反^运算符进行解密

Question

I'm trying to reverse the following code in order to provide a function which takes the buffer and decrypts it.

void crypt_buffer(unsigned char *buffer, size_t size, char *key) {
    size_t i;
    int j;

    j = 0;
    for(i = 0; i < size; i++) {
        if(j >= KEY_SIZE)
            j = 0;
        buffer[i] ^= key[j];
        j++;
    }
}

I was wordering if a simple buffer[i] ^= (1/key[j]); will be enough to decrypt the encrypted file. But there will be some issues with truncation, isn't it?

Answer 1

This is not a power operator. It is the XOR operator. The thing that you notice for the XOR operator is that x ^ k ^ k == x . That means that your encryption function is already the decryption function when called with the same key and the ciphertext instead of the plaintext.

Answer 2

In C, the ^ is not the “power” operator (there is no such operator) but the “xor” operator.

Which is great news for you (I assume), as the reverse operation of “xor” is “xor” itself :-)