std::vector<uint8_t> modify_block(const std::vector<uint8_t> ciphertext, uint64_t blocknumer, uint16_t delta){
  std::vector<uint8_t>::const_iterator first = ciphertext.begin() + blocknumer*sha1::block_size;
  std::vector<uint8_t>::const_iterator last = ciphertext.begin() + (blocknumer+1)*sha1::block_size;
  std::vector<uint8_t> block(first, last);


  block[block.size()-2] = block[block.size()-2] ^ delta;
  block[block.size()-1] = block[block.size()-1] ^ delta;

  return block;
}