Re: /dev/random in 2.4.6

[Theodore Tso <tytso@mit.edu>]

On Mon, Aug 20, 2001 at 04:00:30PM +0000, David Wagner wrote:
> 
> I don't see why not.  Apply this change, and use /dev/urandom.
> You'll never block, and the outputs should be thoroughly unpredictable.
> What's missing?

Absolutely.  And if /dev/urandom is not unpredictable, that means
someone has broken SHA-1 in a pretty complete way, in which case it's
very likely that most of the users of the randomness are completely
screwed, since they probably depend on SHA-1 (or some other MAC which
is probably in pretty major danger if someone has indeed managed to
crack SHA-1).

> (I don't see why so many people use /dev/random rather than /dev/urandom.
> I harbor suspicions that this is a misunderstanding about the properties
> of pseudorandom number generation.)

Probably.  /dev/random is probably appropriate when you're trying to
get randomness for a long-term RSA/DSA key, but for session key
generation which is what most server boxes will be doing, /dev/urandom
will be just fine.

Of course, then you have the crazies who are doing Monte Carlo
simulations, and then send me mail asking why using /dev/urandom is so
slow, and how can they the reseed /dev/urandom so they can get
repeatable, measureable results on their Monte Carlo sinulations....

					- Ted