@Override
  public byte[] unwrap(byte[] incoming, int offset, int len) throws SaslException {
    throwIfNotComplete();
    throw new IllegalStateException("PLAIN supports neither integrity nor privacy");
  }
This class was copied from Hadoop Common (3.1.2) and subsequently modified.
hbase-examples/src/main/java/org/apache/hadoop/hbase/security/provider/example/SaslPlainServer.java