On Strengthening Authentication Protocols to Foil Cryptanalysis

Cryptographic protocols have usually been designed at an abstract level without concern for the cryptographic algorithms used in implementation. In this paper it is shown that the abstract protocol definition can have an important effect on the ability of an attacker to mount a successful attack on an implementation. In particular, it will be determined whether an adversary is able to generate corresponding pairs of plaintext and ciphertext to use as a lever in compromising secret keys. The ideas are illustrated by analysis of two well-known authentication systems which have been used in practice. They are Kerberos and KryptoKnight. It is shown that for the Kerberos protocol, an adversary can acquire at will an unlimited number of known plaintext-ciphertext pairs. Similarly, an adversary in the KryptoKnight system can acquire an unlimited number of data pairs which, by a less direct means, can be seen to be cryptanalytically equivalent to known plaintext-ciphertext pairs. We propose new protocols, using key derivation techniques, which achieve the same end goals as these others without this undesirable feature.