Difficult: It still seems to me that making the key public like this is a bad idea, though the book makes an adequate argument about the decryption key still being secure. If Eve intercepts the encryption key (e and n), then she will be able to encode her own message and replace one intercepted from Alice. In other systems, we have discussed this as insecure, and I do not see a good way to protect against it here. Also, how is the message changed into numbers in the first place? I get that we are now numbering 01 to 26, but then the message is turned into a number by just jamming the numbers from each letter together? That seems like it could get confusing. In the example used, there is no big deal. Since there are only 26 letters, the 30 in 30120 cannot be mistaken for a 30. But what if we encrypted bav? Then we would have 20122 which could also be seen as 20 12 2, or tlb. These two groups of letters do not make words, so there is not much danger, but it seems that there should be some words that might cause problems.
Reflective: I suppose that this encryption method was not particularly feasible without computers. It says that "finding large primes is not difficult", but that is really "not difficult with a computer." Also, I'd love to see Bob calculating c^d on page 166 without a computer, even using the binary method. I'm guessing after all that work he'd be a bit miffed to discover that the message was just "cat".
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