On Mon, Feb 14, 2011 at 4:15 PM, Glenn Maynard <[email protected]> wrote:
> On Mon, Feb 14, 2011 at 6:43 PM, Shabsi Walfish <[email protected]> wrote: > >> OpenSSL is not exactly a reliable source of cryptographic best practices. >> :) In any case, see here http://linux.die.net/man/4/urandom : > > > No single implementation is; neither are Linux manpages. The question is > whether there are security issues when generating long-term keys from a > secure PRNG (RC4, Yarrow, Fortuna) from an entropy pool that's been seeded > but exhausted. I suspect that question has been examined at great length by > others in the past, so I doubt there's new ground for us to cover on this. > It would be interesting if anyone knows of any competent analysis of this > question (preferably in a form written for non-cryptographers). > Hmm... if there is a guarantee that /dev/urandom was successfully seeded at some point in the past, then I'm happy with it. Is there such a guarantee? I don't see that documented anywhere, and I'm not sure how it would be provided. Since /dev/urandom never blocks, I'm assuming it will return something based mostly on weak entropy sources (like system time and/or or a very small number of real bits of entropy) in the event that the system hasn't really had a chance to seed the pool yet. I can live with not reliably refreshing the pool, but its pretty scary if you think about what happens when a user boots their phone up for the first time, etc. and there is just no entropy there yet. Shabsi > > In any case, an API which returns random data with a guarantee of entropy > inherently must block, like /dev/random does. That implies an asynchronous > API, taking a callback which is called when the requested data is > available. Even if that's ultimately wanted, it would be a separate API. > > (Of course, if that API is created later, then it should be similar to this > one--an asynchronous version of this synchronous API. I can think of some > minor speed bumps to making an async version of this API--you don't want to > write to the array asynchronously, while other code is running--but nothing > unreasonable.) > > -- > Glenn Maynard >
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