§ 2.3, due Sept 16

This section dealt with the Vigenère cipher. Basically, it partitions the letters into their position (mod n), and performs a caesar shift on each of the groups of letters. We discussed how to defeat this method of encryption.

1. (Difficulty) I was a little hazy at first why a vector (representing the distribution of letters in the English language) dotted with itself is higher than a dot product with a "displaced" version of it:
   After thinking about it for a bit, I am satisfied with the explanation of the book. Perhaps I should show more it rigorously, but this is okay for me for now.
2. (Reflective) I liked the "second method" for breaking the Vigenère cipher. It is more algorithmic and a little less exciting than figuring out by hand how much of a shift the letters had. However, I was trying to think of a similar kind of method earlier.An analogy is having a broken piece of pottery or something. Two small pieces can be placed together in many ways, but by shifting them for a while, you finally find a "sweet spot" where they fit snugly. So it is with this cipher—the distribution of letters in the ciphertext and the English language will fit nicely (i.e. have a large dot product) when you find the right shift of the letters.