CHANGING CIPHERS

Length-Doubling Ciphers and Tweakable Ciphers

We motivate and describe a mode of operation HEM (resp., THEM) that turns a n-bit blockcipher into a variable-input-length cipher (resp., tweakable cipher) that acts on strings of [n..2n − 1] bits. Both HEM and THEM are simple and intuitive and use only two blockcipher calls, while prior work at least takes three. We prove them secure in the sense of strong PRP and tweakable strong PRP, assumin...

Changing probabilities of differentials and linear sums via isomorphisms of ciphers

Ciphers y = C(x, k) and  = (, ) are isomorphic if there exists invertible computable in both directions map y ↔ , x ↔ , k ↔ . Cipher is vulnerable if and only if isomorphic cipher is vulnerable. Instead of computing the key of a cipher it is sufficient to find suitable isomorphic cipher and compute its key. If φ is arbitrary substitution and T is round substitution, its conjugate  = φTφ...

Stream ciphers

Stream ciphers and the eSTREAM project

Stream ciphers are an important class of symmetric cryptographic algorithms. The eSTREAM project contributed significantly to the recent increase of activity in this field. In this paper, we present a survey of the eSTREAM project. We also review recent time/memory/data and time/memory/key trade-offs relevant for the generic attacks on stream ciphers.

Stream Ciphers