1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
5 // Utility class for calculating the HMAC for a given message. We currently
6 // only support SHA1 for the hash algorithm, but this can be extended easily.
11 #include "base/basictypes.h"
12 #include "base/compiler_specific.h"
13 #include "base/memory/scoped_ptr.h"
14 #include "base/strings/string_piece.h"
15 #include "crypto/crypto_export.h"
19 // Simplify the interface and reduce includes by abstracting out the internals.
20 struct HMACPlatformData
;
23 class CRYPTO_EXPORT HMAC
{
25 // The set of supported hash functions. Extend as required.
31 explicit HMAC(HashAlgorithm hash_alg
);
34 // Returns the length of digest that this HMAC will create.
35 size_t DigestLength() const;
37 // TODO(abarth): Add a PreferredKeyLength() member function.
39 // Initializes this instance using |key| of the length |key_length|. Call Init
40 // only once. It returns false on the second or later calls.
42 // NOTE: the US Federal crypto standard FIPS 198, Section 3 says:
43 // The size of the key, K, shall be equal to or greater than L/2, where L
44 // is the size of the hash function output.
45 // In FIPS 198-1 (and SP-800-107, which describes key size recommendations),
46 // this requirement is gone. But a system crypto library may still enforce
47 // this old requirement. If the key is shorter than this recommended value,
49 bool Init(const unsigned char* key
, size_t key_length
) WARN_UNUSED_RESULT
;
51 // Initializes this instance using |key|. Call Init
52 // only once. It returns false on the second or later calls.
53 bool Init(SymmetricKey
* key
) WARN_UNUSED_RESULT
;
55 // Initializes this instance using |key|. Call Init only once. It returns
56 // false on the second or later calls.
57 bool Init(const base::StringPiece
& key
) WARN_UNUSED_RESULT
{
58 return Init(reinterpret_cast<const unsigned char*>(key
.data()),
62 // Calculates the HMAC for the message in |data| using the algorithm supplied
63 // to the constructor and the key supplied to the Init method. The HMAC is
64 // returned in |digest|, which has |digest_length| bytes of storage available.
65 bool Sign(const base::StringPiece
& data
, unsigned char* digest
,
66 size_t digest_length
) const WARN_UNUSED_RESULT
;
68 // Verifies that the HMAC for the message in |data| equals the HMAC provided
69 // in |digest|, using the algorithm supplied to the constructor and the key
70 // supplied to the Init method. Use of this method is strongly recommended
71 // over using Sign() with a manual comparison (such as memcmp), as such
72 // comparisons may result in side-channel disclosures, such as timing, that
73 // undermine the cryptographic integrity. |digest| must be exactly
74 // |DigestLength()| bytes long.
75 bool Verify(const base::StringPiece
& data
,
76 const base::StringPiece
& digest
) const WARN_UNUSED_RESULT
;
78 // Verifies a truncated HMAC, behaving identical to Verify(), except
79 // that |digest| is allowed to be smaller than |DigestLength()|.
81 const base::StringPiece
& data
,
82 const base::StringPiece
& digest
) const WARN_UNUSED_RESULT
;
85 HashAlgorithm hash_alg_
;
86 scoped_ptr
<HMACPlatformData
> plat_
;
88 DISALLOW_COPY_AND_ASSIGN(HMAC
);
93 #endif // CRYPTO_HMAC_H_