Quadratic Congruences

Abstract

Here we shall see how the Chinese Remainder Theorem allows us to solve quadratic congruences for composite moduli. Quadratic congruences play a role in such digital communication tasks as certified receipts, remote signing of contracts, and coin tossing — or playing poker over the telephone (discussed in Chap. 19). Finally, quadratic congruences are needed in the definition of pseudoprimes, which were once almost as important as actual primes in digital encryption (see Chap. 19).