#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

typedef uint32_t u32;
typedef uint8_t u8;

void decrypt_bin(u8 *binary, u32 size)
{
  u8 magic[4] = { 0x6C, 0x37, 0x51, 0x73 };
  u32 idx, window;
  for ( window=0, idx=0; idx < size; idx++, window++ )
  {
    window &= 3;
    binary[idx] ^= magic[window];
  }
}

int main(int argc, char *argv[])
{
  if ( argc < 3 )
  {
    fprintf(stderr, "Usage:\n\t%s in.bin out.bin\n", argv[0]);
    return EXIT_FAILURE;
  }

  FILE *fp = fopen(argv[1], "rb");
  if ( fp == NULL )
    return EXIT_FAILURE;
  fseek(fp, 0, SEEK_END);
  u32 size = ftell(fp);
  fseek(fp, 0, SEEK_SET);
  u8 *binary = malloc(size);
  if ( binary == NULL )
  {
    fclose(fp);
    return EXIT_FAILURE;
  }
  fread(binary, size, 1, fp);
  fclose(fp);

  decrypt_bin(binary, size);

  fp = fopen(argv[2], "wb");
  if ( fp == NULL )
  {
    free(binary);
    return EXIT_FAILURE;
  }
  fwrite(binary, size, 1, fp);
  fclose(fp);
  free(binary);
  return EXIT_SUCCESS;
}