Tabula Recta

Substation cipher encrypt a message by substituting a character with another this mixing up the original message. A polyalphabetic substitution replace the same character by more than 1 alphabets so making it harder to break. Tabula recta uses a matrix of alphabets and a key phrase. To encrypt, the message is aligned with the secret key which is used an index to look up from the aplhabet matrix the corresponding character to substitute with. The key shorter than the message is repeatedly used. Another trick is to expand the key. The key can be an excerpt of text from a book that is as long as the message.