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    SubjectRe: Why does reading from /dev/urandom deplete entropy so much?
    (Why hasn't anyone been cc:ing Matt on this?)

    On Dec 4, 2007 8:18 AM, Adrian Bunk <> wrote:
    > On Tue, Dec 04, 2007 at 12:41:25PM +0100, Marc Haber wrote:
    > > While debugging Exim4's GnuTLS interface, I recently found out that
    > > reading from /dev/urandom depletes entropy as much as reading from
    > > /dev/random would. This has somehow surprised me since I have always
    > > believed that /dev/urandom has lower quality entropy than /dev/random,
    > > but lots of it.
    > man 4 random
    > > This also means that I can "sabotage" applications reading from
    > > /dev/random just by continuously reading from /dev/urandom, even not
    > > meaning to do any harm.
    > >
    > > Before I file a bug on bugzilla,
    > >...
    > The bug would be closed as invalid.
    > No matter what you consider as being better, changing a 12 years old and
    > widely used userspace interface like /dev/urandom is simply not an
    > option.

    You seem to be confused. He's not talking about changing any userspace
    interface, merely how the /dev/urandom data is generated.

    For Matt's benefit, part of the original posting:

    > Before I file a bug on bugzilla, can I ask why /dev/urandom wasn't
    > implemented as a PRNG which is periodically (say, every 1024 bytes or
    > even more) seeded from /dev/random? That way, /dev/random has a much
    > higher chance of holding enough entropy for applications that really
    > need "good" entropy.

    A PRNG is clearly unacceptable. But roughly restated, why not have
    /dev/urandom supply merely cryptographically strong random numbers,
    rather than a mix between the 'true' random of /dev/random down to the
    cryptographically strong stream it'll provide when /dev/random is
    tapped? In principle, this'd leave more entropy available for
    applications that really need it, especially on platforms that don't
    generate a lot of entropy in the first place (servers).


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