Quantum Stream Ciphers: Impossibility of Unconditionally Strong Algorithms

Stream ciphers is one of two large classes of ciphers with private keys in classical cryptography. In this paper, we introduce the concept of a quantum stream cipher. Special types of quantum stream ciphers were proposed earlier by numerous researchers. We prove a general result on the nonexistence of an absolutely strong quantum stream cipher if the length of the message is much longer than the length of the key. We analyze individual and collective attacks against the quantum stream cipher. A relationship between the problem of guessing the key by the opponent and the problem of distinguishing of random quantum states is established.