This is the fourth in a series about cryptography; an extremely important aspect of computer science and cyber security. It covers the XOR logical operation, that is the exclusive OR operation, explaining how it can be used to encrypt and decrypt a sequence of binary digits. XOR is an important process that is employed by many modern day ciphers. Using a spreadsheet, this video demonstrates how the XOR logical operation can be applied to a single character ASCII code to encrypt and decrypt it using the same symmetric key, and the same method.