keyfunc returning jwt.UnsafeAllowNoneSignatureType: <nil>

• Public key used as an HMAC secret

  rejected

  An HS256 token keyed with the PEM text failed against a keyfunc returning *ecdsa.PublicKey (HMAC verify expects []byte), but verified when the keyfunc returned the PEM bytes.

  Recorded by the check

  fact jwt-hmac-public-key = rejected
  evidence jwt-hmac-public-key: keyfunc returning *ecdsa.PublicKey: token signature is invalid: key is of invalid type: HMAC verify expects []byte
  evidence jwt-hmac-public-key: keyfunc returning the PEM bytes (a keyfunc that loads keys as bytes): <nil>

• Curve checked against alg

  enforced

  Signing ES256 with a P-384 key fails with key is invalid.

  Recorded by the check

  fact jwt-ecdsa-curve-check = enforced
  evidence jwt-ecdsa-curve-check: key is invalid

• Default clock skew

  0s

  No leeway by default: a token that expired three seconds earlier was rejected.

  Recorded by the check

  fact jwt-clock-skew = 0s

• Expiry claim required

  no

  A token without exp was valid; WithExpirationRequired() makes it fail with token is missing required claim: exp claim is required.

  Recorded by the check

  fact jwt-exp-required = no
  evidence jwt-exp-required: with WithExpirationRequired: token has invalid claims: token is missing required claim: exp claim is required

Usage

package main

import (
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"fmt"
	"time"

	"github.com/golang-jwt/jwt/v5"
)

func main() {
	key, _ := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	claims := jwt.RegisteredClaims{Subject: "user-42", Issuer: "https://auth.example.com", ExpiresAt: jwt.NewNumericDate(time.Now().Add(10 * time.Minute))}
	signed, _ := jwt.NewWithClaims(jwt.SigningMethodES256, claims).SignedString(key)

	// The keyfunc returns a typed key; the options pin the method and the claims.
	token, err := jwt.ParseWithClaims(signed, &jwt.RegisteredClaims{},
		func(*jwt.Token) (any, error) { return &key.PublicKey, nil },
		jwt.WithValidMethods([]string{"ES256"}),
		jwt.WithIssuer("https://auth.example.com"),
		jwt.WithExpirationRequired(),
		jwt.WithLeeway(30*time.Second),
	)
	fmt.Println("valid:", err == nil && token.Valid, "sub:", token.Claims.(*jwt.RegisteredClaims).Subject)
}