linux-kernel - Re: TPMs and random numbers

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Message-ID: <20130913021349.GB9445@thunk.org>
Date:	Thu, 12 Sep 2013 22:13:49 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Jörn Engel <joern@...fs.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Jeff Garzik <jgarzik@...ox.com>,
	David Safford <safford@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Leonidas Da Silva Barbosa <leosilva@...ux.vnet.ibm.com>,
	Ashley Lai <ashley@...leylai.com>,
	Rajiv Andrade <mail@...jiv.net>,
	Marcel Selhorst <tpmdd@...horst.net>,
	Sirrix AG <tpmdd@...rix.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kent Yoder <key@...ux.vnet.ibm.com>,
	David Safford <safford@...son.ibm.com>,
	Mimi Zohar <zohar@...ibm.com>,
	"Johnston, DJ" <dj.johnston@...el.com>
Subject: Re: TPMs and random numbers

On Thu, Sep 12, 2013 at 06:23:09PM -0400, Jörn Engel wrote:
> It is worse in three ways:
> - it costs performance,
> - it may create a false sense of safety and
> - it actively does harm if we credit it as entropy.
> 
> How much weight you assign to each of those is up to you.  So long as
> we don't credit any of it as entropy, I am not too adverse to mixing
> it in.  But I can equally see benefit in burning the bridges.

Well, mixing it in and using /dev/[u]random is certainly better than
blindly using the output from the RNG from the TPM directly as a
key.

I'm not sure what you mean by "burning the bridges"; what is the
alternative that you are suggesting?

						- Ted
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