Summary list for section 5.1, description of block ciphers

[joedoyle]

First, thanks for writing this. I'm smack in the middle of your target audience, an everyday programmer without much knowledge of cryptography, and reading the first few sections has proved enlightening.

The last sentence in section 5.1 includes "as long as it doesn’t sacrifice any of the above properties". When I read that, "which properties?" came into mind, so maybe it'd be useful to explicitly summarize which properties you're referring to. I picked out two, 1) that a different key must lead to a completely different set of arrows, and 2) knowing many (input,output) pairs shouldn't give any information about any other (input, output) pair.

[lvh]

Thanks! Glad to hear you're enjoying the book.

That sentence is definitely a total cop-out and should be addressed :) There is another big review ticket open (#80) that suggests some changes, but it also suggests many other things. I'm trying to get that one broken up and that section cleaned up, and then we can clarify that sentence without introducing inconsistencies.

Thanks again!

[joedoyle]

Regarding #80, for what little my opinion is worth, I think your point about only a tiny fraction of all permutations being used is important. It's useful to first think about how many possible relations there could be between P and C, but then further realize that a given key can only select one of them, and so the number of permutations that could be used is limited to the number of possible keys.

[lvh]

I think your opinion is very valuable, and yes, I agree. He has a point in that it is not sufficient, though :)