1. The part of this reading that I had the most trouble understanding was the affine cipher. I understand the encryption method, but the decryption got me confused. I understand congruence, but I didn't understand why we were finding a multiplicative inverse of x (mod 26). Could there be more than one desired inverse?
2. I have always been fascinated with substitution ciphers, so I was happy to read 2.4. I loved the reference to U.S. historical figures, and their uses of substitution ciphers. More specifically, I'm glad that the book went through an example of trying to break a cipher through frequency counts. Although knowing which letter e is replaced with is helpful, it still does not tell us the remainder of the cipher. Unless someone has access to a computer (which almost everyone does these days) or they really want to know the message, the substitution cipher is a decent deterrent to discovering the message.
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