C++ program using <bitset> running pretty slow

Question

int main(){
  double one, zero, bias;
  double max = -2.0;
  string plaintext, ciphertext, firstFourStr, lastFourStr;
  bitset<16> V;
  bitset<16> base("0000110000001011");
  bitset<16> targetKey;
  for(int k=0; k<16; k++){
    ifstream plaintexts ("plaintexts.txt");
    ifstream ciphertexts ("ciphertext07.txt");
    bitset<12> tmpBs(k);
    bitset<16> add(tmpBs.to_string()+"0000");
    bitset<16> guessedKey1(add^base);
    for(int j=0; j<16; j++){
      bitset<4> tmpBs2(j);
      bitset<16> add2(tmpBs2.to_string()+"000000000000");
      bitset<16> guessedKey(guessedKey1^add2);
      one = zero = 0.0;
      for(int i=0; i<20000; i++){
        getline(plaintexts, plaintext);
        getline(ciphertexts, ciphertext);
        bitset<16> pTxt(plaintext);
        bitset<16> cTxt(ciphertext);
        V = guessedKey^cTxt;
        bitset<4> firstFour((V.to_string()).substr(0,4));
        bitset<4> secondFour((V.to_string()).substr(4,4));
        bitset<4> thirdFour((V.to_string()).substr(8,4));
        bitset<4> lastFour((V.to_string()).substr(12,4));
        bitset<16> U(sBoxInverse(firstFour)+sBoxInverse(secondFour)+sBoxInverse(thirdFour)+sBoxInverse(lastFour));
        if((U[2]^U[6]^U[10]^U[14]^pTxt[4]^pTxt[7]^pTxt[12]^pTxt[15]) == 0) zero++;
        else one++;
      }
      plaintexts.close();
      ciphertexts.close();
      bias = zero/(zero+one)-0.5;
      if(bias < 0) bias *= -1;
      if(max <= bias){
        max = bias;
        targetKey = guessedKey;
      }
      cout << bias << endl;
    }
  }
  cout << targetKey << ": " << max << endl;
}

I was doing my cryptography assignment and this is the program I wrote to get the key. But it runs pretty slow. I tried to replace some lines of code with simpler ones to check if operations like to_string() or set() caused the problem but it seems that they have nothing to do with the problem. So what causes the program to run slow?

Answer 1

Probably the thing which is slowest in that code is not the std::bitsets, but rather the extensive (and unnecessary) use of std::strings to manipulate them. For example:

 bitset<16> add2(tmpBs2.to_string()+"000000000000");

requires creating a std::string from a bitset, making a new std::string by concatenating that with a constant C string, and then reinterpreting the result as a std::bitset.

You'd be much better off manipulating the std::bitsets as unsigned integers and using bitwise operators and shifts.