A powerful cipher that uses a keyword to create a series of Caesar ciphers — difficult to solve without the keyword, and easy if you have it.
A Caesar cipher shifts letters in the alphabet by a determined number, so ABCD with a shift of 3 would become DEFG. The Vigenère uses several different Caesar shifts based on a keyword. The keyword (e.g. Password) is repeated over and over again during the message (e.g. passwordpasswordpasswor…). Each letter is paired with the encrypted message to produce a different shift based on a tabla recta — shown above and easily replicated. As a result, the letter z might mean an t at one point in the code but it might mean an h at another point. Without the keyword (or at least knowledge of how long the word even is), it is nearly impossible to crack.
According to Wikipedia, the cipher dates back to 1553 in the writing of crypologist Giovan Battista Bellaso but was incorrectly attributed to Blaise de Vigenère and named for him.
This was another cipher taught to me by the great minds behind the Mystery Experiences Company.