Abstract
Since quantum key distribution protocol was invented in 1984, huge number of papers on quantum cryptography have been published. However, they have so many serious technical problems which should overcome difficulties. On the other hand, it is reasonable to develop a quantum version of the conventional mathematical symmetric key cipher. In 2000, Yuen protocol was invented to realize a quantum version of stream cipher. In this paper, we introduce the principle and recent progress on quantum stream cipher by Yuen 2000 protocol.
