Hill Ciphers
Richard E. Klima · 2012
Recall that substitution ciphers, which encrypt characters one at a time with 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. 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 more than one character at a time. As we will see, while Playfair ciphers are only designed to encrypt two characters at a time, Hill ciphers are designed to allow for encryption of any number of characters at a time.