Thoughts on Efficient Instantiations of Tweakable Block Ciphers and Refinements to Modes OCB and PMAC

Author: Phillip Rogaway

Summary:
Tweakable block ciphers are ciphers that take three inputs (key, tweak, block) instead of the usual two (key, block) such that different tweaks can create different permutations that are all still secure. This eliminates the need for changing keys if we want to have a different block cipher. The paper describes how efficient tweakable block ciphers can be constructed from regular block ciphers such that tweaks can be incremented cheaply, while keeping the tweaked cipher secure.

The paper then describes changes to OCB and PMAC that make the two algorithms easier to understand and their security simpler to prove.

The Good:
I'm not a cryptographer, so the fact that I understood most of the paper is very significant. The paper is well written, and somewhat easy to understand. I didn't go through the proofs, so I can't say anything about them. It's a good introduction to tweakable block ciphers, and what can be done with them. The constructions they use for making tweakable block ciphers easy and efficient to construct are nice and seem to be quite useful.

The Bad:
I didn't really get what else I can do using these tweakable block ciphers and the instantiations Rogaway came up with. I wish he explained more plainly what these can be used for other than improving OCB and PMAC.

The Ugly:
The notation was a little annoying. It was difficult to keep track of what the tildas and bars meant. While consistent, the notation was difficult to follow because there were too many symbols introduced or used in place of others "to simplify the notation".