  The previous work showed that the Y00 protocol could stay secure with the
eavesdropper's guessing probability on the secret keys being strictly less than
one under an unlimitedly long known-plaintext attack with quantum memory.
However, an assumption that at least a fast correlation attack is completely
disabled by irregular mapping. The present study shows that the Y00 protocol
can be information-theoretic secure under any quantum-computational
crypto-analyses if the Y00 system is well designed. The Y00 protocol directly
encrypts messages with short secret keys expanded into pseudo-random running
keys unlike One-Time Pad. However, it may offer information-theoretic security
beyond the Shannon limit of cryptography.
