On Tweaking Luby-Rackoff Blockciphers

Tweakable blockciphers, first formalized by Liskov, Rivest, and Wagner [12], are blockciphers with an additional input, the tweak, which allows for variability. An open problem proposed by Liskov et al. is how to construct tweakable blockciphers without using a pre-existing blockcipher. There are many natural questions in this area: is it significantly more efficient to incorporate a tweak directly? How do direct constructions compare to existing techniques? Are these direct constructions optimal and for what levels of security? How large of a tweak can be securely added? In this work, we explore these questions for LubyRackoff blockciphers. We show that tweakable blockciphers can be created directly from Luby-Rackoff ciphers, and in some cases show that direct constructions of tweakable blockciphers are more efficient than previously known constructions.