The Vigenère Cipher is a classic cipher which was invented by Giovan Battista Bellaso in 1553 and later misattributed to Blaise de Vigenère As late as 1868 the mathematician Charles Lutwidge Dodgson (Lewis Carroll) called the Vigenère cipher unbreakable. The French referred to the Vigenère as le chiffre indéchiffrable, meaning “the indecipherable cipher.”
The Wikipedia article I link to above states that “Kasiski entirely broke the cipher and published the technique in the 19th century.” This is an exaggeration. There are many Java Applets online which allow the user to use the Kasiski Examination and they only work when the ciphertext is vastly longer than the key. For example, Simon Singh has a cracking tool based on the Kasiski Examination and offers an example with 655 characters of ciphertext and a seven-character key. That is over 93 times more ciphertext than key! The Confederate use of the Vigenère during the American Civil War was indeed compromised by Kasiski-style attacks, but they fought the entire war using only three keys, so there were thousands of ciphertext characters for each character of the key.
In sharp contrast, my technique, which does not employ the Kasiski Examination or the related Friedman Test, can typically break Vigenère ciphers with as little as four times more ciphertext than key.
I challenge anybody using the Kasiski Examination or the Friedman Test to break these examples:
Test Case #1
lymog ssqqc tfymb qrzfz aaglo mmmes sszfe yhjwm pjnsh colou ukw
Test Case #2
zxlpd xywtg hswhi stuzb exxax wjkpr djeyw fuwyf lgpdx yxxou gsgac uzbvw unxfy jubcv dtpcy p
If you wish to encipher your own messages for the purpose of testing my Applet, a guy calling himself Sharky has an Applet for encoding Vigenère ciphers. Sharky actually seems to take the Vigenère seriously. He writes, “Vigenère ciphers with moderate keys are appropriate for situations that do not require failproof security.” This is like saying that cars without brakes are appropriate for situations that do not require stopping.
Here is an Applet for a cipher that really does work.