1. The only real difficult thing about today's reading was the description of the last two attacks in each section, Chosen Plaintext, Chosen Ciphertext. I didn't quite understand what it meant when it said "Choose the letter a as the plaintext," and "Choose the letter A as ciphertext." Does this mean the decoder is to simply choose a letter and by doing so is able to crack the cipher? Or does this mean that the attacker only needs to know one letter of the corresponding text? The wording is throwing me off I guess. This was similar for both sections on substitution and affine ciphers.
2. The most interesting thing about the two ciphers is there different difficulties. As the text points out, the simple shift-substitution method only has 26 possible keys, so a brute force attack wouldn't be too hard. In contrast, the affine cipher has 312 possible keys, making a brute force attack much more un-realistic. However, it occurred to me that an individual still only needs one letter from the plaintext and its corresponding letter in the ciphertext. When even one letter is obtained, the rest of the text in either cipher method is easy to obtain through simple mathematical operations.
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