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// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "crypto/hmac.h"
#include <algorithm>
#include "base/logging.h"
namespace crypto {
// Performs a constant-time comparison of two strings, returning true if the
// strings are equal.
//
// For cryptographic operations, comparison functions such as memcmp() may
// expose side-channel information about input, allowing an attacker to
// perform timing analysis to determine what the expected bits should be. In
// order to avoid such attacks, the comparison must execute in constant time,
// so as to not to reveal to the attacker where the difference(s) are.
// For an example attack, see
// http://groups.google.com/group/keyczar-discuss/browse_thread/thread/5571eca0948b2a13
static bool SecureMemcmp(const void* s1, const void* s2, size_t n) {
const unsigned char* s1_ptr = reinterpret_cast<const unsigned char*>(s1);
const unsigned char* s2_ptr = reinterpret_cast<const unsigned char*>(s2);
unsigned char tmp = 0;
for (size_t i = 0; i < n; ++i, ++s1_ptr, ++s2_ptr)
tmp |= *s1_ptr ^ *s2_ptr;
return (tmp == 0);
}
size_t HMAC::DigestLength() const {
switch (hash_alg_) {
case SHA1:
return 20;
case SHA256:
return 32;
default:
NOTREACHED();
return 0;
}
}
bool HMAC::Verify(const base::StringPiece& data,
const base::StringPiece& digest) const {
if (digest.size() != DigestLength())
return false;
return VerifyTruncated(data, digest);
}
bool HMAC::VerifyTruncated(const base::StringPiece& data,
const base::StringPiece& digest) const {
if (digest.empty())
return false;
size_t digest_length = DigestLength();
scoped_array<unsigned char> computed_digest(
new unsigned char[digest_length]);
if (!Sign(data, computed_digest.get(), static_cast<int>(digest_length)))
return false;
return SecureMemcmp(digest.data(), computed_digest.get(),
std::min(digest.size(), digest_length));
}
} // namespace crypto
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