Using the Vigenere cipher requires the cryptographer and recipient to share a keyword. The cryptographer and the recipient use this keyword, along with the Trimethius tableau, to encrypt and decrypt messages.
- Decide on a keyword
Prior to sending a message, the cryptographer and message recipient must decide on a word that encrypts and decrypts their messages.
- Align the keyword and the message
Write the keyword on top of the message to be encoded or decoded by lining up each word one-to-one. Use the keyword repeatedly until the end of the message. For example, if the keyword is blue, and the message is "How are you today?," the line-up is: blu ebl ueb luebl How are you today.
- Encrypt the message
The keyword letters correspond to the rows and the message letters correspond to the columns in the Trimethius tableau. To encrypt the first letter of the message, find the intersection of the first letters of the keyword and message. The letter at this intersection is the first letter of the encrypted message.
- Decrypt the message
To decrypt the message, locate the row corresponding to the first letter of the keyword, and find the first letter of the encrypted message along that row. The column containing the enciphered letter is the first letter of the original message.