
This lecture describes joint work with James Hirschfeld and David Wehlau. In the lecture I focus on 2 main topics:
Time permitting I will skirmish with the case of infinite fields.
Let A be a square (0,1) v by v matrix with constant row and column sums. Assume furthermore that A has no (λ,2) isolated zeroes. That is, assume A has no λ by 2 submatrix containing exactly one 0. Under suitable conditions we obtain a lower bound on v and characterize the case of equality.
Let m ≤ n be positive integers. An marrangement from an alphabet X of size n is a permutation of m distinct elements from X. Regarding them as words, the Hamming distance (as usual) measures the number of disagreeing positions between two marrangements.
Define an nary arrangement code of length m and minimum distance d to be a set Γ of marrangements from an nset such that all pairs of different words in Γ have Hamming distance ≥ d. Note that when n=m, one recovers (the more familiar) permutation codes.
This talk will survey my preliminary observations on this topic.
We study the functional codes of order h defined by G. Lachaud on a nondegenerate Hermitian variety. We exhibit a divisibility condition satisfied by all the weights of this code. In the case this functional code is defined by evaluating quadratic functions on the nondegenerate Hermitian surface, we list the first five weights, we describe the geometric structure of the corresponding quadrics and give a positive answer to a conjecture formulated on this question. We will present two new conjectures. The first is about the divisor (largest integer dividing all the weights) of the functional code. The second is on its minimum distance and the distribution of the codewords of its first 2h+1 weights.
This is a joint work with San Ling (NTU, Singapore) and Chaoping Xing (NTU, Singapore).
In cryptographic systems, faults can occur naturally and/or due to some malicious acts of an attacker. In the past, researchers showed how an attacker could exploit computational errors to break some popular cryptosystems. This has made the task of verification of the correctness of cryptographic computations quite important. In some applications, further robustness in term of the ability to continue performing correct computations in presence of certain faults is also sought.
In elliptic curve cryptography, a wellknown technique to detect errors in its group operation is to verify whether or not the operation output is a point on the curve. This point verification scheme has however been shown to be insufficient against certain types of fault based attacks. In this talk, errordetecting schemes for elliptic curve scalar multiplication, which is fundamental to elliptic curve cryptosystems, are considered. We present structures based on recomputation and parallel computation along with point verification. These structures use encoding techniques that rely on proprieties of elliptic curves and provide a high probability of error detection.
This is joint work with A. DominguezOviedo.
The security of publickey cryptosystems using quadratic fields is based on two types of discrete logarithm problem. In the imaginary quadratic case, the discrete logarithm problem in the ideal class group is used, whereas in the real quadratic case the principal ideal problem (also known as the infrastructure discrete logarithm problem) is used instead. In this talk, we describe recent improvements to the best known algorithms for solving these two problems. Our numerical results are presented, as well as extrapolations leading to recommendations for parameter sizes providing approximately the same level of security as block ciphers with 80, 112, 128, 192, and 256bit symmetric keys.
An isogeny between elliptic curves is an algebraic morphism which is a group homomorphism. Many applications in cryptography require evaluating large degree isogenies between elliptic curves efficiently. For ordinary curves of the same endomorphism ring, the previous best known algorithm has a worst case running time which is cubic exponential in the length of the input. We show that this problem can be solved in subexponential time under reasonable heuristics, and we present examples of evaluated isogenies which well exceed the previous world record for the evaluation of general prime degree isogenies, including examples of cryptographic size. Time permitting, we will also discuss applications to elliptic curve cryptography and the discrete logarithm problem.
Joint work with Vladimir Soukharev.
Bent functions are a class of functions from GF(p^{n}) to GF(p) that are in a precise sense as far as possible from any affine function. This makes them resistant to some wellknown cryptographic attacks. There are other features desired in the cryptographic applications, such as a high algebraic degree of the function, and the talk focuses on classes of bent functions that perform well with respect to such additional criteria.
We show how tools from different areas, such as arithmetic geometry (elliptic and hyperelliptic curves), finite fields (character sums) and combinatorics (spreads, relative difference sets, Desarguesian planes) are all used in the study of bent functions. The results are theoretical (characterizations, necessary conditions, constructions) and algorithmic (polynomial time certification of bentness).
Quantum key distribution (QKD) promises secure key agreement by using quantum mechanical systems. Assuming practical and affordable QKD systems become widely available, what role will QKD play in future cryptographic infrastructures?
QKD can provide longterm confidentiality for encrypted information without reliance on computational assumptions. Will anyone really care?
Like classical key establishment protocols, QKD requires authentication to prevent maninthemiddle attacks. If authentication is achieved using a short shared secret key, one can regard QKD as a form of key expansion. If one is willing to accept some conservative computational assumptions, does QKD still add any value?
I will discuss these and related questions and present situations where QKD can add practical value. I will ask the audience some questions too.
NTRU is a cryptosystem proposed by Hoffstein, Pipher and Silverman in 1996. It is based on polynomials with integer coefficients, where secrecy is obtained by performing operations modulo two different primes in Z. We propose a variant of NTRU, in which Z is replaced by the Eisenstein integers Z[ω]. In this talk, we describe this variant, and show how the key property which makes NTRU over Z[ω] so efficient and secure is its hexagonal lattice structure in C.
This is joint work with Ali Miri (Ryerson), Camelia Karimianpour (Ottawa) and most recently, Katherine Jarvis (Ottawa).
We introduce and study a set of new interleavers for Turbo codes based on permutation functions with known inverses over finite fields. We use monomial, Dickson, Mobius and Redei permutation functions. Our process requires information on the cycle structure of these permutation functions. We use known information on the cycles of monomials, Dickson and Mobius functions. As a byproduct, we provide the cycle structure of Redei functions, as well as an expression for the inverse of any Redei function. Finally, selfinverse permutation functions are used to construct interleavers, that are their own deinterleavers, and are useful for turbo codes.
Joint work with Amin Sakzad and MohammadReza Sadeghi.
Quantum secret sharing schemes deal with the distribution of a quantum state among a set of n players, so that only authorized subsets can reconstruct the secret. While the connections between codes, secret sharing schemes and matroids have been subject of extensive investigations, their analogues in the context of quantum secret sharing schemes have not been studied as much, in particular no associations have been made with matroids. In this talk we give the first steps toward establishing the connections between matroids and quantum secret sharing schemes. In addition to providing a new perspective on quantum secret sharing schemes, this characterization has important benefits; they enable us to construct efficient quantum secret sharing schemes for many general access structures. We show that an identically selfdual matroid that is representable over a finite field induces a quantum secret sharing scheme with information rate one. Using the theory of quantum stabilizer codes, we make this association constructive which additionally elaborates on the correspondence between quantum codes and secret sharing schemes.
This is a joint work with Robert Raussendorf (University of British Columbia).
A key ingredient in hyperelliptic curve arithmetic is divisor reduction. Standard reduction methods take as input a nonreduced divisor in Mumford representation and iteratively generate a sequence of divisors until a reduced one is obtained. In this computationally expensive process, the degrees of the intermediate Mumford coefficients gradually decrease down to the genus of the curve. This talk will illustrate how to replace this costly procedure by the computation of just two linear recurrences which allow the recovery of the Mumford polynomials of the reduced target divisor at the end. The two scenarios under consideration are a large input divisor that could have been obtained via an inexpensive tupling procedure for example, and an input divisor that is the sum of two reduced divisors, as encountered in scalar multiplication using nonadjacent form.
In 2008, Kedlaya and Umans introduced the first quasilinear time algorithm to compute the modular composition of univariate polynomials, namely, f(g) modulo h.
I will describe an extension of this idea to a multivariate setting, and its application to computations with modular polynomials, as seen for instance in the SEA algorithm for elliptic curve point counting.
I will highlight a polynomialtime reduction to translate the problem of factoring a product N of two primes into the computation of some multiple series with analytic coefficients. In particular, I conjecture that these series can be computed in O ( exp(log^{ϵ} N) ) bit operations for any ϵ > 0, therefore providing a similar estimate for the inferred subexponential deterministic factoring algorithm. Unlike previous leading subexponentialtime factoring algorithms related to the Quadratic Sieve, this one does not use "Fermattype" equalities. It rather finds an approximation to the value of a carefully chosen multiplicative function related to the sum of divisors of N (hence its deterministic character). Analytic number theoretic techniques and especially the Riemann zeta function play here an essential role.
Let B be an alphabet and suppose that there is a set of noisy channels on symbol sets B. We will associate a family of graphs, G with the channels: if (x,y) ∈ E(G) for G ∈ G then in the channel associated to G, the letters x and y are distinguishable with positive probability. There is a corresponding notion when the graphs are directed. If we are given such a family of noisy channels, what is the best code to use if we do not know which precise channel we will get. The answer is to find a largest possible set of sequences where each pair is distinguishable in any given channel for at least one of their indices. In the directed graph case this code corresponds to a covering array which is an object that generalizes orthogonal arrays and is more typically encountered in software engineering and reliability testing. This talk does not present any new results but surveys this interesting connection.
A construction of a rateless code family for communication over additive white Gaussian noise (AWGN) channels is presented. The proposed code structure is based on transmission of information in the form of multiple redundant data streams. The decoder separates the received multiple layers of data using parallel low complexity detection and then decodes each layer individually. The impact of the density of design rate points on code performance is examined. It is also demonstrated how the proposed codes can be applied for communication over multiple access and interference channels.
We consider the node scheduling problem for tcovered and connected sensor networks. Some combinatorial methods are proposed to allocate all nodes in the sensor network into k(k ≥ t) different groups {0,1,...,k−1} without requiring location information such that each group will be connected and maintaining the coverage ratio as high as possible. Theoretical analysis and simulation results show that the new scheduling method has better performance than previous randomized scheduling scheme. It can be used to prolong the lifetime of sensor networks effectively.