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// Copyright (c) 2012 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.
// Utility class for calculating the HMAC for a given message. We currently
// only support SHA1 for the hash algorithm, but this can be extended easily.
#ifndef CRYPTO_HMAC_H_
#define CRYPTO_HMAC_H_
#include <stddef.h>
#include "base/compiler_specific.h"
#include "base/macros.h"
#include "base/memory/scoped_ptr.h"
#include "base/strings/string_piece.h"
#include "crypto/crypto_export.h"
namespace crypto {
// Simplify the interface and reduce includes by abstracting out the internals.
struct HMACPlatformData;
class SymmetricKey;
class CRYPTO_EXPORT HMAC {
public:
// The set of supported hash functions. Extend as required.
enum HashAlgorithm {
SHA1,
SHA256,
};
explicit HMAC(HashAlgorithm hash_alg);
~HMAC();
// Returns the length of digest that this HMAC will create.
size_t DigestLength() const;
// TODO(abarth): Add a PreferredKeyLength() member function.
// Initializes this instance using |key| of the length |key_length|. Call Init
// only once. It returns false on the second or later calls.
//
// NOTE: the US Federal crypto standard FIPS 198, Section 3 says:
// The size of the key, K, shall be equal to or greater than L/2, where L
// is the size of the hash function output.
// In FIPS 198-1 (and SP-800-107, which describes key size recommendations),
// this requirement is gone. But a system crypto library may still enforce
// this old requirement. If the key is shorter than this recommended value,
// Init() may fail.
bool Init(const unsigned char* key, size_t key_length) WARN_UNUSED_RESULT;
// Initializes this instance using |key|. Call Init
// only once. It returns false on the second or later calls.
bool Init(SymmetricKey* key) WARN_UNUSED_RESULT;
// Initializes this instance using |key|. Call Init only once. It returns
// false on the second or later calls.
bool Init(const base::StringPiece& key) WARN_UNUSED_RESULT {
return Init(reinterpret_cast<const unsigned char*>(key.data()),
key.size());
}
// Calculates the HMAC for the message in |data| using the algorithm supplied
// to the constructor and the key supplied to the Init method. The HMAC is
// returned in |digest|, which has |digest_length| bytes of storage available.
bool Sign(const base::StringPiece& data, unsigned char* digest,
size_t digest_length) const WARN_UNUSED_RESULT;
// Verifies that the HMAC for the message in |data| equals the HMAC provided
// in |digest|, using the algorithm supplied to the constructor and the key
// supplied to the Init method. Use of this method is strongly recommended
// over using Sign() with a manual comparison (such as memcmp), as such
// comparisons may result in side-channel disclosures, such as timing, that
// undermine the cryptographic integrity. |digest| must be exactly
// |DigestLength()| bytes long.
bool Verify(const base::StringPiece& data,
const base::StringPiece& digest) const WARN_UNUSED_RESULT;
// Verifies a truncated HMAC, behaving identical to Verify(), except
// that |digest| is allowed to be smaller than |DigestLength()|.
bool VerifyTruncated(
const base::StringPiece& data,
const base::StringPiece& digest) const WARN_UNUSED_RESULT;
private:
HashAlgorithm hash_alg_;
scoped_ptr<HMACPlatformData> plat_;
DISALLOW_COPY_AND_ASSIGN(HMAC);
};
} // namespace crypto
#endif // CRYPTO_HMAC_H_
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