RAPPOR implementation

components/rappor/byte_vector_utils.h

+// 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.
+
+#ifndef COMPONENTS_RAPPOR_BYTE_VECTOR_UTILS_H_
+#define COMPONENTS_RAPPOR_BYTE_VECTOR_UTILS_H_
+
+#include <vector>
+
+#include "base/basictypes.h"
+#include "components/rappor/rappor_parameters.h"
+#include "crypto/hmac.h"
+
+namespace rappor {
+
+// A vector of 8-bit integers used to store a set of binary bits.
+typedef std::vector<uint8_t> ByteVector;
+
+// Computes a bitwise OR of byte vectors and stores the result in rhs.
+// Returns rhs for chaining.
+ByteVector* ByteVectorOr(const ByteVector& lhs, ByteVector* rhs);
+
+// Merges the contents of lhs and rhs  vectors according to a mask vector.
+// The i-th bit of the result vector will be the i-th bit of either the lhs
+// or rhs vector, based on the i-th bit of the mask vector.
+// Equivalent to (lhs & ~mask) | (rhs & mask).  Stores the result in rhs.
+// Returns rhs for chaining.
+ByteVector* ByteVectorMerge(const ByteVector& mask,
+                            const ByteVector& lhs,
+                            ByteVector* rhs);
+
+// Counts the number of bits set in the byte vector.
+int CountBits(const ByteVector& vector);
+
+// A utility object for generating random binary data with different
+// likelihood of bits being true, using entropy from crypto::RandBytes().
+class ByteVectorGenerator {
+ public:
+  explicit ByteVectorGenerator(size_t byte_count);
+
+  ~ByteVectorGenerator();
+
+  // Generates a random byte vector where the bits are independent random
+  // variables which are true with the given |probability|.
+  ByteVector GetWeightedRandomByteVector(Probability probability);
+
+ protected:
+  // Size of vectors to be generated.
+  size_t byte_count() const { return byte_count_; }
+
+  // Generates a random vector of bytes from a uniform distribution.
+  virtual ByteVector GetRandomByteVector();
+
+ private:
+  size_t byte_count_;
+
+  DISALLOW_COPY_AND_ASSIGN(ByteVectorGenerator);
+};
+
+// A ByteVectorGenerator that uses a psuedo-random function to generate a
+// deterministically random bits.  This class only implements a single request
+// from HMAC_DRBG and streams up to 2^19 bits from that request.
+// Ref: http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf
+// We're using our own PRNG instead of crypto::RandBytes because we need to
+// generate a repeatable sequence of bits from the same seed. Conservatively,
+// we're choosing to use HMAC_DRBG here, as it is one of the best studied
+// and standardized ways of generating deterministic, unpredictable sequences
+// based on a secret seed.
+class HmacByteVectorGenerator : public ByteVectorGenerator {
+ public:
+  // Constructor takes the size of the vector to generate, along with a
+  // |entropy_input| and |personalization_string| to seed the pseudo-random
+  // number generator.  The string parameters are treated as byte arrays.
+  HmacByteVectorGenerator(size_t byte_count,
+                          const std::string& entropy_input,
+                          const std::string& personalization_string);
+
+  ~HmacByteVectorGenerator();
+
+  // Generates a random string suitable for passing to the constructor as
+  // |entropy_input|.
+  static std::string GenerateEntropyInput();
+
+  // Key size required for 128-bit security strength (including nonce).
+  static const size_t kEntropyInputSize;
+
+ protected:
+  // Generate byte vector generator that streams from the next request instead
+  // of the current one.  For testing against NIST test vectors only.
+  explicit HmacByteVectorGenerator(const HmacByteVectorGenerator& prev_request);
+
+  // ByteVector implementation:
+  virtual ByteVector GetRandomByteVector() OVERRIDE;
+
+ private:
+  // HMAC initalized with the value of "Key" HMAC_DRBG_Initialize.
+  crypto::HMAC hmac_;
+
+  // The "V" value from HMAC_DRBG.
+  ByteVector value_;
+
+  // Total number of bytes streamed from the HMAC_DRBG Generate Process.
+  size_t generated_bytes_;
+
+  DISALLOW_ASSIGN(HmacByteVectorGenerator);
+};
+
+}  // namespace rappor
+
+#endif  // COMPONENTS_RAPPOR_BYTE_VECTOR_UTILS_H_