To know: I made a Vigenere Cipher video. Watching is optional. This is meant to explain the cryptanalysis. I felt something was lost in translation during class, so I made an attempt to tease it apart in further detail so you can make sense of it. If you don’t feel really confident about the keylength explanation (i.e. why it works), then go to 3:36 in the video

To know: daily posts won’t be accepted late. Don’t worry, I drop a bunch because life happens to everyone, so it’s ok to miss a few.

Your first daily task today is to be done without any aids or research. Get a blank sheet of paper and, without looking anything up or looking at anything, write down the biggest finite positive integer you can. It must be well-defined. That is, it must be possible, in theory, to compute (with arbitrarily much computation power and memory) the integer. And if you are going to give a famous named number, you must give its definition (no “I remember some big number called X”; you need to define it). The winners — the biggest numbers, if I can determine them — will get some points toward prizes of dubious value at the end of semester; no grades involved. Hand this in on canvas.

For the rest of today I have some exercises for you (not to be handed in; but I can provide solutions upon request). Whenever I assign exercises, do as many as an hour affords for the dailypost, and do the rest as part of your studying for the course, when time permits.

(Exercise 1) Suppose you have a known plaintext situation for affine cipher. The plaintext is HAHAHA and the ciphertext is NONONO. Determine the key. Hint: write down some equations modulo 26 that must be true and try to solve for the key. Use the Crypto Tools Sheet (addition and multiplication tables mod 26) in solving.

(Exercise 2) Suppose you have a known plaintext situation for affine cipher. The plaintext is III (that’s 888, in case there’s any confusion) and the ciphertext is QQQ. Explain why this is not enough information to determine the key.

(Exercise 3) Suppose you have a known plaintext situation for affine cipher. The plaintext is BO and the ciphertext is OB. Explain why this is not enough information to determine the key.

(Exercise 4) I decide to make affine cipher more secure by encrypting first with one key, and then encrypting again using another key. Is there any reason this is more secure? Why or why not?