1 // Copyright 2014 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #include "components/rappor/rappor_metric.h" 6 7 #include "base/logging.h" 8 #include "base/rand_util.h" 9 10 namespace rappor { 11 12 RapporMetric::RapporMetric(const std::string& metric_name, 13 const RapporParameters& parameters, 14 int32_t cohort_seed) 15 : metric_name_(metric_name), 16 parameters_(parameters), 17 sample_count_(0), 18 bloom_filter_(parameters.bloom_filter_size_bytes, 19 parameters.bloom_filter_hash_function_count, 20 (cohort_seed % parameters.num_cohorts) * 21 parameters.bloom_filter_hash_function_count) { 22 DCHECK_GE(cohort_seed, 0); 23 DCHECK_LT(cohort_seed, RapporParameters::kMaxCohorts); 24 } 25 26 RapporMetric::~RapporMetric() {} 27 28 void RapporMetric::AddSample(const std::string& str) { 29 ++sample_count_; 30 // Replace the previous sample with a 1 in sample_count_ chance so that each 31 // sample has equal probability of being reported. 32 if (base::RandGenerator(sample_count_) == 0) { 33 bloom_filter_.SetString(str); 34 } 35 } 36 37 ByteVector RapporMetric::GetReport(const std::string& secret) const { 38 // Generate a deterministically random mask of fake data using the 39 // client's secret key + real data as a seed. The inclusion of the secret 40 // in the seed avoids correlations between real and fake data. 41 // The seed isn't a human-readable string. 42 const std::string personalization_string = metric_name_ + 43 std::string(bytes().begin(), bytes().end()); 44 HmacByteVectorGenerator hmac_generator(bytes().size(), secret, 45 personalization_string); 46 const ByteVector fake_mask = 47 hmac_generator.GetWeightedRandomByteVector(parameters().fake_prob); 48 ByteVector fake_bits = 49 hmac_generator.GetWeightedRandomByteVector(parameters().fake_one_prob); 50 51 // Redact most of the real data by replacing it with the fake data, hiding 52 // and limiting the amount of information an individual client reports on. 53 const ByteVector* fake_and_redacted_bits = 54 ByteVectorMerge(fake_mask, bytes(), &fake_bits); 55 56 // Generate biased coin flips for each bit. 57 ByteVectorGenerator coin_generator(bytes().size()); 58 const ByteVector zero_coins = 59 coin_generator.GetWeightedRandomByteVector(parameters().zero_coin_prob); 60 ByteVector one_coins = 61 coin_generator.GetWeightedRandomByteVector(parameters().one_coin_prob); 62 63 // Create a randomized response report on the fake and redacted data, sending 64 // the outcome of flipping a zero coin for the zero bits in that data, and of 65 // flipping a one coin for the one bits in that data, as the final report. 66 return *ByteVectorMerge(*fake_and_redacted_bits, zero_coins, &one_coins); 67 } 68 69 void RapporMetric::SetBytesForTesting(const ByteVector& bytes) { 70 bloom_filter_.SetBytesForTesting(bytes); 71 } 72 73 } // namespace rappor 74
