This method- the Vignere Cipher- was fascinating. It's such a great variation of the substitution cipher that is easy to understand but very difficult to crack. I think it's interesting that two plaintext letters can map to the same ciphertext letter but still there is a unique encryption. Although I didn't understand it entirely, I also found the connection to vectors to be intriguing. One reason this class was attractive to me was that it appeared to be applied mathematics, and applied abstract algebra in particular. The Vignere Cipher is evidence of that application indeed.
The hardest part was forming and multiplying the probability vectors. I don't really understand what the letter frequency numbers mean. For example, the chart on page 17 says "a = .082" but I have no idea what the .082 represents. I know its higher than b=.015 so 'e' appears more frequently than 'b' but until I get some context for the numbers the vectors won't make much sense. (Upon writing that I am thinking that if I added up all the decimals they would sum to 1, meaning that there is a 8.2% chance that any given letter is an 'e'.) The other related hard part for me was figuring out why to multiply A (sub j) and A (sub i). Just in general the probability vectors were the hardest part for me.
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