components/rappor/byte_vector_utils.h

   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 #ifndef COMPONENTS_RAPPOR_BYTE_VECTOR_UTILS_H_
   6 #define COMPONENTS_RAPPOR_BYTE_VECTOR_UTILS_H_
   7
   8 #include <vector>
   9
  10 #include "base/basictypes.h"
  11 #include "base/macros.h"
  12 #include "components/rappor/rappor_parameters.h"
  13 #include "crypto/hmac.h"
  14
  15 namespace rappor {
  16
  17 // A vector of 8-bit integers used to store a set of binary bits.
  18 typedef std::vector<uint8_t> ByteVector;
  19
  20 // Computes a bitwise AND of byte vectors and stores the result in rhs.
  21 // Returns rhs for chaining.
  22 ByteVector* ByteVectorAnd(const ByteVector& lhs, ByteVector* rhs);
  23
  24 // Computes a bitwise OR of byte vectors and stores the result in rhs.
  25 // Returns rhs for chaining.
  26 ByteVector* ByteVectorOr(const ByteVector& lhs, ByteVector* rhs);
  27
  28 // Merges the contents of lhs and rhs  vectors according to a mask vector.
  29 // The i-th bit of the result vector will be the i-th bit of either the lhs
  30 // or rhs vector, based on the i-th bit of the mask vector.
  31 // Equivalent to (lhs & ~mask) | (rhs & mask).  Stores the result in rhs.
  32 // Returns rhs for chaining.
  33 ByteVector* ByteVectorMerge(const ByteVector& mask,
  34                             const ByteVector& lhs,
  35                             ByteVector* rhs);
  36
  37 // Counts the number of bits set in the byte vector.
  38 int CountBits(const ByteVector& vector);
  39
  40 // A utility object for generating random binary data with different
  41 // likelihood of bits being true, using entropy from crypto::RandBytes().
  42 class ByteVectorGenerator {
  43  public:
  44   explicit ByteVectorGenerator(size_t byte_count);
  45
  46   ~ByteVectorGenerator();
  47
  48   // Generates a random byte vector where the bits are independent random
  49   // variables which are true with the given |probability|.
  50   ByteVector GetWeightedRandomByteVector(Probability probability);
  51
  52  protected:
  53   // Size of vectors to be generated.
  54   size_t byte_count() const { return byte_count_; }
  55
  56   // Generates a random vector of bytes from a uniform distribution.
  57   virtual ByteVector GetRandomByteVector();
  58
  59  private:
  60   size_t byte_count_;
  61
  62   DISALLOW_COPY_AND_ASSIGN(ByteVectorGenerator);
  63 };
  64
  65 // A ByteVectorGenerator that uses a psuedo-random function to generate a
  66 // deterministically random bits.  This class only implements a single request
  67 // from HMAC_DRBG and streams up to 2^19 bits from that request.
  68 // Ref: http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf
  69 // We're using our own PRNG instead of crypto::RandBytes because we need to
  70 // generate a repeatable sequence of bits from the same seed. Conservatively,
  71 // we're choosing to use HMAC_DRBG here, as it is one of the best studied
  72 // and standardized ways of generating deterministic, unpredictable sequences
  73 // based on a secret seed.
  74 class HmacByteVectorGenerator : public ByteVectorGenerator {
  75  public:
  76   // Constructor takes the size of the vector to generate, along with a
  77   // |entropy_input| and |personalization_string| to seed the pseudo-random
  78   // number generator.  The string parameters are treated as byte arrays.
  79   HmacByteVectorGenerator(size_t byte_count,
  80                           const std::string& entropy_input,
  81                           const std::string& personalization_string);
  82
  83   ~HmacByteVectorGenerator();
  84
  85   // Generates a random string suitable for passing to the constructor as
  86   // |entropy_input|.
  87   static std::string GenerateEntropyInput();
  88
  89   // Key size required for 128-bit security strength (including nonce).
  90   static const size_t kEntropyInputSize;
  91
  92  protected:
  93   // Generate byte vector generator that streams from the next request instead
  94   // of the current one.  For testing against NIST test vectors only.
  95   explicit HmacByteVectorGenerator(const HmacByteVectorGenerator& prev_request);
  96
  97   // ByteVector implementation:
  98   virtual ByteVector GetRandomByteVector() OVERRIDE;
  99
 100  private:
 101   // HMAC initalized with the value of "Key" HMAC_DRBG_Initialize.
 102   crypto::HMAC hmac_;
 103
 104   // The "V" value from HMAC_DRBG.
 105   ByteVector value_;
 106
 107   // Total number of bytes streamed from the HMAC_DRBG Generate Process.
 108   size_t generated_bytes_;
 109
 110   DISALLOW_ASSIGN(HmacByteVectorGenerator);
 111 };
 112
 113 }  // namespace rappor
 114
 115 #endif  // COMPONENTS_RAPPOR_BYTE_VECTOR_UTILS_H_