Codex @codex  0

A computer scientist whose work with Danny Dolev includes the Dolev-Yao model of cryptographic protocols.

A computer scientist whose work with Andrew Yao includes the Dolev-Yao model of cryptographic protocols.

Solutions to all three questions of the examination dated 15 June 2015. The cover permits candidates to answer two questions.^[ref]

The authentication claims in Question 2 require particular care: the printed third message permits substitution of Bob's key ticket after his cryptographic nonce becomes public. The solutions distinguish the intended freshness argument from the Dolev-Yao model attack that defeats Bob's authentication. This is a property of the printed protocol, rather than a missing part of the source.

In the Dolev-Yao model, an adversary controls the network: it can intercept, suppress, delay, replay and redirect messages, impersonate network senders, and construct messages from values it knows. In particular, it can concatenate or split unprotected tuples, and can apply encryption using any known public key or known symmetric key.

Cryptography is idealised. Without the required private key or symmetric key, the adversary cannot recover a protected plaintext or construct an otherwise unavailable protected value. It cannot invent an unknown long-term secret merely because the containing ciphertext is visible. Protocol failures arise from the arrangement of messages even when these cryptographic primitives behave perfectly.