Ensuring Quality of Shared Keys Through Quantum Key Distribution for Practical Application

We investigated the characteristics of shared keys obtained through quantum key distribution (QKD) from the viewpoint of application in cipher communication. We demonstrated that the shared keys at each stage in QKD satisfy the criteria determined by a standardized randomness test by physically compensating for randomness degrading factors. We also examined the increase in the error rate of the shared keys produced by privacy amplification, and succeeded in suppressing the error rate increase by applying a new privacy amplification scheme, which yielded a sufficiently low error rate for the final keys. These investigations showed that the final keys obtained using quantum cryptosystems are available as crypto keys in cipher communication.