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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:
// CheckMAC reports whether messageMAC is a valid HMAC tag for message.
func CheckMAC(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))
type hmac struct {
size int
blocksize int
opad, ipad []byte
outer, inner hash.Hash
func (h *hmac) Sum(in []byte) []byte {
origLen := len(in)
in = h.inner.Sum(in)
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.size }
func (h *hmac) BlockSize() int { return h.blocksize }
func (h *hmac) Reset() {
// New returns a new HMAC hash using the given hash.Hash type and key.
func New(h func() hash.Hash, key []byte) hash.Hash {
hm := new(hmac)
hm.outer = h()
hm.inner = h()
hm.size = hm.inner.Size()
hm.blocksize = hm.inner.BlockSize()
hm.ipad = make([]byte, hm.blocksize)
hm.opad = make([]byte, hm.blocksize)
if len(key) > hm.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 len(mac1) == len(mac2) && subtle.ConstantTimeCompare(mac1, mac2) == 1