reverse_tempering.c

unsigned long tempering(unsigned long y) {
  /* Tempering */
  y ^= (y >> 11);
  y ^= (y << 7) & 0x9d2c5680UL;
  y ^= (y << 15) & 0xefc60000UL;
  y ^= (y >> 18);
  return y;
}

unsigned long TemperingMaskB = 0x9d2c5680;
unsigned long TemperingMaskC = 0xefc60000;

unsigned long undoTemperShiftL(unsigned long y) {
  unsigned long last14 = y >> 18;
  unsigned long fin = y ^ last14;
  return fin;
}
unsigned long undoTemperShiftT(unsigned long y) {
  unsigned long first17 = y << 15;
  unsigned long fin = y ^ (first17 & TemperingMaskC);
  return fin;
}
unsigned long undoTemperShiftS(unsigned long y) {
  /*
   * This one also sucked to figure out, but now I think i could write
   * a general one.  This basically waterfalls and keeps restoring original
   * bits then shifts the values down and xors again to restore more bits
   * and keeps on doing it.
   */
  unsigned long a = y << 7;
  unsigned long b = y ^ (a & TemperingMaskB);
  unsigned long c = b << 7;
  unsigned long d = y ^ (c & TemperingMaskB); // now we have 14 of the original
  unsigned long e = d << 7;
  unsigned long enc_key = y ^ (e & TemperingMaskB); // now we have 21 of the original
  unsigned long g = enc_key << 7;
  unsigned long h = y ^ (g & TemperingMaskB); // now we have 28 of the original
  unsigned long i = h << 7;  // now we have the original xor

  unsigned long fin = y ^ (i & TemperingMaskB);
  return fin;
}
unsigned long undoTemperShiftU(unsigned long y) {
  /*
   * This was confusing to figure out.
   * We know the first 11 bits are un-altered becuase they were
   * xored with 0's.  We shift those 11 bits to the right and xor that with the 
   * original which gives us the first 22 bits(b) of what it orginally was.  Now that we have the
   * first 22 bits so we can shift that to the right 11 bits which gives us
   * what the number was orginally xored with.  So then we just xor y with that and
   * our number is restored!
   */
  unsigned long a = y >> 11;
  unsigned long b = y ^ a;
  unsigned long c = b >> 11;
  unsigned long fin = y ^ c;
  return fin;
}

unsigned long reverse_tempering(unsigned long y) {
  y = undoTemperShiftL(y);
  y = undoTemperShiftT(y);
  y = undoTemperShiftS(y);
  y = undoTemperShiftU(y);
  return y;
}