libgo/go/crypto/hmac/hmac.go

   1 // Copyright 2009 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 /*
   6 Package hmac implements the Keyed-Hash Message Authentication Code (HMAC) as
   7 defined in U.S. Federal Information Processing Standards Publication 198.
   8 An HMAC is a cryptographic hash that uses a key to sign a message.
   9 The receiver verifies the hash by recomputing it using the same key.
  10
  11 Receivers should be careful to use Equal to compare MACs in order to avoid
  12 timing side-channels:
  13
  14         // CheckMAC reports whether messageMAC is a valid HMAC tag for message.
  15         func CheckMAC(message, messageMAC, key []byte) bool {
  16                 mac := hmac.New(sha256.New, key)
  17                 mac.Write(message)
  18                 expectedMAC := mac.Sum(nil)
  19                 return hmac.Equal(messageMAC, expectedMAC)
  20         }
  21 */
  22 package hmac
  23
  24 import (
  25         "crypto/subtle"
  26         "hash"
  27 )
  28
  29 // FIPS 198-1:
  30 // http://csrc.nist.gov/publications/fips/fips198-1/FIPS-198-1_final.pdf
  31
  32 // key is zero padded to the block size of the hash function
  33 // ipad = 0x36 byte repeated for key length
  34 // opad = 0x5c byte repeated for key length
  35 // hmac = H([key ^ opad] H([key ^ ipad] text))
  36
  37 type hmac struct {
  38         size         int
  39         blocksize    int
  40         opad, ipad   []byte
  41         outer, inner hash.Hash
  42 }
  43
  44 func (h *hmac) Sum(in []byte) []byte {
  45         origLen := len(in)
  46         in = h.inner.Sum(in)
  47         h.outer.Reset()
  48         h.outer.Write(h.opad)
  49         h.outer.Write(in[origLen:])
  50         return h.outer.Sum(in[:origLen])
  51 }
  52
  53 func (h *hmac) Write(p []byte) (n int, err error) {
  54         return h.inner.Write(p)
  55 }
  56
  57 func (h *hmac) Size() int { return h.size }
  58
  59 func (h *hmac) BlockSize() int { return h.blocksize }
  60
  61 func (h *hmac) Reset() {
  62         h.inner.Reset()
  63         h.inner.Write(h.ipad)
  64 }
  65
  66 // New returns a new HMAC hash using the given hash.Hash type and key.
  67 // Note that unlike other hash implementations in the standard library,
  68 // the returned Hash does not implement encoding.BinaryMarshaler
  69 // or encoding.BinaryUnmarshaler.
  70 func New(h func() hash.Hash, key []byte) hash.Hash {
  71         hm := new(hmac)
  72         hm.outer = h()
  73         hm.inner = h()
  74         hm.size = hm.inner.Size()
  75         hm.blocksize = hm.inner.BlockSize()
  76         hm.ipad = make([]byte, hm.blocksize)
  77         hm.opad = make([]byte, hm.blocksize)
  78         if len(key) > hm.blocksize {
  79                 // If key is too big, hash it.
  80                 hm.outer.Write(key)
  81                 key = hm.outer.Sum(nil)
  82         }
  83         copy(hm.ipad, key)
  84         copy(hm.opad, key)
  85         for i := range hm.ipad {
  86                 hm.ipad[i] ^= 0x36
  87         }
  88         for i := range hm.opad {
  89                 hm.opad[i] ^= 0x5c
  90         }
  91         hm.inner.Write(hm.ipad)
  92         return hm
  93 }
  94
  95 // Equal compares two MACs for equality without leaking timing information.
  96 func Equal(mac1, mac2 []byte) bool {
  97         // We don't have to be constant time if the lengths of the MACs are
  98         // different as that suggests that a completely different hash function
  99         // was used.
 100         return subtle.ConstantTimeCompare(mac1, mac2) == 1
 101 }