We have an embedded product, which we are carrying for several hardware iterations since more than 5 years ago. We have all the source code, most of it nicely documented. As the product is actively sold and needs an upgrade, I have been tasked to design next generation improved version.

While we have all the source code, and most of it is well documented, the security part is not, especially its slowest part, a 128-bit hash function with 72 rounds. I can step trough it in our debugger, execute it, and the code works fine except its slowness. If I would know which cryptography standard it is, I could attempt to upgrade it to be executed in hardware, which would drastically speed it up. (The original consultant who wrote this function is not with our company and I cannot find her).

Is there any logical way to find out which encryption are we executing? I do not need a help to do my work, I am asking for an advice if there is any strategy how to look at code ('unoptimized mess'); learning the principle of it (like this one has 72 rounds and hashes 128 bits); then finding candidate standards doing the same thing.

I would think that after finding a documented standard, I should be able to hash various numbers by using both our hash and the standard hash, and simply see if the results match?

I looked on the web for information about known functions using 72 rounds, like Skein algorithm, but could not find anything similar.

Are there any known hash values for known standards, for example results of hashing 0? This hash converts 0 to these 16 hexadecimal bytes:

ae c5 40 44 df 2d 91 1c 87 ab 1a ff 59 09 aa b7

Function beginning and end is below, as some of you requested, the full function is at: full_function_code

It looks like the previous programmer let the old C compiler to convert it, and then used it in 'assembly' style. Maybe it was due to avoid compiler optimizing, just my guess.

There are 72 rounds, each of which ends with a call to function TricoreDextr(); which is the C equivalent of Infineon TriCore DEXTR instruction. Think of DEXTR as splitting a 64-bit number into two parts (at the selected bit location) and swapping them.

Any concept explanations is a great help, thank you!

/*
 * @param in_arr    16 byte long array, input data (128 bits)
 * @param out_arr   16 byte long array, output data (128 bits)
 */
void Hash16bytes(unsigned char *in_arr, unsigned char *out_arr) 
{
  long d0, d1, d2, d3, d4, d5, d6, d7, d8, d9, d10, d11, d12, d13, d15;
  long IN[16];

  d4 = 0xB12E8FB5;

  int i;
  for (i = 0; i < 16; i++)
  {
      IN[i] = (long) (*(in_arr + i));
  }

  d11 = (IN[3] << 24) | (IN[2] << 16) | (IN[1] << 8) | IN[0];
  d10 = (IN[7] << 24) | (IN[6] << 16) | (IN[5] << 8) | IN[4];
  d12 = (IN[0xB] << 24) | (IN[0xA] << 16) | (IN[9] << 8) | IN[8];
  d13 = (IN[0xF] << 24) | (IN[0xE] << 16) | (IN[0xD] << 8) | IN[0xC];

  /* block like this below repeats 72 times... */
  d3 = 0x7025BD47;
  d8 = 0xCCEE4EFE;
  d8 += d13; 
  d8 += d3;
  d8 = TricoreDextr(d8, 6);

  /* 
     etc., TricoreDextr is called total of 72 times 
   */

  d0 += d15;
  d0 += 0x5C4E0000;
  d0 -= 0x2EDC;
  d0 = TricoreDextr(d0, 5);

  *(out_arr + 3) = (unsigned char) ((d5 >> 24) & 0xFF);
  *(out_arr + 2) = (unsigned char) ((d5 >> 16) & 0xFF);
  *(out_arr + 1) = (unsigned char) ((d5 >> 8) & 0xFF);
  *(out_arr + 0) = (unsigned char) ((d5) & 0xFF);

  *(out_arr + 7) = (unsigned char) ((d6 >> 24) & 0xFF);
  *(out_arr + 6) = (unsigned char) ((d6 >> 16) & 0xFF);
  *(out_arr + 5) = (unsigned char) ((d6 >> 8) & 0xFF);
  *(out_arr + 4) = (unsigned char) ((d6) & 0xFF);

  d8 += d0;
  d8 += 0x10320000;
  d8 += 0x5476;

  *(out_arr + 0xB) = (unsigned char) ((d8 >> 24) & 0xFF);
  *(out_arr + 0xA) = (unsigned char) ((d8 >> 16) & 0xFF);
  *(out_arr + 9) = (unsigned char) ((d8 >> 8) & 0xFF);
  *(out_arr + 8) = (unsigned char) ((d8) & 0xFF);

  *(out_arr + 0xF) = (unsigned char) ((d7 >> 24) & 0xFF);
  *(out_arr + 0xE) = (unsigned char) ((d7 >> 16) & 0xFF);
  *(out_arr + 0xD) = (unsigned char) ((d7 >> 8) & 0xFF);
  *(out_arr + 0xC) = (unsigned char) ((d7) & 0xFF);
}