Cryptanalysis of Vigenere cipher: not just how, but why it works

The Vigenere cipher, dating from the 1500's, was still used during the US civil war. We introduce the cipher and explain a standard method of cryptanalysis based on frequency analysis and the geometry of vectors. We focus on visual intuition to explain why it works. The only background needed is some familiarity with vectors and probabilities. For more on this method: _Introduction_to_Cryptography_with_Coding_Theory_ by Wade Trappe and Lawrence C. Washington, Second Edition, Section 2.3. Here the method is exactly as in the video, because this is the text I teach my course from. _Secret_History:_The_Story_of_Cryptology_ by Craig P. Bauer, Section 3.3. Here the method is done with more mathematical sophistication (so it looks slightly different). This is a very valuable and detailed textbook for anyone interested in old ciphers and their cryptanalysis. I also have some cryptanalysis tools here you can use to implement the method (which I do with my students in the classroom): https://crypto.katestange.net/cryptanalysis-tools/