  So far, quantum key distribution (QKD) has been the main subject in the field
of quantum cryptography, but that is not quantum cryptographic communication,
it is only the ability to send keys for cryptographic purposes. To complete
cryptographic communication, a technique for encrypting data is necessary, and
the conventional cryptographic technique of mathematical symmetric key cipher
or One Time Pad (OTP) is adopted in the discussion so far. However, OTP is not
the ultimate cipher for data encryption, because it does not satisfy security
conditions in the modern cryptology. Around 2000, a new quantum stream cipher
was proposed as a technique to challenge the possibility of overcoming
drawbacks of OTP in practical use. Recently, we have published some review
papers on it in Entropy (Open access journal), and others. This paper
introduces an overview and a back ground of our paper that is entitled Quantum
stream cipher based on Holevo-Yuen theory.
