A Discrete Logarithm-based Approach to Compute Low-Weight Multiples of Binary Polynomials

Being able to compute efficiently a low-weight multiple of a given binary polynomial is often a key ingredient of correlation attacks to LFSR-based stream ciphers. The best known general purpose algorithm is based on the generalized birthday problem. We describe an alternative approach which is based on discrete logarithms and has much lower memory complexity requirements with a comparable time complexity.Comment: 12 page

Burgess's Bounds for Character Sums

We prove that Burgess's bound gives an estimate not just for a single character sum, but for a mean value of many such sums.Comment: Minor changes and addition of reference to Gallagher & Montgomer

Heat kernel expansions on the integers and the Toda lattice hierarchy

We consider the heat equation $u_t=Lu$ where $L$ is a second-order difference operator in a discrete variable $n$. The fundamental solution has an expansion in terms of the Bessel functions of imaginary argument. The coefficients $\alpha_k(n,m)$ in this expansion are analogs of Hadamard's coefficients for the (continuous) Schrodinger operator. We derive an explicit formula for $\alpha_k$ in terms of the wave and the adjoint wave functions of the Toda lattice hierarchy. As a first application of this result, we prove that the values of these coefficients on the diagonals $n=m$ and $n=m+1$ define a hierarchy of differential-difference equations which is equivalent to the Toda lattice hierarchy. Using this fact and the correspondence between commutative rings of difference operators and algebraic curves we show that the fundamental solution can be summed up, giving a finite formula involving only two Bessel functions with polynomial coefficients in the time variable $t$, if and only if the operator $L$ belongs to the family of bispectral operators constructed in [18].Comment: corrected typo

Extremal Transitions in Heterotic String Theory

In this paper we study extremal transitions between heterotic string compactifications, i.e., transitions between pairs (M,V) where M is a Calabi-Yau manifold and V a gauge bundle. Bundle transitions are described using language recently espoused by Friedman, Morgan, Witten. In addition, partly as a check on our methods, we also study how small instantons are described in the same language, and also describe the sheaves corresponding to small instantons.Comment: 26 pages, LaTex, 3 figures, references adde