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.
4
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.
7
8#ifndef CRYPTO_HMAC_H_
9#define CRYPTO_HMAC_H_
10
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"
16
17namespace crypto {
18
19// Simplify the interface and reduce includes by abstracting out the internals.
20struct HMACPlatformData;
21class SymmetricKey;
22
23class CRYPTO_EXPORT HMAC {
24 public:
25  // The set of supported hash functions. Extend as required.
26  enum HashAlgorithm {
27    SHA1,
28    SHA256,
29  };
30
31  explicit HMAC(HashAlgorithm hash_alg);
32  ~HMAC();
33
34  // Returns the length of digest that this HMAC will create.
35  size_t DigestLength() const;
36
37  // TODO(abarth): Add a PreferredKeyLength() member function.
38
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.
41  //
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,
48  // Init() may fail.
49  bool Init(const unsigned char* key, size_t key_length) WARN_UNUSED_RESULT;
50
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;
54
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()),
59                key.size());
60  }
61
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;
67
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;
77
78  // Verifies a truncated HMAC, behaving identical to Verify(), except
79  // that |digest| is allowed to be smaller than |DigestLength()|.
80  bool VerifyTruncated(
81      const base::StringPiece& data,
82      const base::StringPiece& digest) const WARN_UNUSED_RESULT;
83
84 private:
85  HashAlgorithm hash_alg_;
86  scoped_ptr<HMACPlatformData> plat_;
87
88  DISALLOW_COPY_AND_ASSIGN(HMAC);
89};
90
91}  // namespace crypto
92
93#endif  // CRYPTO_HMAC_H_
94