Affiliation: Sinister Greenend Organization
New proofs for old modes
We study the standard block cipher modes of operation: CBC, CFB, and OFB and analyse their security. We don't look at ECB other than briefly to note its insecurity, and we have no new results on counter mode. Our results improve over those previously published in that (a) our bounds are better, (b) our proofs are shorter and easier, (c) the proofs correct errors we discovered in previous work, or some combination of these. We provide a new security notion for symmetric encryption which turns out to be rather useful when analysing block cipher modes. Finally, we pay attention to different methods for selecting initialization vectors for the block cipher modes, and prove security for a number of different selection policies. In particular, we introduce the concept of a `generalized counter', and prove that generalized counters suffice for security in (full-width) CFB and OFB modes and that generalized counters encrypted using the block cipher (with the same key) suffice for all three modes.
The Wrestlers Protocol: A simple, practical, secure, deniable protocol for key-exchange
We describe and prove (in the random-oracle model) the security of a simple but efficient zero-knowledge identification scheme, whose security is based on the computational Diffie-Hellman problem. Unlike other recent proposals for efficient identification protocols, we don't need any additional assumptions, such as the Knowledge of Exponent assumption. From this beginning, we build a simple key-exchange protocol, and prove that it achieves `SK-security' -- and hence security in Canetti's Universal Composability framework. Finally, we show how to turn the simple key-exchange protocol into a slightly more complex one which provides a number of valuable `real-life' properties, without damaging its security.