Caesar and Vigenère Ciphers

These two ciphers are simple enough: they require only that letters are substituted with others in the alphabet. The Caesar cipher involves shifting each letter of a text a fixed number of places along the alphabet. The Vigenère cipher adds another layer of complexity by employing a keyword to unblock encryption.

The Caesar Shift

Named after Julius caesar, the Caesar cipher is an extremely simple, and insecure, form of encryption. It is said that Caesar used a shift of three letters to encode his personal communication, so we’ll take that as an example here.

Applying a shift of three to a text results in every letter moving three places along the alphabet; so an A becomes a D, a B becomes an E and so on. In this way, the message “The party will be on November ninth” would be encoded as “Wkh sduwb zloo eh rq qryhpehu qlqwk”.

Increasing the Complexity

The Vigenère cipher takes this a step further, employing a keyword to determine the extent of the shift. In this example, we’ll use three-letter abbreviations of the months of the year as possible keywords:Jan, Feb, Mar, etc.

To begin the encryption, a ‘tabula recta’ is needed. This is simply the alphabet written out 26 times, each time beginning with a different letter. As you can see on the table below, the first line, listed as ‘A’ in the table, begins with the letter A. The second line begins with B and is listed as ‘B’ and so on.

Assume that we use ‘Sep’ as our key. This directs us to rows ‘s’, ‘e’ and ‘p’ of the table. If encrypting a text, we would use row ‘s’ to encode the first letter, row ‘e’ for the second letter and ‘p’ for the third. For the fourth letter, we revert back to ‘s’, repeating the process thus.

To take a line of text as an example, ‘our cover is blown, we need to make new plans’, becomes ‘gyg uskwv xk fagac, oi cwis ls bsot fil hppfw’.

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