mceliece.cpp

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/*
 * (C) Copyright Projet SECRET, INRIA, Rocquencourt
 * (C) Bhaskar Biswas and  Nicolas Sendrier
 *
 * (C) 2014 cryptosource GmbH
 * (C) 2014 Falko Strenzke fstrenzke@cryptosource.de
 *
 * Botan is released under the Simplified BSD License (see license.txt)
 *
 */

#include <botan/internal/mce_internal.h>
#include <botan/mceliece.h>
#include <botan/internal/code_based_util.h>
#include <botan/internal/bit_ops.h>

namespace Botan {

namespace {

secure_vector<uint8_t> concat_vectors(const secure_vector<uint8_t>& a,
                                      const secure_vector<uint8_t>& b,
                                      size_t dimension,
                                      size_t codimension)
   {
   secure_vector<uint8_t> x(bit_size_to_byte_size(dimension) + bit_size_to_byte_size(codimension));

   const size_t final_bits = dimension % 8;

   if(final_bits == 0)
      {
      const size_t dim_bytes = bit_size_to_byte_size(dimension);
      copy_mem(&x[0], a.data(), dim_bytes);
      copy_mem(&x[dim_bytes], b.data(), bit_size_to_byte_size(codimension));
      }
   else
      {
      copy_mem(&x[0], a.data(), (dimension / 8));
      size_t l = dimension / 8;
      x[l] = static_cast<uint8_t>(a[l] & ((1 << final_bits) - 1));

      for(size_t k = 0; k < codimension / 8; ++k)
         {
         x[l] ^= static_cast<uint8_t>(b[k] << final_bits);
         ++l;
         x[l] = static_cast<uint8_t>(b[k] >> (8 - final_bits));
         }
      x[l] ^= static_cast<uint8_t>(b[codimension/8] << final_bits);
      }

   return x;
   }

secure_vector<uint8_t> mult_by_pubkey(const secure_vector<uint8_t>& cleartext,
                                      std::vector<uint8_t> const& public_matrix,
                                      size_t code_length, size_t t)
   {
   const size_t ext_deg = ceil_log2(code_length);
   const size_t codimension = ext_deg * t;
   const size_t dimension = code_length - codimension;
   secure_vector<uint8_t> cR(bit_size_to_32bit_size(codimension) * sizeof(uint32_t));

   const uint8_t* pt = public_matrix.data();

   for(size_t i = 0; i < dimension / 8; ++i)
      {
      for(size_t j = 0; j < 8; ++j)
         {
         if(cleartext[i] & (1 << j))
            {
            xor_buf(cR.data(), pt, cR.size());
            }
         pt += cR.size();
         }
      }

   for(size_t i = 0; i < dimension % 8 ; ++i)
      {
      if(cleartext[dimension/8] & (1 << i))
         {
         xor_buf(cR.data(), pt, cR.size());
         }
      pt += cR.size();
      }

   secure_vector<uint8_t> ciphertext = concat_vectors(cleartext, cR, dimension, codimension);
   ciphertext.resize((code_length+7)/8);
   return ciphertext;
   }

secure_vector<uint8_t> create_random_error_vector(size_t code_length,
                                                  size_t error_weight,
                                                  RandomNumberGenerator& rng)
   {
   secure_vector<uint8_t> result((code_length+7)/8);

   size_t bits_set = 0;

   while(bits_set < error_weight)
      {
      gf2m x = random_code_element(static_cast<uint16_t>(code_length), rng);

      const size_t byte_pos = x / 8;
      const size_t bit_pos = x % 8;

      const uint8_t mask = (1 << bit_pos);

      if(result[byte_pos] & mask)
         continue; // already set this bit

      result[byte_pos] |= mask;
      bits_set++;
      }

   return result;
   }

}

void mceliece_encrypt(secure_vector<uint8_t>& ciphertext_out,
                      secure_vector<uint8_t>& error_mask_out,
                      const secure_vector<uint8_t>& plaintext,
                      const McEliece_PublicKey& key,
                      RandomNumberGenerator& rng)
   {
   const uint16_t code_length = static_cast<uint16_t>(key.get_code_length());

   secure_vector<uint8_t> error_mask = create_random_error_vector(code_length, key.get_t(), rng);

   secure_vector<uint8_t> ciphertext = mult_by_pubkey(plaintext, key.get_public_matrix(),
                                                      key.get_code_length(), key.get_t());

   ciphertext ^= error_mask;

   ciphertext_out.swap(ciphertext);
   error_mask_out.swap(error_mask);
   }

}