ABSTRACT
Recall that substitution ciphers, in which characters are encrypted one at a time using a single cipher alphabet, are susceptible to attack by frequency analysis. However, recall also that substitution ciphers would be less susceptible to attack if plaintext characters were encrypted in pairs (i.e., digraphs) rather than one at a time. This is the idea that forms the basis for Playfair ciphers, which we considered in Chapter 2. In this chapter, we will consider Hill ciphers, which can loosely be thought of as doing for mathematical substitution ciphers what Playfair ciphers do for nonmathematical substitution ciphers. Specifically, Hill ciphers are designed to allow for mathematical encryption of characters in groups of more than one at a time. As we will see, while Playfair ciphers are designed to allow for encryption of characters only in pairs, Hill ciphers are designed to allow for encryption of characters in groups of any size.
