Teach the GCM Driver how to decrypt incoming messages.

components/gcm_driver/crypto/gcm_message_cryptographer.cc

 // Copyright 2015 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.
 
 #include "components/gcm_driver/crypto/gcm_message_cryptographer.h"
 
 #include <algorithm>
 
 #include "base/big_endian.h"
 #include "base/logging.h"
 #include "crypto/hkdf.h"
 
 namespace gcm {
 namespace {
 
 // Size, in bytes, of the nonce for a record. This must be at least the size
 // of a uint64_t, which is used to indicate the record sequence number.
 const uint64_t kNonceSize = 12;
 
 // The default record size as defined by draft-thomson-http-encryption-01.
 const size_t kDefaultRecordSize = 4096;
 
 // Key size, in bytes, of a valid AEAD_AES_128_GCM key.
 const size_t kContentEncryptionKeySize = 16;
 
-// Salt size, in bytes, that will be used together with the key to create a
-// unique content encryption key for a given message.
-const size_t kSaltSize = 16;
-
 }  // namespace
 
 const size_t GCMMessageCryptographer::kAuthenticationTagBytes = 16;
+const size_t GCMMessageCryptographer::kSaltSize = 16;
 
 GCMMessageCryptographer::GCMMessageCryptographer() {}
 
 GCMMessageCryptographer::~GCMMessageCryptographer() {}
 
 bool GCMMessageCryptographer::Encrypt(const base::StringPiece& plaintext,
                                       const base::StringPiece& key,
                                       const base::StringPiece& salt,
                                       size_t* record_size,
                                       std::string* ciphertext) const {
@@ skipping 103 matching lines @@
                     0   /* subkey_secret_bytes_to_generate */);
 
   // draft-thomson-http-encryption-01 defines that the result should be XOR'ed
   // with the record's sequence number, but because Web Push encryption is
   // limited to a single record we do not have to do that.
 
   return hkdf.client_write_key().as_string();
 }
 
 }  // namespace gcm