Diffie-Hellman Protocol as a Symmetric Cryptosystem

摘要

The Diffie-Hellman protocol is commonly considered as an asymmetric cryptosystem. However, this paper shows that the current versions of the Diffie-Hellman protocol can, in both directions of communications, be interpreted as a series of two consecutive encryptions using a symmetric cipher. The cipher used is distinguished by a specific feature: the result of a series of several encryptions is independent of the keys' order. Each of the communicating parties encrypts the default message with its own secret key. Then, both parties send their cryptograms to each other, and each of them encrypts the received cryptogram with its own key. This way, both parties obtain the same resulting cryptogram, which is then used as a seed. The interpretation introduced above leads to a “heretical” question of whether the Diffie-Hellman protocol falls into the asymmetric or symmetric cryptosystem category.