ARZL

ARZL is a (standard ?) compressed format which is used by the secure bootloader to load the non-secure bootloader and the secure kernel.

Obfuscation

The raw decompressed ARZL output is obfuscated. Although there are three versions of the obfuscation, the basic operation is the same. The obfuscation is just bit swaps as well as some deterministic changes using information from the offset.

int arzl_deobfuscate(unsigned char *buffer, int len)
{
  unsigned char *buf, *bufend;
  uint32_t data;
  int change_stride;

  buf = buffer;
  bufend = &buffer[len];

  do
  {
    data = *(uint32_t *)buf;
    buf += 4;
    change_stride = (data & 0xF800F800) >> 27;
    if ( (data & 0xF800F800) == 0xF800F000 )
    {
      data = (((data >> 16) & 0xFFC007FF) | ((data & 0x7FF) << 11)) - ((buf - buffer) >> 1);
      *((uint32_t *)buf - 1) = ((((data & 0x7FF) << 16) | 0xF800F000) & 0xFFFFF800) | ((data >> 11) & 0x7FF);
    }
    else if ( change_stride == 30 )
    {
      buf -= 2;
    }
  }
  while ( bufend > buf );
}

Version 1

The only change is that the offset information is added instead of subtracted.

data = (((data >> 16) & 0xFFC007FF) | ((data & 0x7FF) << 11)) + ((buf - buffer) >> 1);

Version 2

Version 2 is the same as version 0 but in addition, there's an additional operation to swap two nibbles in certain conditions. The condition is found through a learning process and may be overfitted.

else if ( (data & 0x8000FBF0) == 0x0000F2C0 )
      {
        data = (data & 0xF0FFFFF0) | ((data & 0xF) << 24) | ((data >> 24) & 0xF);
        *((uint32_t *)buf - 1) = data;
      }