The Vigenere cipher system is a 15th Century system that was thought to be
unbreakable until late in the 19th Century, when none other than Charles Babbage
figured out how to break it!
The way it works is as follows: the key is actually a "key word" whose letters are used to determine the amount that each letter in the plain text is to be shifted. As an example, if the key word is BED, and the plain text is PICK UP THE TRASH, the B in BED will cause P to be encoded as Q (B is the 1st letter after A), the E in BED will cause I to be encoded as M (E is 4th after A, so M is the 4th after I), and so on.
Here's the entire process. The 1st line is plain text, the third the encryption.
Note that as the key word, BED gets reused!
PICK UP THE TRASH
BEDB ED BED BEDBE
QMFL YS ULH UVDTL
See if you can detect weaknesses in this approach! Babbage obviously did.