void pamaury_xor_it(uint8_t compute_checksum(uint8_t *buf, int size, uint8_t t[20]) { memset(t, 0, sizeof(t)); 20); for(int i = 0; i < size; i++) t[i % 20] ^= buf[i]; for(int i = 0; i < 20; i++) t[i] = ~t[i]; } int check_block(uint8_t *buf, uint8_t ref[20], unsigned size) { uint8_t t[20]; compute_checksum(buf, size, t); return memcmp(ref, t, 20); } 

Block A

The A block contains mainy information about what looks like a decompression algorithm. This block is 1024 bytes long and is located at block blockA+2 (multiply this by 512 to get the offset). This block is XOR encoded using a key found in the block itself. The following procedure shows how to decode it. It uses a 1024 byte tables called g_check_block_A_table in the code below.

static int decode_block_A(uint8_t block[1020])
{
    uint8_t *p = &g_check_block_A_table[32 * (block[998] & 0x1f)];
    uint8_t key[32];

    for(int i = 0; i < 20; i++)
    {
        block[1000 + i] ^= p[i];
        key[i] = block[1000 + i];
    }
    for(int i = 20; i < 32; i++)
        key[i] = key[i - 20];
    
    for(int i = 0; i < 992; i++)
        block[i] ^= key[i % 32] ^ g_check_block_A_table[i];
    
    return check_block(block - 1, block + 1000, 1001);
}