Decrypting Text - code breaking software
The Colossus at Bletchley

Vignère decryption

Vignère ciphertext can be difficult to decrypt. It is a combination of a Caesar shift combined with a keyword. The length of the keyword determines the number of different encryptions that are applied to the plaintext. For example, if the keyword is 4 chars in length then the plaintext is divided into 4 subtexts and a seperate Caesar shift is applied to each subtext depending on the value of the corresponding letter in the keyword. The cipher is polyalphabetic which means that a character can be enciphered in different ways. A frequency analysis of Vignère often shows a fairly balanced set of frequencies - because the keyword is used to switch between different cipher alphabets.
Charles Babbage worked out a method to decrypt this cipher.
This program uses a slightly shortened approach. If you don't enter in a keyword then it can break down the text into subtexts so that you can see whether a pattern is present. It will suggest a keyword. It assumes that the most common letter in the subtext is "e" which is not always the case.
Here is an example ciphertext which uses a 3 letter key. You can probably work out the keyword from the suggestion - (hint: computer company). Conversely, you can paste into this box any ciphertext of your own that you want to decipher.
Enter your ciphertext below
 keyword - leave this empty if you want the program to show suggestions