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static int guncon3_decode(unsigned char *data, const unsigned char *key) {
    int x, y, key_index;
    unsigned char bkey, keyr, byte;
    s32 a_sum, b_sum;
    s32 key_offset;

    b_sum = ((data[13] ^ data[12]) + data[11] + data[10] - data[9] - data[8]) ^ data[7];
    b_sum&=0xFF;
    a_sum = (((data[6] ^ b_sum) - data[5] - data[4]) ^ data[3]) + data[2] + data[1] - data[0];
    a_sum &=0xFF;
    if (a_sum != key[7]) {
        if (debug)
            printk(KERN_ERR "checksum mismatch: %02x %02x\n", a_sum, key[7]);
        return -1;
    }

    key_offset = (((((key[1] ^ key[2]) - key[3] - key[4]) ^ key[5]) + key[6] - key[7]) ^ data[14]) + (unsigned char)0x26;
    key_offset &= 0xFF;
    key_index = 4;

    //byte E is part of the key offset
    // byte D is ignored, possibly a padding byte - make the checksum workout
    for (x = 12; x >= 0; x--) {
        byte = data[x];
        for (y = 4; y > 1; y--) { // loop 3 times
            key_offset--;

            bkey = KEY_TABLE[key_offset + 0x41];
            keyr = key[key_index];
            if (--key_index == 0)
                key_index = 7;

            if ((bkey & 3) == 0)
                byte =(byte - bkey) - keyr;
            else if ((bkey & 3) == 1)
                byte = ((byte + bkey) + keyr);
            else
                byte = ((byte ^ bkey) ^ keyr);
        }
        data[x] = byte;
    }
    return 0;
}