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Date: Wed, 11 Sep 2013 16:28:31 -0400
From: "Theodore Ts'o" <tytso@mit.edu>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>, Jeff Garzik <jgarzik@pobox.com>,
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        David Safford <safford@watson.ibm.com>, Mimi Zohar <zohar@us.ibm.com>,
        "Johnston, DJ" <dj.johnston@intel.com>
Subject: Re: TPMs and random numbers
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On Wed, Sep 11, 2013 at 12:25:48PM -0700, H. Peter Anvin wrote:
> This of course has been a long-running debate.  Similarly, we could
> make much better use of RDRAND if instead of doing data reduction in
> rngd we could feed it to the pool and just credit fractional bits.
> The FIPS tests that rngd runs are weak and obsoleted, but perhaps
> better than nothing (now when we don't shut down rngd due to false
> positives.)

/dev/urandom is using RDRAND already, and that's what most of the
applications which are generating ssh host keys, session keys, etc.,
are using.

/dev/random is using RDRAND as well, but we're not giving any entropy
credit, so it will take longer to get the necessary randomness to
generate a GPG key.

The rason why it would be good to use TPM to fetch randomness is for
those platforms is (a) for pre-RDRAND capable x86 systems, and (c)
non-x86 platforms that might be using a TPM which don't have a RDRAND
function.

Also, in general, it's better to use as many entropy sources as
possible.

Cheers,

					- Ted
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