Re: random numbers

From: Michael Robinson (robinson@netrinsics.com)
Date: 03/04/01 

Date: Sun, 4 Mar 2001 12:56:28 +0800 (+0800)
From: Michael Robinson <robinson@netrinsics.com>
To: freebsd-security@freebsd.org

>Would /dev/urandom be acceptable for use in a one time pad encryption
>system? Such a system is only as strong as the random number generator used
>to generate the keys.
>
>I get the feeling that /dev/random would be a much better choice, but key
>generation with that would be much slower.

Caveat: last I checked, the /dev/[u]random device in -CURRENT was completely
broken for crypto-grade randomness (it said as much in the source).

>Does anyone know of any hardware that isn't to expensive and generates good
>random numbers?

Technically speaking, if you don't have one bit of entropy for each bit of
pad, you don't have a true one-time pad. If you want to generate a lot of
entropy cheaply, the common way to do it is take the digitized input of a
sound card, make a conservative estimate of the number of bits of entropy
per sample, and run as many samples as necessary through a cryptographic hash
(e.g. SHA-1) until you have as many entropy bits in as hash bits out.

If you aren't so insistant on a true one-time pad, you can always use the
hash output to seed a Blum-Blum-Shub PRNG.

        -Michael Robinson

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