Small world

[maradydd]

There's a post up on BoingBoing today (ok, yesterday for me) about open vs. closed search algorithms, suggesting that the search algorithms used by Google, Yahoo et al are bad because of their lack of transparency. It invokes a comparison to an important concept in computer security: "security through obscurity" is dangerous because an effective encryption scheme should be equally hard to break whether you know the internals of the algorithm that generated the ciphertext or whether you don't.

I think comparing this to search is a bad (or at best misleading) idea, and expounded on this in the comments. But I'm far more entertained by the fact that the two best comments on the post so far come from two sources with whom I am tangentially familiar, albeit from totally different directions: jrtom and radtea. Small damn world!

Comments

[jrtom.livejournal.com]

pwnd

Oh, quite. I rather expected that. :)

I should have clarified: I know that LCGs are not, shall we say, precisely the cutting edge of PRNGs. I'd assumed that a successful attack would at least be facilitated by being able to observe an uninterrupted sequence of outputs; perhaps that's irrelevant.

I still believe that, due to certain practical concerns, that subverting Algorithm A would be at least very difficult (in the same way that breaking the output of a good crypto algorithm via brute force is very difficult), but I acknowledge that I may not have technically demonstrated that it's _impossible_.

Rest assured, if I ever need a good (i.e., cryptographically sound) source of randomness, I'm not going to assume that I already know how to do that. :)

[maradydd.livejournal.com]

Not necessarily an uninterrupted sequence of outputs; ISTR that polynomial interpolation is a common first step in figuring out a PRNG's seeds. In any case, any attack that works on a stream cipher will frequently be useful against a PRNG as well (maybe not in the general case, but there are equivalences between certain stream ciphers and certain PRNGs).

[enochsmiles.livejournal.com]

Rest assured, if I ever need a good (i.e., cryptographically sound)
source of randomness, I'm not going to assume that I already know how to
do that. :)

Alas, many of your colleagues will.

(And you're right that it makes it harder on the attacker if they don't have an uninterrupted stream, but you're vastly overestimating how much harder, especially in the LCG case.)