Paper 2004/294

Solving Systems of Differential Equations of Addition and Cryptanalysis of the Helix Cipher

Souradyuti Paul and Bart Preneel

Abstract

Mixing addition modulo 2^n (+) and exclusive-or has a host of applications in symmetric cryptography as the operations are fast and nonlinear over GF(2). We deal with a frequently encountered equation (x+y)XOR((x XOR a)+(y XOR b))=c$. The difficulty of solving an arbitrary system of such equations -- named differential equations of addition (DEA) -- is an important consideration in the evaluation of the security of many ciphers against differential attacks. This paper shows that the satisfiability of an arbitrary set of DEA -- which has so far been assumed \emph{hard} for large $n$ -- is in the complexity class P. We also design an efficient algorithm to obtain all solutions to an arbitrary system of DEA with running time linear in the number of solutions. Our second contribution is solving DEA in an adaptive query model where an equation is formed by a query (a,b) and oracle output c. The challenge is to optimize the number of queries to solve (x+y)XOR((x XOR a)+(y XOR b))=c. Our algorithm solves this equation with only 3 queries in the worst case. Another algorithm solves the equation (x+y)XOR(x+(y XOR b))=c$ with (n-t-1) queries in the worst case (t is the position of the least significant `1' of x), and thus, outperforms the previous best known algorithm by Muller -- presented at FSE~'04 -- which required 3(n-1) queries. Most importantly, we show that the upper bounds, for our algorithms, on the number of queries match worst case lower bounds. This, essentially, closes further research in this direction as our lower bounds are optimal. We used our results to cryptanalyze a recently proposed cipher Helix, which was a candidate for consideration in the 802.11i standard. We are successful in reducing the data complexity of a DC attack on the cipher by a factor of 3 in the worst case (a factor of 46.5 in the best case).

Note: A proper subset of the results of this report under the title "Optimal Lower Bounds on the Number of Queries for Solving Differential Equations of Addition" had resided here till April 20, 2005.