Sometime in the 1460's, Alberti was wandering through the gardens of the Vatican, when he bumped into his friend Leonardo Dato, the pontifical secretary, who began chatting to him about some of the finer points of cryptography. This casual conversation prompted Alberti to write an essay on the subject, outlining what he believed to be a new form of cipher. Up until this point, a substitution cipher involved establishing a single cipher alphabet for encrypting each message. However, Alberti proposed using two or more cipher alphabets and switching between them during encipherment, thereby confusing potential cryptanalysts.

Here we have two possible cipher alphabets, and we can encrypt a message by alternating between them. The first letter of the plaintext message is encrypted using Ciphertext Alphabet 1, the second letter of the message is encrypted using Ciphertext Alphabet 2. We encrypt the third letter of the message by returning to Ciphertext Alphabet 1, the fourth letter is encrypted using Ciphertext Alphabet 2, and so on.

To try out this cipher, type your message into the box labelled Plaintext, then click the "Encipher Plaintext" button.

 


 
 This is much stronger than using just one cipher alphabet because a common letter like E is encrypted in two different ways, which makes it much harder to spot in the cipher text.