103 lines
2.6 KiB
Go
103 lines
2.6 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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/*
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Package hmac implements the Keyed-Hash Message Authentication Code (HMAC) as
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defined in U.S. Federal Information Processing Standards Publication 198.
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An HMAC is a cryptographic hash that uses a key to sign a message.
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The receiver verifies the hash by recomputing it using the same key.
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Receivers should be careful to use Equal to compare MACs in order to avoid
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timing side-channels:
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// CheckMAC returns true if messageMAC is a valid HMAC tag for message.
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func CheckMAC(message, messageMAC, key []byte) bool {
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mac := hmac.New(sha256.New, key)
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mac.Write(message)
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expectedMAC := mac.Sum(nil)
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return hmac.Equal(messageMAC, expectedMAC)
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}
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*/
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package hmac
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import (
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"crypto/subtle"
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"hash"
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)
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// FIPS 198:
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// http://csrc.nist.gov/publications/fips/fips198/fips-198a.pdf
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// key is zero padded to the block size of the hash function
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// ipad = 0x36 byte repeated for key length
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// opad = 0x5c byte repeated for key length
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// hmac = H([key ^ opad] H([key ^ ipad] text))
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type hmac struct {
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size int
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blocksize int
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key, tmp []byte
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outer, inner hash.Hash
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}
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func (h *hmac) tmpPad(xor byte) {
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for i, k := range h.key {
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h.tmp[i] = xor ^ k
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}
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for i := len(h.key); i < h.blocksize; i++ {
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h.tmp[i] = xor
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}
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}
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func (h *hmac) Sum(in []byte) []byte {
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origLen := len(in)
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in = h.inner.Sum(in)
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h.tmpPad(0x5c)
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copy(h.tmp[h.blocksize:], in[origLen:])
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h.outer.Reset()
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h.outer.Write(h.tmp)
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return h.outer.Sum(in[:origLen])
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}
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func (h *hmac) Write(p []byte) (n int, err error) {
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return h.inner.Write(p)
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}
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func (h *hmac) Size() int { return h.size }
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func (h *hmac) BlockSize() int { return h.blocksize }
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func (h *hmac) Reset() {
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h.inner.Reset()
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h.tmpPad(0x36)
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h.inner.Write(h.tmp[:h.blocksize])
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}
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// New returns a new HMAC hash using the given hash.Hash type and key.
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func New(h func() hash.Hash, key []byte) hash.Hash {
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hm := new(hmac)
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hm.outer = h()
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hm.inner = h()
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hm.size = hm.inner.Size()
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hm.blocksize = hm.inner.BlockSize()
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hm.tmp = make([]byte, hm.blocksize+hm.size)
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if len(key) > hm.blocksize {
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// If key is too big, hash it.
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hm.outer.Write(key)
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key = hm.outer.Sum(nil)
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}
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hm.key = make([]byte, len(key))
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copy(hm.key, key)
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hm.Reset()
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return hm
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}
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// Equal compares two MACs for equality without leaking timing information.
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func Equal(mac1, mac2 []byte) bool {
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// We don't have to be constant time if the lengths of the MACs are
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// different as that suggests that a completely different hash function
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// was used.
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return len(mac1) == len(mac2) && subtle.ConstantTimeCompare(mac1, mac2) == 1
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}
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