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.