private static String calCode(String imei) throws NoSuchAlgorithmException {
    /* Get binary md5 hash of IMEI number */
    MessageDigest messagedigest = MessageDigest.getInstance("MD5");
    messagedigest.update(imei.getBytes());
    byte imeiMD5[] = messagedigest.digest();

    /* Unlock code is 8 digits long */
    byte unlockCode[] = new byte[8];

    /* Calculate each unlock character */
    for(int i = 0; i < 8; i++) {
        int magicalSum = (imeiMD5[i] + imeiMD5[i + 8]) & 0xff; // Get last byte of two md5 byte sum
        unlockCode[i] = (byte) ((magicalSum * 9) / 255 + 48); // We need to add 48 ('0') here to get readable digit in ASCII
    }

    return new String(unlockCode);
}