Section 3.8 and Sections 2.5-2.8

Difficult: I thought the inverted matrices using modular arithmetic was difficult because I've never used matrices in the context of modular arithmetic before and modular arithmetic is already time-consuming and sometimes confusing to me still anyway.

Interesting: I loved the Sherlock Holmes story and that they included it in the textbook!! It does make me wonder if Elsie knew how to decrypt it, but had just forgotten or was embarrassed about it. I also thought it was interesting that the 2.6 ciphers used matrices and that they were used during WWI