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private String getChecksumWithPrefix(String prefix)
    {
        MessageDigest md;
        try
        {
            md = MessageDigest.getInstance("SHA-1");
        }
        catch(NoSuchAlgorithmException e)
        {
            throw new RuntimeException("Could not find SHA-1 algorithm");
        }
        md.update(prefix.getBytes());
        for(int i = 0; i < 6; i++)
            md.update((byte)(barrel_values[i] + 48));

        byte b[] = md.digest();
        StringBuffer hexString = new StringBuffer();
        for(int i = 0; i < b.length; i++)
        {
            String o = Integer.toHexString(0xff & b[i]);
            if(o.length() < 2)
                o = "0" + o;
            hexString.append(o);
        }

        String h = hexString.toString();
        return hexString.toString();
    }