What is a substitution cipher?

A simple and easy-to-understand version of symmetric encryption is what is known as a substitution cipher. The key for a substitution cipher is a table that maps every possible character in the plaintext message to a different value. To encrypt a message, the sender replaces all characters in the plaintext with their value in the lookup table. To decrypt a message, the recipient reverses the process. Unfortunately, simple substitution ciphers are extremely insecure and subject to a variety of simple attacks. Credits: Talking: Geoffrey Challen (Assistant Professor, Computer Science and Engineering, University at Buffalo). Producing: Greg Bunyea (Undergraduate, Computer Science and Engineering, University at Buffalo). Part of the https://www.internet-class.org online internet course. A blue Systems Research Group (https://blue.cse.buffalo.edu) production.