  This research note II introduces a way to understand a basic concept of the
quantum enigma cipher. The conventional cipher is designed by a mathematical
algorithm and its security is evaluated by the complexity of the algorithm in
security analysis and ability of computers. This kind of cipher can be
decrypted with probability one in principle by the Brute force attack in which
an eavesdropper tries all the possible keys based on the correct ciphertext and
some known plaintext. A cipher with quantum effects in physical layer may
protect the system from the Brute force attack by means of the quantum no
cloning theorem and randomizations based on quantum noise effect. The
randomizations for the ciphertext which is the output from the mathematical
encryption box is crucial to realize a quantum enigma cipher. Especially, by
randomizations, it is necessary to make a substantial difference in accuracy of
ciphertext in eavesdropper's observation and legitimate user's observation. The
quantum illumination protocol can make a difference in error performance of the
legitimate's receiver and the eavesdropper's receiver. This difference is due
to differences in ability of the legitimate's receiver with entanglement and
the eavesdropper's receiver without entanglement. It is shown in this note that
the quantum illumination can be employed as an element of the most simple
quantum enigma cipher.
