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// 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"
#include "base/rand_util.h"
namespace rappor {
RapporMetric::RapporMetric(const std::string& metric_name,
const RapporParameters& parameters,
int32_t cohort_seed)
: metric_name_(metric_name),
parameters_(parameters),
sample_count_(0),
bloom_filter_(parameters.bloom_filter_size_bytes,
parameters.bloom_filter_hash_function_count,
(cohort_seed % parameters.num_cohorts) *
parameters.bloom_filter_hash_function_count) {
DCHECK_GE(cohort_seed, 0);
DCHECK_LT(cohort_seed, RapporParameters::kMaxCohorts);
// Since cohort_seed is in the range [0, kMaxCohorts), num_cohorts should
// divide kMaxCohorts for each cohort to have equal weight.
DCHECK_EQ(0, RapporParameters::kMaxCohorts % parameters.num_cohorts);
}
RapporMetric::~RapporMetric() {}
void RapporMetric::AddSample(const std::string& str) {
++sample_count_;
// Replace the previous sample with a 1 in sample_count_ chance so that each
// sample has equal probability of being reported.
if (base::RandGenerator(sample_count_) == 0) {
bloom_filter_.SetString(str);
}
}
ByteVector RapporMetric::GetReport(const std::string& secret) const {
// Generate a deterministically random mask of fake data using the
// client's secret key + real data as a seed. The inclusion of the secret
// in the seed avoids correlations between real and fake data.
// The seed isn't a human-readable string.
const std::string personalization_string = metric_name_ +
std::string(bytes().begin(), bytes().end());
HmacByteVectorGenerator hmac_generator(bytes().size(), secret,
personalization_string);
const ByteVector fake_mask =
hmac_generator.GetWeightedRandomByteVector(parameters().fake_prob);
ByteVector fake_bits =
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* fake_and_redacted_bits =
ByteVectorMerge(fake_mask, bytes(), &fake_bits);
// Generate biased coin flips for each bit.
ByteVectorGenerator coin_generator(bytes().size());
const ByteVector zero_coins =
coin_generator.GetWeightedRandomByteVector(parameters().zero_coin_prob);
ByteVector one_coins =
coin_generator.GetWeightedRandomByteVector(parameters().one_coin_prob);
// Create a randomized response report on the fake and redacted data, sending
// the outcome of flipping a zero coin for the zero bits in that data, and of
// flipping a one coin for the one bits in that data, as the final report.
return *ByteVectorMerge(*fake_and_redacted_bits, zero_coins, &one_coins);
}
void RapporMetric::SetBytesForTesting(const ByteVector& bytes) {
bloom_filter_.SetBytesForTesting(bytes);
}
} // namespace rappor
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