RAPPOR implementation

components/rappor/rappor_reporter.cc

+// Copyright (c) 2013 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 <assert.h>
+#include <limits.h>
+#include <math.h>
+
+#include <bitset>
+#include <map>
+#include <string>
+#include <vector>
+
+#include "base/logging.h"
+#include "base/strings/string_number_conversions.h"
+#include "components/rappor/bytevector.h"
+#include "components/rappor/rappor.h"
+#include "components/rappor/rappor_reporter.h"
+#include "crypto/hmac.h"
+#include "crypto/random.h"
+
+#define HMAC_SEED 10
+
+namespace rappor {
+
+namespace {
+
+// A utility object for generating random binary data with different
+// likelihood of bits being true.
+class ByteVectorGenerator {
+ public:
+  explicit ByteVectorGenerator(size_t byte_count);
+  ByteVector GetRandomByteVector();
+  ByteVector GetWeightedRandomByteVector(int probIndex);
+
+ protected:
+  virtual uint8_t RandByte();
+
+ private:
+  size_t byte_count_;
+};
+
+ByteVectorGenerator::ByteVectorGenerator(size_t byte_count)
+    : byte_count_(byte_count) {}
+
+uint8_t ByteVectorGenerator::RandByte() {
+  uint8_t randomBits;
+  crypto::RandBytes(&randomBits, sizeof(uint8_t));
+  return randomBits;
+}
+
+ByteVector ByteVectorGenerator::GetRandomByteVector() {

[jwd, 2013/12/17 23:00:25]

Mention that it's expected 50% 1s or something like that.

Actually, isn't this dependent on the implementation of RandByte? Making the
comments in GetWeightedRandomByteVector not guaranteed?

[Steven Holte, 2013/12/18 02:06:12]

Added comments about RandByte and RandBytes being expected to have uniform
distributions of bytes.  It should be approximately true for the pseudorandom
generator.

+  ByteVector bytes(byte_count_);
+  for (size_t i = 0; i < byte_count_; i++) {
+    bytes[i] = RandByte();
+  }
+  return bytes;
+}
+
+ByteVector ByteVectorGenerator::GetWeightedRandomByteVector(int probIndex) {
+  switch (probIndex) {
+    // 87.5% ones
+    case -8:

[jwd, 2013/12/17 23:00:25]

What determines these constants? As in, why -8,...,8

[Steven Holte, 2013/12/18 02:06:12]

Changed these to Probability enum values.

+      return GetRandomByteVector().Or(GetRandomByteVector()).Or(
+               GetRandomByteVector());
+    // 75% ones
+    case -4:
+      return GetRandomByteVector().Or(GetRandomByteVector());
+    // 50% ones
+    case 2:
+      return GetRandomByteVector();
+    // 25% ones
+    case 4:
+      return GetRandomByteVector().And(GetRandomByteVector());
+    // 12.5% ones
+    case 8:
+      return GetRandomByteVector().And(GetRandomByteVector()).And(
+             GetRandomByteVector());
+    default:
+      // Invalid probability index "probIndex" for coin flips
+      abort();
+  }
+}
+
+// A ByteVectorGenerator that uses a psuedo-random function to generate
+// deterministicly random bits.
+class HmacByteVectorGenerator : public ByteVectorGenerator {
+ public:
+  HmacByteVectorGenerator(size_t byte_count, const std::string& secret);
+
+ protected:
+  virtual uint8_t RandByte();
+
+ private:
+  crypto::HMAC hmac_;
+  uint64_t hmac_state_;
+};
+
+HmacByteVectorGenerator::HmacByteVectorGenerator(size_t byte_count,
+                                                 const std::string& secret)
+    : ByteVectorGenerator(byte_count), hmac_(crypto::HMAC::SHA256) {
+  if (!hmac_.Init(secret)) {
+    abort();
+  };
+  assert(hmac_.DigestLength() > sizeof(uint64_t));
+  hmac_state_ = uint64_t(HMAC_SEED);
+}
+
+uint8_t HmacByteVectorGenerator::RandByte() {
+  uint8_t randomBits;
+  std::string state = base::Uint64ToString(hmac_state_);
+  if (!hmac_.Sign(state, &randomBits, sizeof(uint8_t))) {
+    abort();
+  }
+  ++hmac_state_;
+  return randomBits;
+}
+
+}  // namespace
+
+RapporReporter::RapporReporter(const std::string& secret) : secret_(secret) {}
+
+std::vector<uint8_t> RapporReporter::GetReport(const Rappor& rappor) {
+  const ByteVector realbits(rappor.GetBytes());
+  size_t size = realbits.size();
+  HmacByteVectorGenerator hmacGenerator(size, secret_ + rappor.rappor_name());
+  const ByteVector fakebits =
+      hmacGenerator.GetWeightedRandomByteVector(rappor.fake_prob_index());
+  const ByteVector fakeones =
+      hmacGenerator.GetWeightedRandomByteVector(rappor.fake_one_prob_index());
+  const ByteVector onebits = (realbits.And(fakebits.Not())).Or(
+        fakeones.And(fakebits));
+
+  ByteVectorGenerator coinGenerator(size);
+  const ByteVector zero_coins = coinGenerator.GetWeightedRandomByteVector(
+      rappor.zero_honesty_prob_index());
+  const ByteVector one_coins = coinGenerator.GetWeightedRandomByteVector(
+      rappor.one_honesty_prob_index());
+  return (zero_coins.And(onebits.Not())).Or(one_coins.And(onebits));
+
+  return std::vector<uint8_t>();
+}
+
+}  // namespace rappor