This post is Part 1 of a three part series:
Using the Vignere Cipher to Encrypt a Message (Part 1) <– You are here
Using the Vignere Cipher to Encrypt a Message (Part 2)
Using the Vignere Cipher to Encrypt a Message (Part 3)

The Vignère Cipher is a “method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of the keyword.” Read more about it here, it has a really cool history. It’s called a “polyalphabetic cipher” because it uses two or more cipher alphabets to encrypt the original message.

In the next couple of posts, I’m going to walk through my solution to implementing a Vignère Cipher in Javascript. I’m sure there are plenty of ways to do it, and I’ll probably find this solution pretty basic when I look at it again in a year, but for now, I’m satisfied. I thought this problem was a lot of fun to solve and was a great extension of the Caesar Cipher, which I covered in my last post.

### How it Works

The process goes like this:

Step 1 Get the message you want to encrypt (no spaces, all lower case), for example “thisisamessage.”

``````message: thisisamessage
``````

Step 2 Choose a keyword to encrypt the message with, i.e. “lemon”, and repeat it over and over until you get a string that is the same length as your message.

``````keyword: lemonlemonlemo
``````

Step 3 This step is very similar to the Caesar Cipher. Add each letter of the original message to the letter of the keyword string to produce a new letter. For example, `t` is index 19, and `l` is index 11. If you combine them, and wrap back around to the beginning of the alphabet, you end up with the letter `e` at index 4. After you do that for every letter, you’ll get the cipher text. In this example,

``````ciphertext: elugvdeysfdess
``````

Alternative Step 3 Instead of combining the indices of the letters in the message string and the keyword string, you can actually use a Vignère table to look up the new letter. This is a bit harder to implement in code, but it’s a fun exercise for sure.

I’ll go through both solutions in this series. I’ll cover the Caesar Cipher solution first since it’s a bit easier in my opinion.

Continue to Part 2 to see the solution without the table or skip to Part 3 to see the solution with the table.