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Date:   Thu, 9 Jan 2020 14:30:48 -1000
From:   Andy Lutomirski <>
To:     Kurt Roeckx <>
Cc:     "Theodore Y. Ts'o" <>,
        Stephan Mueller <>,
        Andy Lutomirski <>,
        LKML <>,
        Linux API <>,
        Kees Cook <>,
        "Jason A. Donenfeld" <>,
        "Ahmed S. Darwish" <>,
        Lennart Poettering <>,
        "Eric W. Biederman" <>,
        "Alexander E. Patrakov" <>,
        Michael Kerrisk <>,
        Willy Tarreau <>,
        Matthew Garrett <>,
        Ext4 Developers List <>,
        linux-man <>
Subject: Re: [PATCH v3 0/8] Rework random blocking

> On Jan 9, 2020, at 12:02 PM, Kurt Roeckx <> wrote:

> If the kernel provides a good RNG, the only reason I can see why
> you would like to have direct access to a hwrng is to verify that
> it's working correctly. That might mean that you put it in some
> special mode where it returns raw unprocessed values. If the device
> is in such a mode, it's output will not provide the same entropy
> per bit, and so I would expect the kernel to stop using it directly.

I disagree.

If I buy a ChaosKey or a fancy EAL4FIPSOMG key, I presumably have it for a reason and I want to actually use the thing for real. Maybe it’s for some certification reason and maybe it’s just because it’s really cool.

As for “direct” access,  I think AMD provides an interface to read raw output from the on-die entropy source. Exposing this to user space is potentially quite useful for anyone who wants to try to characterize it.  I don’t really think people should use a raw sample interface as a source of production random numbers, though.

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