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@dgrijalva @zhyuri @simonjefford @kofalt @martinlindhe @aussiegeek
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package jwt
import (
// Implements the HMAC-SHA family of signing methods signing methods
// Expects key type of []byte for both signing and validation
type SigningMethodHMAC struct {
Name string
Hash crypto.Hash
// Specific instances for HS256 and company
var (
SigningMethodHS256 *SigningMethodHMAC
SigningMethodHS384 *SigningMethodHMAC
SigningMethodHS512 *SigningMethodHMAC
ErrSignatureInvalid = errors.New("signature is invalid")
func init() {
// HS256
SigningMethodHS256 = &SigningMethodHMAC{"HS256", crypto.SHA256}
RegisterSigningMethod(SigningMethodHS256.Alg(), func() SigningMethod {
return SigningMethodHS256
// HS384
SigningMethodHS384 = &SigningMethodHMAC{"HS384", crypto.SHA384}
RegisterSigningMethod(SigningMethodHS384.Alg(), func() SigningMethod {
return SigningMethodHS384
// HS512
SigningMethodHS512 = &SigningMethodHMAC{"HS512", crypto.SHA512}
RegisterSigningMethod(SigningMethodHS512.Alg(), func() SigningMethod {
return SigningMethodHS512
func (m *SigningMethodHMAC) Alg() string {
return m.Name
// Verify the signature of HSXXX tokens. Returns nil if the signature is valid.
func (m *SigningMethodHMAC) Verify(signingString, signature string, key interface{}) error {
// Verify the key is the right type
keyBytes, ok := key.([]byte)
if !ok {
return ErrInvalidKeyType
// Decode signature, for comparison
sig, err := DecodeSegment(signature)
if err != nil {
return err
// Can we use the specified hashing method?
if !m.Hash.Available() {
return ErrHashUnavailable
// This signing method is symmetric, so we validate the signature
// by reproducing the signature from the signing string and key, then
// comparing that against the provided signature.
hasher := hmac.New(m.Hash.New, keyBytes)
if !hmac.Equal(sig, hasher.Sum(nil)) {
return ErrSignatureInvalid
// No validation errors. Signature is good.
return nil
// Implements the Sign method from SigningMethod for this signing method.
// Key must be []byte
func (m *SigningMethodHMAC) Sign(signingString string, key interface{}) (string, error) {
if keyBytes, ok := key.([]byte); ok {
if !m.Hash.Available() {
return "", ErrHashUnavailable
hasher := hmac.New(m.Hash.New, keyBytes)
return EncodeSegment(hasher.Sum(nil)), nil
return "", ErrInvalidKeyType
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