RAPPOR implementation

components/rappor/rappor_metric.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/rappor_metric.h"
+
+#include "base/logging.h"
+
+namespace rappor {
+
+RapporMetric::RapporMetric(const RapporParameters& parameters,
+                           int32_t cohort)
+    : parameters_(parameters),
+      bloom_(parameters.bloom_filter_size_bytes,
+             parameters.bloom_filter_hash_function_count,
+             cohort * parameters.bloom_filter_hash_function_count) {
+    DCHECK_GE(cohort, 0);
+}
+
+RapporMetric::~RapporMetric() {}
+
+void RapporMetric::AddSample(const std::string& str) { bloom_.AddString(str); }
+
+ByteVector RapporMetric::GetReport(const std::string& secret) const {
+  // Start with the real bloom filter data.
+  const ByteVector real_bits(bytes());
+
+  // Generate a deterministically random mask of fake data using the
+  // client's secret key + real data as a seed.
+  std::string seed = secret + parameters()->rappor_name +
+      std::string(real_bits.begin(), real_bits.end());
+  HmacByteVectorGenerator hmac_generator(real_bits.size(), seed);
+  const ByteVector fake_mask =
+      hmac_generator.GetWeightedRandomByteVector(parameters()->fake_prob);
+  ByteVector fake_ones =
+      hmac_generator.GetWeightedRandomByteVector(parameters()->fake_one_prob);
+
+  // Redact most of the real data by replacing it with the fake data, hiding
+  // and limiting the amount of information an individual client reports on.
+  const ByteVector* redacted_bits =
+      ByteVectorMerge(fake_mask, real_bits, &fake_ones);
+
+  // Generate biased coin flips for each bit.
+  ByteVectorGenerator coin_generator(real_bits.size());
+  const ByteVector zero_coins =
+      coin_generator.GetWeightedRandomByteVector(parameters()->zero_coin_prob);
+  ByteVector one_coins =
+      coin_generator.GetWeightedRandomByteVector(parameters()->one_coin_prob);
+
+  // Use the redacted data to select which coin type is used for each bit in
+  // the final report.
+  const ByteVector* output =
+      ByteVectorMerge(*redacted_bits, zero_coins, &one_coins);
+
+  return *output;
+}
+
+}  // namespace rappor