This article describes Cryptography, defined as the science of encrypting and deciphering messages written in secret codes, it has played a vital role in securing information since ancient times. There are several cryptographic techniques and many make extensive use of mathematics to secure information. The author discusses an activity built around one of the techniques that illustrates an application of matrices. Secondary school teachers may use this activity to consolidate their students' learning of certain concepts of matrices such as the algorithm for matrix multiplication and the concept of the multiplicative inverse of a matrix. It is hoped that this activity, not only offers teachers great opportunities to either introduce or consolidate certain mathematical concepts and algorithms, but also to convince their students that mathematics plays an important role in various walks of life and hence is a useful and meaningful field of study. (Contains 1 table.)