The most interesting thing for me in these sections was learning about the variety of crypto-systems. I was very interested to learn about the varieties of systems that have been invented, and then cracked. It seems that just as much ingenuity, creativity, and intelligence is required to make up a code system as is required to crack one. And I don't just mean cracking it once; I mean figuring out a systematic method for always breaking an encrypted message given the encryption method. When I first read about Kerckhoff's principle it seemed fairly insignificant to me, but now it is increasingly making more sense.
One thing that confused me about the inverse matrices was how the fractions related to their inverses. For example, the example in the book finds that 5 is the inverse of -2 mod 11. Then they make the conclusion that we can replace -1/2 by 5 in mod 11. How did they jump from -2 to -1/2? Another thing that was difficult for me was the ADFGX cipher. I didn't really understand how to formulate the matrices.
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