Avoid some string copies in RAPPOR code.

components/rappor/byte_vector_utils.cc

 // Copyright 2014 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/rappor/byte_vector_utils.h"
 
 #include <algorithm>
 #include <string>
 
 #include "base/logging.h"
 #include "base/rand_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "crypto/random.h"
 
 namespace rappor {
 
 namespace {
 
 // Reinterpets a ByteVector as a StringPiece.
 base::StringPiece ByteVectorAsStringPiece(const ByteVector& lhs) {
   return base::StringPiece(reinterpret_cast<const char *>(&lhs[0]), lhs.size());
 }
 
 // Concatenates parameters together as a string.
-std::string Concat(const ByteVector& value, char c, const std::string& data) {
-  return std::string(value.begin(), value.end()) + c + data;
+std::string Concat(const ByteVector& value, char c, base::StringPiece data) {
+  std::string result(value.begin(), value.end());
+  result += c;
+  data.AppendToString(&result);
+  return result;
 }
 
 // Performs the operation: K = HMAC(K, data)
 // The input "K" is passed by initializing |hmac| with it.
 // The output "K" is returned by initializing |result| with it.
 // Returns false on an error.
 bool HMAC_Rotate(const crypto::HMAC& hmac,
                  const std::string& data,
                  crypto::HMAC* result) {
   ByteVector key(hmac.DigestLength());
@@ skipping 10 matching lines @@
   return hmac.Sign(ByteVectorAsStringPiece(*value),
                    &(*value)[0], value->size());
 }
 
 // Implements (Key, V) = HMAC_DRBG_Update(provided_data, Key, V)
 // See: http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf
 // "V" is read from and written to |value|.
 // The input "Key" is passed by initializing |hmac1| with it.
 // The output "Key" is returned by initializing |out_hmac| with it.
 // Returns false on an error.
-bool HMAC_DRBG_Update(const std::string& provided_data,
+bool HMAC_DRBG_Update(base::StringPiece provided_data,
                       const crypto::HMAC& hmac1,
                       ByteVector* value,
                       crypto::HMAC* out_hmac) {
   // HMAC_DRBG Update Process
   crypto::HMAC temp_hmac(crypto::HMAC::SHA256);
   crypto::HMAC* hmac2 = provided_data.size() > 0 ? &temp_hmac : out_hmac;
   // 1. K = HMAC(K, V || 0x00 || provided_data)
   if (!HMAC_Rotate(hmac1, Concat(*value, 0x00, provided_data), hmac2))
     return false;
   // 2. V = HMAC(K, V)
@@ skipping 89 matching lines @@
     case PROBABILITY_0:
       return ByteVector(byte_count_);
   }
   NOTREACHED();
   return ByteVector(byte_count_);
 }
 
 HmacByteVectorGenerator::HmacByteVectorGenerator(
     size_t byte_count,
     const std::string& entropy_input,
-    const std::string& personalization_string)
+    base::StringPiece personalization_string)
     : ByteVectorGenerator(byte_count),
       hmac_(crypto::HMAC::SHA256),
       value_(hmac_.DigestLength(), 0x01),
       generated_bytes_(0) {
   // HMAC_DRBG Instantiate Process
   // See: http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf
   // 1. seed_material = entropy_input + nonce + personalization_string
   // Note: We are using the 8.6.7 interpretation, where the entropy_input and
   // nonce are acquired at the same time from the same source.
   DCHECK_EQ(kEntropyInputSize, entropy_input.size());
-  std::string seed_material(entropy_input + personalization_string);
+  std::string seed_material(entropy_input);
+  personalization_string.AppendToString(&seed_material);
   // 2. Key = 0x00 00...00
   crypto::HMAC hmac1(crypto::HMAC::SHA256);
   if (!hmac1.Init(std::string(hmac_.DigestLength(), 0x00)))
     NOTREACHED();
   // 3. V = 0x01 01...01
   // (value_ in initializer list)
 
   // 4. (Key, V) = HMAC_DRBG_Update(seed_material, Key, V)
   if (!HMAC_DRBG_Update(seed_material, hmac1, &value_, &hmac_))
     NOTREACHED();
@@ skipping 46 matching lines @@
     bytes_to_go -= n;
     generated_bytes_ += n;
     // Check max_number_of_bits_per_request from 10.1 Table 2
     // max_number_of_bits_per_request == 2^19 bits == 2^16 bytes
     DCHECK_LT(generated_bytes_, 1U << 16);
   }
   return bytes;
 }
 
 }  // namespace rappor