The square C*2of a linear error correcting code C is the linear code spanned by the component-wise products of every pair of (non-necessarily distinct) words in C. Squares of codes have gained attention for several applications mainly in the area of cryptography, and typically in those applications, one is concerned about some of the parameters (dimension and minimum distance) of both C*2and C. In this paper, motivated mostly by the study of this problem in the case of linear codes defined over the binary field, squares of cyclic codes are considered. General results on the minimum distance of the squares of cyclic codes are obtained, and constructions of cyclic codes C with a relatively large dimension of C and minimum distance of the square C*2are discussed. In some cases, the constructions lead to codes C such that both C and C*2simultaneously have the largest possible minimum distances for their length and dimensions.

In this paper, we consider linear secret sharing schemes over a finite field Fq, where the secret is a vector in Fℓqand each of the n shares is a single element of Fq. We obtain lower bounds on the so-called threshold gap g of such schemes, defined as the quantity r-t where r is the smallest number such that any subset of r shares uniquely determines the secret and t is the largest number such that any subset of t shares provides no information about the secret. Our main result establishes a family of bounds which are tighter than previously known bounds for ℓ ≳ 2 . Furthermore, we also provide bounds, in terms of n and q , on the partial reconstruction and privacy thresholds, a more fine-grained notion that considers the amount of information about the secret that can be contained in a set of shares of a given size. Finally, we compare our lower bounds with known upper bounds in the asymptotic setting.

An OT-combiner takes n candidate implementations of the oblivious transfer (OT) functionality, some of which may be faulty, and produces a secure instance of oblivious transfer as long as a large enough number of the candidates are secure. We see an OT-combiner as a 2-party protocol that can make several black-box calls to each of the n OT candidates, and we want to protect against an adversary that can corrupt one of the parties and a certain number of the OT candidates, obtaining their inputs and (in the active case) full control of their outputs. In this work we consider perfectly (unconditionally, zero-error) secure OT-combiners and we focus on minimizing the number of calls to the candidate OTs. First, we construct a single-use (one call per OT candidate) OT-combiner which is perfectly secure against active adversaries corrupting one party and a constant fraction of the OT candidates. This extends a previous result by Ishai et al. (ISIT 2014) that proves the same fact for passive adversaries. Second, we consider a more general asymmetric corruption model where an adversary can corrupt different sets of OT candidates depending on whether it is Alice or Bob who is corrupted. We give sufficient and necessary conditions for the existence of an OT combiner with a given number of calls to the candidate OTs in terms of the existence of secret sharing schemes with certain access structures and share-lengths. This allows in some cases to determine the optimal number of calls to the OT candidates which are needed to construct an OT combiner secure against a given adversary.

Given a linear error correcting code C, its m-th power is defined as the linear span of the set of all coordinate-wise products of m (not necessarily distinct) codewords in C. The study of powers of codes (and especially squares) is relevant in a number of recent results in several areas of cryptography where we need to bound certain parameters (such as the dimension and the minimum distance) of both a linear code and some power of it simultaneously. These areas include most notably secret sharing and multiparty computation, but also two-party cryptography and public key cryptography. In this paper, some of these applications will be discussed together with several recent results and some open challenges.

Multiplicative linear secret sharing is a fundamental notion in the area of secure multiparty computation and, since recently, in the area of two-party cryptography as well. In a nutshell, this notion guarantees that the product of two secrets is obtained as a linear function of the vector consisting of the coordinatewise product of two respective share-vectors. This paper focuses on the following foundational question, which is novel to the best of our knowledge. Suppose we abandon the latter linearity condition and instead require that this product is obtained by some, not-necessarily-linear “product reconstruction function.” Is the resulting notion equivalent to multiplicative linear secret sharing? We show the (perhaps somewhat counterintuitive) result that this relaxed notion is strictly more general. Concretely, fix a finite field ${\mathbb F}_q$ as the base field over which linear secret sharing is considered. Then we show there exists an (exotic) linear secret sharing scheme with an unbounded number of players $n$ such that it has $t$-privacy with $t = \Omega(n)$ and such that it does admit a product reconstruction function, yet this function is necessarily nonlinear. In addition, we determine the minimum number of players for which those exotic schemes exist. Our proof is based on combinatorial arguments involving quadratic forms. It extends to similar separation results for important variations, such as strongly multiplicative secret sharing.

Given a linear code C, one can define the dth power of C as the span of all componentwise products of d elements of C. A power of C may quickly fill the whole space. Our purpose is to answer the following question: does the square of a code typically fill the whole space? We give a positive answer, for codes of dimension k and length roughly (1/2)k2or smaller. Moreover, the convergence speed is exponential if the difference k(k+1)/2-n is at least linear in k. The proof uses random coding and combinatorial arguments, together with algebraic tools involving the precise computation of the number of quadratic forms of a given rank, and the number of their zeros.

The Ihara limit (or constant) A(q) has been a central problem of study in the asymptotic theory of global function fields (or equivalently, algebraic curves over finite fields). It addresses global function fields with many rational points and, so far, most applications of this theory do not require additional properties. Motivated by recent applications, we require global function fields with the additional property that their zero class divisor groups contain at most a small number of d -torsion points. We capture this with the notion of torsion limit, a new asymptotic quantity for global function fields. It seems that it is even harder to determine values of this new quantity than the Ihara constant. Nevertheless, some nontrivial upper bounds are derived. Apart from this new asymptotic quantity and bounds on it, we also introduce Riemann-Roch systems of equations. It turns out that this type of equation system plays an important role in the study of several other problems in each of these areas: arithmetic secret sharing, symmetric bilinear complexity of multiplication in finite fields, frameproof codes, and the theory of error correcting codes. Finally, we show how our new asymptotic quantity, our bounds on it and Riemann-Roch systems can be used to improve results in these areas.

We consider the class of secret sharing schemes where there is no a priori bound on the number of players n but where each of the n share-spaces has fixed cardinality q. We show two fundamental lower bounds on the threshold gap of such schemes. The threshold gap g is defined as r-t, where r is minimal and t is maximal such that the following holds: for a secret with arbitrary a priori distribution, each r-subset of players can reconstruct this secret from their joint shares without error ( r-reconstruction) and the information gain about the secret is nil for each t-subset of players jointly ( t-privacy). Our first bound, which is completely general, implies that if , then g ≥ [( n-t+1)/q] independently of the cardinality of the secret-space. Our second bound pertains to \BBF q-linear schemes with secret-space \BBF qk( k ≥ 2). It improves the first bound when k is large enough. Concretely, it implies that g ≥ [( n-t+1)/ q]+f(q,k,t,n), for some function f that is strictly positive when k is large enough. Moreover, also in the \BBF q-linear case, bounds on the threshold gap independent of t or r are obtained by additionally employing a dualization argument. As an application of our results, we answer an open question about the asymptotics of arithmetic secret sharing schemes and prove that the asymptotic optimal corruption tolerance rate is strictly smaller than 1.

In this invited talk,1we introduce the notion of arithmetic codex, or codex for short. It encompasses several well-established notions from cryptography (arithmetic secret sharing schemes, which enjoy additive as well as multiplicative properties) and algebraic complexity theory (bilinear complexity of multiplication) in a natural mathematical framework. Arithmetic secret sharing schemes have important applications to secure multi-party computation and even to two-party cryptography. Interestingly, several recent applications to two-party cryptography rely crucially on the existing results on “asymptotically good families” of suitable such schemes. Moreover, the construction of these schemes requires asymptotically good towers of function fields over finite fields: no elementary (probabilistic) constructions are known in these cases. Besides introducing the notion, we discuss some of the constructions, as well as some limitations.

In 1986, D. V. Chudnovsky and G. V. Chudnovsky first employed algebraic curves over finite fields to construct bilinear multiplication algorithms implicitly through supercodes introduced by Shparlinski-Tsfasman-Vladuţ, or equivalently, multiplication-friendly codes that we will introduce in this paper. This idea was further developed by Shparlinski-Tsfasman-Vladuţ in order to study the asymptotic behavior of multiplication complexity in extension fields. Later on, Ballet et al. further investigated the method and obtained some improvements. Recently, Ballet and Pieltant made use of curves over an extension field of to obtain an improvement on the complexity of multiplications in extensions of the binary field. In this paper, we develop the multiplication-friendly splitting technique and then apply this technique to study asymptotic behavior of multiplications in extension fields. By combining this with the idea of using algebraic function fields, we are able to improve further the asymptotic results of multiplication complexity. In particular, the improvement for small fields such as the binary and ternary fields is substantial.