code: public static RRSIGRecord
  sign(RRset rrset, DNSKEYRecord key, PrivateKey privkey,
       Date inception, Date expiration) throws DNSSECException
  {
  	return sign(rrset, key, privkey, inception, expiration, null);
  }
docs: Generate a DNSSEC signature.  key and privateKey must refer to the
  same underlying cryptographic key.
  @param rrset The data to be signed
  @param key The DNSKEY record to use as part of signing
  @param privkey The PrivateKey to use when signing
  @param inception The time at which the signatures should become valid
  @param expiration The time at which the signatures should expire
  @throws UnsupportedAlgorithmException The algorithm is unknown
  @throws MalformedKeyException The key is malformed
  @throws DNSSECException Some other error occurred.
  @return The generated signature