Date: Wed, 3 May 2000 14:28:43 -0700 From: Steve Reid <sreid@sea-to-sky.net> To: James Wyatt <jwyatt@rwsystems.net> Cc: security@FreeBSD.ORG Subject: Re: Cryptographic dump(8) Message-ID: <20000503142843.A411@grok.localnet> In-Reply-To: <Pine.BSF.4.10.10005031304120.21115-100000@bsdie.rwsystems.net>; from James Wyatt on Wed, May 03, 2000 at 01:06:04PM -0500 References: <200005031718.KAA63329@apollo.backplane.com> <Pine.BSF.4.10.10005031304120.21115-100000@bsdie.rwsystems.net>
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On Wed, May 03, 2000 at 01:06:04PM -0500, James Wyatt wrote:
> How can you tell how much entropy is in the 'pool' for /dev/random and
> it's about to start being not-so-random? Pull a sample once in a while and
> Chi test it? I like being able to know. - Jy@
A Chi test won't work. As I understand it, the random device repeatedly
uses MD5 over a pool of bits, which means the output will pass all of
the usual statistical tests. I think (but am not sure) that the function
is cryptographically strong. If it is then you should be "okay" as long
as there was lots of entropy when you started. Since you're using 3DES
you're already depending on cryptographic security; it's not like you're
generating a "one time pad" for perfect security (/dev/random wouldn't
suffice for that anyway because of the way it uses MD5).
If you use /dev/random it will EOF when the "entropy counter" (a bogus
concept but what can you do) reaches zero. This can be a VERY BAD
THING(tm) if your application isn't expecting it because you may end up
using only a partial key! Always check return values and handle them
accordingly.
You could just use /dev/urandom ("u" for "unlimited"?) which never EOFs
and hope that the output is cryptographically strong enough. But check
return values on IO calls anyway, just because it's a good idea.
Most importantly, RTFM! `man 4 random`
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