code: 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();
  }
docs: 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.