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diff --git a/crypto/hmac.h b/crypto/hmac.h
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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 "base/basictypes.h"
+#include "base/compiler_specific.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_