Finite congruence-simple semirings and their application to public-key cryptography

**Speaker:** Jens Zumbraegel (UCD/CSI)

**Time: **4:00PM

**Date: **Mon 2nd February 2009

**Location: **Mathematical Sciences Seminar Room

**Abstract**

A set with two binary operations (R,+,*) is called a semiring if (R,+) is a commutative semigroup, (R,*) is a semigroup, and both distributive laws hold. We assume that R has always a zero-element which is neutral in (R,+) and absorbing in (R,*). Semirings arise in numerous occasions, the natural numbers (N={0,1,2,dots},+,*) probablybeing the most well-known example. The structure of a semiring is in a sense the most general over which matrix operations can be defined. Problems from graph theory like shortest path have concisedescriptions using certain semirings.

Finite semirings can be applied in public-key cryptography to construct semigroup actions that may serve as a basis for generalised Diffie-Hellman and ElGamal cryptosystems. For cryptographic purposes it is important that the semiring in use is congruence-simple, meaning that it cannot be homomorphically mapped onto a smaller semiring.This leads to the question whether useful congruence-simple semiringsexist.

In the talk a full classification of finite congruence-simple semirings will be presented and the proof will be sketched. The result generalises the classical Wedderburn-Artin theorem on the classification of finite simple rings. A substantial notion in the proof is that of (strongly) irreducible semimodules over semirings. Key results are that 1) any finite congruence-simple semiring R

admits an irreducible semimodule M, and 2) a density result stating that R is then a "dense" subsemiring of the endomorphism semiring End(M) of the commutative monoid (M,+).

(This talk is part of the Algebra/Claude Shannon Institute series.)

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