Reduce string building complexity when needing to add a NUL byte

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 <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 39 matching lines @@
   ~WebPushEncryptionDraft03() override = default;
 
   // GCMMessageCryptographer::EncryptionScheme implementation.
   std::string DerivePseudoRandomKey(
       const base::StringPiece& /* recipient_public_key */,
       const base::StringPiece& /* sender_public_key */,
       const base::StringPiece& ecdh_shared_secret,
       const base::StringPiece& auth_secret) override {
     const char kInfo[] = "Content-Encoding: auth";
 
-    std::string info;
-    info.reserve(sizeof(kInfo) + 1);
-    info.append(kInfo);
-    info.append(1, '\0');
+    // This deliberately copies over the NUL terminus.
+    std::string info(kInfo, sizeof(kInfo));

[eroman, 2017/05/23 18:20:08]

How about using a StringPiece instead? (avoids copying; HKDF looks to take
StringPiece input).

[Peter Beverloo, 2017/05/23 19:01:21]

Done.

 
     crypto::HKDF hkdf(ecdh_shared_secret, auth_secret, info,
                       32, /* key_bytes_to_generate */
                       0,  /* iv_bytes_to_generate */
                       0 /* subkey_secret_bytes_to_generate */);
 
     return hkdf.client_write_key().as_string();
   }
 
   // Creates the info parameter for an HKDF value for the given
@@ skipping 116 matching lines @@
       const base::StringPiece& recipient_public_key,
       const base::StringPiece& sender_public_key,
       const base::StringPiece& ecdh_shared_secret,
       const base::StringPiece& auth_secret) override {
     DCHECK_EQ(recipient_public_key.size(), 65u);
     DCHECK_EQ(sender_public_key.size(), 65u);
 
     const char kInfo[] = "WebPush: info";
 
     std::string info;
-    info.reserve(sizeof(kInfo) + 1 + 65 + 65);
-    info.append(kInfo);
-    info.append(1, '\0');
+    info.reserve(sizeof(kInfo) + 65 + 65);
+
+    // This deliberately copies over the NUL terminus.
+    info.append(kInfo, sizeof(kInfo));
 
     recipient_public_key.AppendToString(&info);
     sender_public_key.AppendToString(&info);
 
     crypto::HKDF hkdf(ecdh_shared_secret, auth_secret, info,
                       32, /* key_bytes_to_generate */
                       0,  /* iv_bytes_to_generate */
                       0 /* subkey_secret_bytes_to_generate */);
 
     return hkdf.client_write_key().as_string();
@@ skipping 270 matching lines @@
 
   // https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-02
   // defines that the result should be XOR'ed with the record's sequence number,
   // however, Web Push encryption is limited to a single record per
   // https://tools.ietf.org/html/draft-ietf-webpush-encryption-03.
 
   return hkdf.client_write_key().as_string();
 }
 
 }  // namespace gcm