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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 returns true if messageMAC is a valid HMAC tag for message.
func CheckMAC(message, messageMAC, key []byte) bool {
mac := hmac.New(sha256.New, key)
mac.Write(message)
expectedMAC := mac.Sum(nil)
return hmac.Equal(messageMAC, expectedMAC)
}
*/
package hmac
import (
"crypto/subtle"
"hash"
)
// FIPS 198:
// http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf
// 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
key, tmp []byte
outer, inner hash.Hash
}
func (h *hmac) tmpPad(xor byte) {
for i, k := range h.key {
h.tmp[i] = xor ^ k
}
for i := len(h.key); i < h.blocksize; i++ {
h.tmp[i] = xor
}
}
func (h *hmac) Sum(in []byte) []byte {
origLen := len(in)
in = h.inner.Sum(in)
h.tmpPad(0x5c)
copy(h.tmp[h.blocksize:], in[origLen:])
h.outer.Reset()
h.outer.Write(h.tmp)
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() {
h.inner.Reset()
h.tmpPad(0x36)
h.inner.Write(h.tmp[:h.blocksize])
}
// 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.tmp = make([]byte, hm.blocksize+hm.size)
if len(key) > hm.blocksize {
// If key is too big, hash it.
hm.outer.Write(key)
key = hm.outer.Sum(nil)
}
hm.key = make([]byte, len(key))
copy(hm.key, key)
hm.Reset()
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
}