Don't enforce the old HMAC key size requirement in FIPS 198 Sec. 3.

crypto/hmac.h

 // 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_
 #pragma once
@@ skipping 22 matching lines @@
   ~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.
   // TODO(abarth): key_length should be a size_t.
+  //
+  // 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, int 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()),
@@ skipping 26 matching lines @@
  private:
   HashAlgorithm hash_alg_;
   scoped_ptr<HMACPlatformData> plat_;
 
   DISALLOW_COPY_AND_ASSIGN(HMAC);
 };
 
 }  // namespace crypto
 
 #endif  // CRYPTO_HMAC_H_