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/mem_ops.h>
#include <botan/internal/bit_ops.h>
#include <botan/internal/code_based_util.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);  // NOLINT(*container-data-pointer)
      copy_mem(&x[dim_bytes], b.data(), bit_size_to_byte_size(codimension));
   } else {
      copy_mem(&x[0], a.data(), (dimension / 8));  // NOLINT(*container-data-pointer)
      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));
      }
      if(const size_t remaining_codim_bits = codimension % 8) {
         const uint8_t final_codim_byte = b[codimension / 8];
         x[l] ^= static_cast<uint8_t>(final_codim_byte << final_bits);
         if(final_bits + remaining_codim_bits > 8) {
            ++l;
            x[l] = static_cast<uint8_t>(final_codim_byte >> (8 - final_bits));
         }
      }
   }
 
   return x;
}
 
secure_vector<uint8_t> mult_by_pubkey(const secure_vector<uint8_t>& cleartext,
                                      const std::vector<uint8_t>& 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));
 
   BOTAN_ARG_CHECK(cleartext.size() == bit_size_to_byte_size(dimension), "Invalid McEliece plaintext length");
   BOTAN_ARG_CHECK(public_matrix.size() == dimension * cR.size(), "Invalid McEliece public matrix length");
 
   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)) != 0) {
            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)) != 0) {
         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) {
      const 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) != 0) {
         continue;  // already set this bit
      }
 
      result[byte_pos] |= mask;
      bits_set++;
   }
 
   return result;
}
 
}  // namespace
 
void mceliece_encrypt(secure_vector<uint8_t>& ciphertext_out,
                      secure_vector<uint8_t>& error_mask_out,
                      const secure_vector<uint8_t>& plaintext,
                      const McEliece_PublicKeyInternal& key,
                      RandomNumberGenerator& rng) {
   const uint16_t code_length = static_cast<uint16_t>(key.code_length());
 
   secure_vector<uint8_t> error_mask = create_random_error_vector(code_length, key.t(), rng);
 
   secure_vector<uint8_t> ciphertext = mult_by_pubkey(plaintext, key.public_matrix(), key.code_length(), key.t());
 
   ciphertext ^= error_mask;
 
   ciphertext_out.swap(ciphertext);
   error_mask_out.swap(error_mask);
}
 
}  // namespace Botan