XS-circuits in Block Ciphers

XS-circuits describe block ciphers that utilize 2 operations: X) bitwise modulo 2 addition of binary words and S) substitution of words using key-dependent S-boxes with possibly complicated internal structure. We propose a model of XS-circuits which, despite the simplicity, covers a rather wide range of block ciphers. In our model, several instances of a simple round circuit, which contains only one S~operation, are linked together and form a compound circuit called a cascade. S operations of a cascade are interpreted as independent round oracles. We deal with diffusion characteristics of cascades. These characteristics are related to the cryptographic strength of corresponding block ciphers. We obtain results on invertibility, transitivity and 2-transitivity of mappings induced by round circuits and their cascades. We provide estimates on the first and second activation times where the i-th activation time is the minimum number of rounds which guarantees that at least i round oracles get different queries while processing two different cascade\u27s inputs. The activation times are related to differential cryptanalysis. We introduce the similarity and duality relations between round circuits. Cascades of related circuits have the same or dual diffusion characteristics. We find canonical representatives of classes of similar circuits and show that the duality between circuits is related to duality between differential and linear attacks against corresponding block ciphers. We discuss families of circuits with growing number of inputs. Such families can be used to build wide-block ciphers

Blind accumulators for e-voting

We present a novel cryptographic primitive, blind accumulator, aimed at constructing e-voting systems. Blind accumulators collect private keys of eligible voters in a decentralized manner not getting information about the keys. Once the accumulation is complete, a voter processes the resulting accumulator deriving a public key that refers to the private key previously added by this voter. Public keys are derived deterministically and can therefore stand as fixed voter pseudonyms. The voter can prove that the derived key refers to some accumulated private key without revealing neither that key nor the voter itself. The voter uses the accumulated private key to sign a ballot. The corresponding public key is used to verify the signature. Since the public key is fixed, it is easy to achieve verifiability, to protect against multiple submissions of ballots by the same voter or, conversely, to allow multiple submissions but count only the last one. We suggest a syntax of blind accumulators and security requirements for them. We embed blind accumulators in the Pseudonymous Key Generation (PKG) protocol which details the use of accumulators in practical settings close to e-voting. We propose an implementation of the blind accumulator scheme whose main computations resemble the Diffie-Hellman protocol. We justify the security of the proposed implementation

The CTR mode with encrypted nonces and its extension to AE

In the modified CTR (CounTeR) mode known as CTR2, nonces are encrypted before constructing sequences of counters from them. This way we have only probabilistic guarantees for non-overlapping of the sequences. We show that these guarantees, and therefore the security guarantees of CTR2, are strong enough in two standard scenarios: random nonces and non-repeating nonces. We also show how to extend CTR2 to an authenticated encryption mode which we call CHE (Counter-Hash-Encrypt). To extend, we use one invocation of polynomial hashing and one additional block encryption

On the Guaranteed Number of Activations in XS-circuits

XS-circuits describe cryptographic primitives that utilize 2 operations on binary words of fixed length: X) bitwise modulo 2 addition and S) substitution. The words are interpreted as elements of a field of characteristic 2. In this paper, we develop a model of XS-circuits according to which several instances of a simple round circuit containing only one S operation are linked together and form a compound circuit called a cascade. S operations of a cascade are interpreted as independent round oracles. When a cascade processes a pair of different inputs, some round oracles get different queries, these oracles are activated. The more activations, the higher security guarantees against differential cryptanalysis the cascade provides. We introduce the notion of the guaranteed number of activations, that is, the minimum number of activations over all choices of the base field, round oracles and pairs of inputs. We show that the guaranteed number of activations is related to the minimum distance of the linear code associated with the cascade. It is also related to the minimum number of occurrences of units in segments of binary linear recurrence sequences whose characteristic polynomial is determined by the round circuit. We provide an algorithm for calculating the guaranteed number of activations. This algorithm can also be used to deal with linear activations related to linear cryptanalysis

Upper bounding the number of bent functions using 2-row bent rectangles

Using the representation of bent functions by bent rectangles, that is, special matrices with restrictions on columns and rows, we obtain an upper bound on the number of bent functions that improves previously known bounds in a practical range of dimensions. The core of our method is the following fact based on the recent observation by Potapov (arXiv:2107.14583): a 2-row bent rectangle is completely determined by one of its rows and the remaining values in slightly more than half of the columns

Bash-f: another LRX sponge function

We present the Bash family of hashing algorithms based on the sponge paradigm. A core element of this family is the Bash-f sponge function which refers to the LRX (Logical-Rotation-Xor) class of symmetric cryptography schemes. We describe the components of Bash-f: a nonlinear mapping, linear diffusion mappings, a permutation of words of a hash state. For each component, we establish reasonable quality criteria as detailed as possible to make the choice of the component maximally objective and transparent