On 13.05.2014 11:06, Antoine Pitrou wrote:
> 
> Is ~/.rnd any kind of serious? It hasn't been modified since two weeks on my system (which is rebooted every day).

The file is apparently only updated if you use one the OpenSSL commands
which needs random data. grep for "RAND_write_file" in the apps/ subdir
of the OpenSSL distribution. Of course, applications can also use that API,
so there may be other situations where it gets updated as well.

However, when removing that file, OpenSSL still only reads 32 bytes from
/dev/urandom, which suggests that it's either using some other sources
of randomness as well (there are some timing tricks being used in the
code for this), or (more likely) simply doesn't need more random
bytes to start with.

So while the file does have some meaning, it's not why I had thought
it would be.

Here's a more likely explanation:

The OpenSSL random number source only works with hash
function feedback and random data that gets added to it. It's not
an PRNG with provable characteristics.

OpenSSL uses SHA-1 for hashing which has a 20 byte digest size, so an
initial vector of 32 bytes is a good start (though more are always
better ;-)):

http://en.wikipedia.org/wiki/Randomness_extractor

That said and coming back to the question why 32 bytes are enough for
OpenSSL: the OpenSSL RNG is being seeded with a seed from the
full range of possible values (160 bits). It's period is a lot smaller
than the MT one (19937 bits), which is why fewer random bytes are
needed.