RAPPOR implementation

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 <string>
+
+#include "base/logging.h"
+#include "base/strings/string_number_conversions.h"
+#include "crypto/random.h"
+
+namespace rappor {
+
+ByteVector* ByteVectorOr(const ByteVector& lhs, ByteVector* rhs) {
+  DCHECK_EQ(lhs.size(), rhs->size());
+  for (size_t i = 0, len = lhs.size(); i < len; ++i) {
+    (*rhs)[i] = lhs[i] | (*rhs)[i];
+  }
+  return rhs;
+}
+
+ByteVector* ByteVectorMerge(const ByteVector& mask,
+                            const ByteVector& lhs,
+                            ByteVector* rhs) {
+  DCHECK_EQ(lhs.size(), rhs->size());
+  for (size_t i = 0, len = lhs.size(); i < len; ++i) {
+    (*rhs)[i] = (lhs[i] & ~mask[i]) | ((*rhs)[i] & mask[i]);
+  }
+  return rhs;
+}
+
+int CountBits(const ByteVector& vector) {
+  int bit_count = 0;
+  for(size_t i = 0; i < vector.size(); ++i) {
+    uint8_t byte = vector[i];
+    for (int j = 0; j < 8 ; ++j) {
+      if (byte & (1 << j))
+        bit_count++;
+    }
+  }
+  return bit_count;
+}
+
+ByteVectorGenerator::ByteVectorGenerator(size_t byte_count)
+    : byte_count_(byte_count) {}
+
+ByteVectorGenerator::~ByteVectorGenerator() {}
+
+uint8_t ByteVectorGenerator::RandByte() {
+  uint8_t random_bits;
+  crypto::RandBytes(&random_bits, sizeof(uint8_t));
+  return random_bits;
+}
+
+ByteVector ByteVectorGenerator::GetRandomByteVector() {
+  ByteVector bytes(byte_count_);
+  for (size_t i = 0; i < byte_count_; ++i) {
+    bytes[i] = RandByte();
+  }
+  return bytes;
+}
+
+ByteVector ByteVectorGenerator::GetWeightedRandomByteVector(
+    Probability probability) {
+  ByteVector bytes = GetRandomByteVector();
+  switch (probability) {
+    case PROBABILITY_75:
+      return *ByteVectorOr(GetRandomByteVector(), &bytes);
+    case PROBABILITY_50:
+      return bytes;
+  }
+  NOTREACHED();
+  return bytes;
+}
+
+HmacByteVectorGenerator::HmacByteVectorGenerator(size_t byte_count,
+                                                 const std::string& secret)
+    : ByteVectorGenerator(byte_count),
+      hmac_(crypto::HMAC::SHA256) {
+  // "Nothing up my sleeve" number for the initial value of an
+  // HmacByteVectorGenerator's state. The initial digits of pi.
+  const uint64_t kHmacInitialState = 3141592653589793;
+
+  if (!hmac_.Init(secret))
+    NOTREACHED();
+
+  DCHECK_GT(hmac_.DigestLength(), sizeof(uint8_t));
+  hmac_state_ = kHmacInitialState;
+}
+
+HmacByteVectorGenerator::~HmacByteVectorGenerator() {}
+
+uint8_t HmacByteVectorGenerator::RandByte() {
+  uint8_t random_bits;
+  std::string state = base::Uint64ToString(hmac_state_);
+  if (!hmac_.Sign(state, &random_bits, sizeof(uint8_t)))
+    NOTREACHED();
+
+  ++hmac_state_;

[ulfar, 2014/02/07 20:19:02]

Actually, we should do something a bit fancier, to be totally correct.
See NIST SP800-90A,
http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf

Or, more casually, see this:
http://crypto.stackexchange.com/questions/9076/using-a-hash-as-a-secure-prng

[Steven Holte, 2014/02/07 21:08:16]

Will update this next pass.

+  return random_bits;
+}
+
+}  // namespace rappor