This is the second in a series about cryptography; an extremely important aspect of computer science and cyber security. It covers a substitution cipher called the keyword cipher, also known as the Vigenère cipher. It explains how a keyword, or key phrase, can be used to effectively generate several different substitution keys for the same plain text message. The video includes a few examples you can try to encrypt and decrypt yourself using different encryption keys. It also describes how cipher text can be transformed again, with a second keyword, to improve the strength of encryption.