Question

I’m very interested in cryptography, and since I like programming too, I decided to make a little program to encrypt files using XTEA encryption algorithm.
I got inspired from Wikipedia, and so I wrote this function to do the encryption (To save space, I won’t post the deciphering function, as it is almost the same):

void encipher(long *v, long *k)
{
    long v0 = v[0], v1 = v[1];
    long sum = 0;   
    long  delta = 0x9e3779b9;
    short rounds = 32;
    for(uint32 i = 0; i<rounds; i++)
    {
        v0 += (((v1 << 4) ^ (v1 >> 5)) + v1) ^ (sum + k[sum & 3]);
      sum += delta;
        v1 += (((v0 << 4) ^ (v0 >> 5)) + v0) ^ (sum + k[(sum>>11) & 3]);
    }
    v[0] = v1;
    v[1] = v1;
}

Now when I want to use it, I wrote this code:

long data[2]; // v0 and v1, 64bits
data[0] = 1;
data[1] = 1;

long key[4]; // 4 * 4 bytes = 16bytes = 128bits
*key = 123; // sets the key

cout << "READ: \t\t" << data[0] << endl << "\t\t" <<  data[1] << endl;
encipher(data, key);
cout << "ENCIPHERED: \t" << data[0] << endl << "\t\t" << data[1] << endl;

decipher(data, key);
cout << "DECIPHERED: \t" << data[0] << endl <<  "\t\t" <<  data[1] << endl;

I always get either run-time crash or wrong decipher text:

I do understand the basics of the program, but I don’t really know what is wrong with my code. Why is the enciphered data[0] and data1 the same? And why is deciphered data completely different from the starting data? Am I using the types wrong?
I hope you can help me solving my problem 🙂 .

• Jan

Answer 1

The problem is here:

v[0] = v1; // should be v[0] = v0
v[1] = v1;

Also, you only set the first 4 bytes of the key. The remaining 12 bytes are uninitialized.
Try something like this:

key[0] = 0x12345678;
key[1] = 0x90ABCDEF;
key[2] = 0xFEDCBA09;
key[3] = 0x87654321;

The fixed code gives me this output:

READ:           1
                1
ENCIPHERED:     -303182565
                -1255815002
DECIPHERED:     1
                1