public static byte []
digestMessage(SIGRecord sig, Message msg, byte [] previous) {
	DNSOutput out = new DNSOutput();
	digestSIG(out, sig);

	if (previous != null)
		out.writeByteArray(previous);

	msg.toWire(out);
	return out.toByteArray();
}
Creates a byte array containing the concatenation of the fields of the
SIG(0) record and the message to be signed.  This does not perform
a cryptographic digest.
@param sig The SIG record used to sign the rrset.
@param msg The message to be signed.
@param previous If this is a response, the signature from the query.
@return The data to be cryptographically signed.
src/main/java/core/org/xbill/DNS/DNSSEC.java