Advantage of the Key Relay Protocol Over Secure Network Coding

The key relay protocol (KRP) plays an important role in improving the performance and the security of quantum key distribution networks. On the other hand, there is also an existing research field called secure network coding (SNC), which has similar goal and structure. In this article, we analyze differences and similarities between KRPs in general and key relay using SNC schemes (KRPs-by-SNC) rigorously. We found, rather surprisingly, that there is a definite gap in security between KRPs in general and KRPs-by-SNC; that is, certain KRPs achieve better security than any SNC schemes on the same graph. We also found that this gap can be closed if we generalize the notion of SNC by adding authenticated public channels; that is, KRPs are equivalent to KRPs-by-SNC schemes augmented with authenticated public channels.