Re: [PATCH] random: add blocking facility to urandom

Sasha Levin
2011-09-07T18:13:25Z

On Wed, 2011-09-07 at 13:38 -0400, Jarod Wilson wrote:
> Certain security-related certifications and their respective review
> bodies have said that they find use of /dev/urandom for certain
> functions, such as setting up ssh connections, is acceptable, but if and
> only if /dev/urandom can block after a certain threshold of bytes have
> been read from it with the entropy pool exhausted. Initially, we were
> investigating increasing entropy pool contributions, so that we could
> simply use /dev/random, but since that hasn't (yet) panned out, and
> upwards of five minutes to establsh an ssh connection using an
> entropy-starved /dev/random is unacceptable, we started looking at the
> blocking urandom approach.

Can't you accomplish this in userspace by trying to read as much as you
can out of /dev/random without blocking, then reading out
of /dev/urandom the minimum between allowed threshold and remaining
bytes, and then blocking on /dev/random?

For example, lets say you need 100 bytes of randomness, and your
threshold is 30 bytes. You try reading out of /dev/random and get 50
bytes, at that point you'll read another 30 (=threshold) bytes
out /dev/urandom and then you'll need to block on /dev/random until you
get the remaining 20 bytes.

--

Sasha.