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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
Package hmac implements the Keyed-Hash Message Authentication Code (HMAC) as
defined in U.S. Federal Information Processing Standards Publication 198.
An HMAC is a cryptographic hash that uses a key to sign a message.
The receiver verifies the hash by recomputing it using the same key.
Receivers should be careful to use Equal to compare MACs in order to avoid
timing side-channels:
// ValidMAC reports whether messageMAC is a valid HMAC tag for message.
func ValidMAC(message, messageMAC, key []byte) bool {
mac := hmac.New(sha256.New, key)
expectedMAC := mac.Sum(nil)
return hmac.Equal(messageMAC, expectedMAC)
package hmac
import (
// FIPS 198-1:
// key is zero padded to the block size of the hash function
// ipad = 0x36 byte repeated for key length
// opad = 0x5c byte repeated for key length
// hmac = H([key ^ opad] H([key ^ ipad] text))
// Marshalable is the combination of encoding.BinaryMarshaler and
// encoding.BinaryUnmarshaler. Their method definitions are repeated here to
// avoid a dependency on the encoding package.
type marshalable interface {
MarshalBinary() ([]byte, error)
UnmarshalBinary([]byte) error
type hmac struct {
opad, ipad []byte
outer, inner hash.Hash
// If marshaled is true, then opad and ipad do not contain a padded
// copy of the key, but rather the marshaled state of outer/inner after
// opad/ipad has been fed into it.
marshaled bool
func (h *hmac) Sum(in []byte) []byte {
origLen := len(in)
in = h.inner.Sum(in)
if h.marshaled {
if err := h.outer.(marshalable).UnmarshalBinary(h.opad); err != nil {
} else {
return h.outer.Sum(in[:origLen])
func (h *hmac) Write(p []byte) (n int, err error) {
return h.inner.Write(p)
func (h *hmac) Size() int { return h.outer.Size() }
func (h *hmac) BlockSize() int { return h.inner.BlockSize() }
func (h *hmac) Reset() {
if h.marshaled {
if err := h.inner.(marshalable).UnmarshalBinary(h.ipad); err != nil {
// If the underlying hash is marshalable, we can save some time by
// saving a copy of the hash state now, and restoring it on future
// calls to Reset and Sum instead of writing ipad/opad every time.
// If either hash is unmarshalable for whatever reason,
// it's safe to bail out here.
marshalableInner, innerOK := h.inner.(marshalable)
if !innerOK {
marshalableOuter, outerOK := h.outer.(marshalable)
if !outerOK {
imarshal, err := marshalableInner.MarshalBinary()
if err != nil {
omarshal, err := marshalableOuter.MarshalBinary()
if err != nil {
// Marshaling succeeded; save the marshaled state for later
h.ipad = imarshal
h.opad = omarshal
h.marshaled = true
// New returns a new HMAC hash using the given hash.Hash type and key.
// Note that unlike other hash implementations in the standard library,
// the returned Hash does not implement encoding.BinaryMarshaler
// or encoding.BinaryUnmarshaler.
func New(h func() hash.Hash, key []byte) hash.Hash {
hm := new(hmac)
hm.outer = h()
hm.inner = h()
blocksize := hm.inner.BlockSize()
hm.ipad = make([]byte, blocksize)
hm.opad = make([]byte, blocksize)
if len(key) > blocksize {
// If key is too big, hash it.
key = hm.outer.Sum(nil)
copy(hm.ipad, key)
copy(hm.opad, key)
for i := range hm.ipad {
hm.ipad[i] ^= 0x36
for i := range hm.opad {
hm.opad[i] ^= 0x5c
return hm
// Equal compares two MACs for equality without leaking timing information.
func Equal(mac1, mac2 []byte) bool {
// We don't have to be constant time if the lengths of the MACs are
// different as that suggests that a completely different hash function
// was used.
return subtle.ConstantTimeCompare(mac1, mac2) == 1