Help reverse decrypt function (decode -> encode)

Question

I have this java function that decode 8bytes text string(encoded as int[]):

public static int[] decode(int[] text, int[] key) {
   int j = 0x3c; //value 60 in decimal
   int i = 0x33; //value 51 in decimal
   int[] result;

   for ( i = 0x33; ; (text[1]) ^= ((key[j]) ^ (text[4])) + i ){
       j = j - 1;
       text[7] ^= ((key[j--]) ^ (text[3])) + i;
       text[6] ^= ((key[j--]) ^ (text[2])) + i;
       text[5] ^= ((key[j]) ^ (text[1])) + i;
       j = j - 1;
       result = text;
       (text[4]) ^= ((key[j]) ^ text[0]) + i;
       if ( j <= 0 )
           break;
       j = j - 1;
       text[0] ^= ((key[j--]) ^ (text[7])) + i;
       text[3] ^= ((key[j]) ^ (text[6])) + i;
       text[2] ^= ((text[4]) ^ (text[5])) + i;
       j = j - 1;
       //System.out.println("i " +i+" - j "+j);
   }
return result;
}

but if I wanted to do the reverse, how can build the encode function? how do I build the inverse method(from decoded text to encode text)?

Answer 1

For

a = b ^ c

Since the XOR operation is trivial to reverse when any 2 values in a, b, c are known, this will only require you to just reverse the order in which operations were done. I had to do this in C since I don't use Java.

int *encode(int *text, int *key) {
  int j = 0x0;
  int i = 0x33; // value 51 in decimal

  for (i = 0x33;;) {
    text[4] ^= ((key[j++]) ^ (text[0])) + i;
    text[5] ^= ((key[j++]) ^ (text[1])) + i;
    text[6] ^= ((key[j++]) ^ (text[2])) + i;
    text[7] ^= ((key[j++]) ^ (text[3])) + i;
    text[1] ^= ((key[j++]) ^ (text[4])) + i;
    if (j >= 0x3c)
      break;
    text[2] ^= ((text[4]) ^ (text[5])) + i;
    text[3] ^= ((key[j++]) ^ (text[6])) + i;
    text[0] ^= ((key[j++]) ^ (text[7])) + i;
  }
  return text;
}

Here's the full code that works.

#include <stdio.h>

int *decode(int *text, int *key) {
  int j = 0x3c; // value 60 in decimal
  int i = 0x33; // value 51 in decimal

  for (i = 0x33;;) {
    text[1] ^= ((key[j--]) ^ (text[4])) + i;
    text[7] ^= ((key[j--]) ^ (text[3])) + i;
    text[6] ^= ((key[j--]) ^ (text[2])) + i;
    text[5] ^= ((key[j--]) ^ (text[1])) + i;
    text[4] ^= ((key[j--]) ^ (text[0])) + i;
    if (j <= 0)
      break;
    text[0] ^= ((key[j--]) ^ (text[7])) + i;
    text[3] ^= ((key[j--]) ^ (text[6])) + i;
    text[2] ^= ((text[4]) ^ (text[5])) + i;
  }
  return text;
}

int *encode(int *text, int *key) {
  int j = 0x0;
  int i = 0x33; // value 51 in decimal

  for (i = 0x33;;) {
    text[4] ^= ((key[j++]) ^ (text[0])) + i;
    text[5] ^= ((key[j++]) ^ (text[1])) + i;
    text[6] ^= ((key[j++]) ^ (text[2])) + i;
    text[7] ^= ((key[j++]) ^ (text[3])) + i;
    text[1] ^= ((key[j++]) ^ (text[4])) + i;
    if (j >= 0x3c)
      break;
    text[2] ^= ((text[4]) ^ (text[5])) + i;
    text[3] ^= ((key[j++]) ^ (text[6])) + i;
    text[0] ^= ((key[j++]) ^ (text[7])) + i;
  }
  return text;
}

int key[] = {0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41,
             0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41,
             0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41,
             0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41,
             0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41,
             0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41, 0x41};
int text[] = {0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68};

int main(int argc, char **argv) {
  int i, *d, *e;
  for (i = 0; i < 8; i++) {
    printf("%x::", text[i]);
  }
  putchar(10);
  d = decode(text, key);
  for (i = 0; i < 8; i++) {
    printf("%x::", d[i]);
  }
  putchar(10);
  e = encode(text, key);
  for (i = 0; i < 8; i++) {
    printf("%x::", e[i]);
  }
  putchar(10);
}

This produces the following output

61::62::63::64::65::66::67::68::
65a::3e9::64a::bd6::5c2::11::1a9::85e::
61::62::63::64::65::66::67::68::

I was able to get back the original array after a decode and encode.