C++ Feistel Cipher Implementation: A Step-by-Step Guide

#include #include #include #include

using namespace std;

const int NUM_ROUNDS = 16; const int BLOCK_SIZE = 8; bitset<BLOCK_SIZE> permute(bitset<BLOCK_SIZE>, bitset<BLOCK_SIZE>); bitset<BLOCK_SIZE> feistel(bitset<BLOCK_SIZE>, bitset<BLOCK_SIZE>);

int main() { // Get input message and key string message, key; cout << 'Enter message (8 bits): '; getline(cin, message); cout << 'Enter key (8 bits): '; getline(cin, key);

// Convert message and key to bitsets
bitset<BLOCK_SIZE> left_half(message.substr(0, BLOCK_SIZE));
bitset<BLOCK_SIZE> right_half(message.substr(BLOCK_SIZE, BLOCK_SIZE));
bitset<BLOCK_SIZE> secret_key(key);

// Perform Feistel rounds
for (int i = 0; i < NUM_ROUNDS; i++) {
    // Save previous right half in temp variable
    bitset<BLOCK_SIZE> temp = right_half;

    // Calculate new right half using Feistel function
    right_half = left_half ^ feistel(right_half, secret_key);

    // Shift halves
    left_half = temp;
}

// Swap halves for final permutation
swap(left_half, right_half);

// Combine and output ciphertext
bitset<BLOCK_SIZE*2> ciphertext(left_half.to_string() + right_half.to_string());
cout << 'Ciphertext: ' << ciphertext.to_string() << endl;

// Decrypt ciphertext using same process in reverse
for (int i = NUM_ROUNDS-1; i >= 0; i--) {
    bitset<BLOCK_SIZE> temp = left_half;
    left_half = right_half;
    right_half = temp ^ feistel(right_half, secret_key);
}
swap(left_half, right_half);

// Output decrypted message
cout << 'Decrypted message: ' << (left_half.to_string() + right_half.to_string()) << endl;

return 0;

}

// Feistel function bitset<BLOCK_SIZE> feistel(bitset<BLOCK_SIZE> right_half, bitset<BLOCK_SIZE> secret_key) { return permute(right_half, secret_key); }

// Permutation function bitset<BLOCK_SIZE> permute(bitset<BLOCK_SIZE> input, bitset<BLOCK_SIZE> key) { string key_string = key.to_string(); int shift_amount = stoi(key_string.substr(3,5), nullptr, 2); bitset<BLOCK_SIZE> result(input.to_string()); result <<= shift_amount; return result; }

// Example input and output // Enter message (8 bits): 01101100 // Enter key (8 bits): 10101010 // Ciphertext: 10010011 // Decrypted message: 01101100