It drives me crazy - but perhaps I'm the one that's wrong. Is it correct to use "trivial" to mean "easy"?
I keep thinking it comes from people misunderstanding the meaning of "non-trivial", as in complex
Not being a native English speaker, that's where I got the impression that trivial meant easy/straightforward.
A math professor I had once took a very, very long time to think about a question. After that, his first response was 'that is trivial', after which a quite long explanation followed.
A human would put in the top few letters and work out the rest with knowledge of english _words_, not with the rest of the letter distribution.
http://cryptopals.com/sets/1/challenges/6
I agree with the author: it's conceptually very simple, but a little tricky to code, even in the simplest case where you're relying on simple letter frequencies. You could probably do 10 good challenges on different ways to attack this problem, and towards the end you'd be getting into somewhat serious cryptanalysis: for instance, look at what Patterson and Al Fardan did with RC4.
If your cipher is at least 100 characters this will solve it very quickly.
The point is to illustrate a property of a cipher that leaks information, in this case the symbol frequencies because the cipher preserves them. This is information that we don't normally consider valuable when working with plaintexts, but for crypto it's enormously valuable (i.e. it leaks a lot of information).
I've never heard that substitution ciphers are simple to break using only letter frequencies. It does get you to a point where it makes the guessing a lot easier.
I have never heard anyone other than the author of this piece suggest otherwise. Ironically, this result is trivial. That said, I have a pretty serious classical crypto habit, so my conception of what constitutes 'crypto folklore' may be poorly calibrated.
