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    SubjectRE: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()
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    > -----Original Message-----
    > From: linux-crypto-owner@vger.kernel.org <linux-crypto-owner@vger.kernel.org> On Behalf Of
    > Jarkko Sakkinen
    > Sent: Wednesday, October 9, 2019 9:33 AM
    > To: Ken Goldman <kgold@linux.ibm.com>
    > Cc: Safford, David (GE Global Research, US) <david.safford@ge.com>; Mimi Zohar
    > <zohar@linux.ibm.com>; linux-integrity@vger.kernel.org; stable@vger.kernel.org; open
    > list:ASYMMETRIC KEYS <keyrings@vger.kernel.org>; open list:CRYPTO API <linux-
    > crypto@vger.kernel.org>; open list <linux-kernel@vger.kernel.org>
    > Subject: Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()
    >
    > On Wed, Oct 09, 2019 at 02:53:39AM +0300, Jarkko Sakkinen wrote:
    > > On Wed, Oct 09, 2019 at 02:49:35AM +0300, Jarkko Sakkinen wrote:
    > > > On Mon, Oct 07, 2019 at 06:13:01PM -0400, Ken Goldman wrote:
    > > > > The TPM library specification states that the TPM must comply with NIST
    > > > > SP800-90 A.
    > > > >
    > > > > https://trustedcomputinggroup.org/membership/certification/tpm-certified-products/
    > > > >
    > > > > shows that the TPMs get third party certification, Common Criteria EAL 4+.
    > > > >
    > > > > While it's theoretically possible that an attacker could compromise
    > > > > both the TPM vendors and the evaluation agencies, we do have EAL 4+
    > > > > assurance against both 1 and 2.
    > > >
    > > > Certifications do not equal to trust.
    > >
    > > And for trusted keys the least trust solution is to do generation
    > > with the kernel assets and sealing with TPM. With TEE the least
    > > trust solution is equivalent.
    > >
    > > Are you proposing that the kernel random number generation should
    > > be removed? That would be my conclusion of this discussion if I
    > > would agree any of this (I don't).
    >
    > The whole point of rng in kernel has been to use multiple entropy
    > sources in order to disclose the trust issue.
    >
    I do understand that, and combining multiple entropy sources, if
    you have them available to get _more_ entropy is a good idea, at
    least in theory. But ...

    How do I know the mixing of entropy happens properly? Especially
    if I'm not capable of judging this by myself.
    And how do I know the SW entropy pool and/or code cannot be influenced
    _somehow_? (either directly or indirectly by influencing one of the
    contributors). More code and/or HW involved means more attack vectors
    and complication of the review process.

    The point is, if you want to certify such an application, you would
    have to have _all_ contributors _plus_ the kernel rng code certified.
    And you would have to have it _recertified_ every time a _single_
    component - including the kernel code itself! - changes.

    > Even with weaker entropy than TPM RNG it is still a better choice for
    > *non-TPM* keys because of better trustworthiness.
    >
    "Even with weaker entropy"? Now that's just silly. If you _know_ and
    _trust_ the TPM to have _better_ entropy, then obviously that is the
    better choice. I guess the key word being the trust you don't have.

    > Using only TPM RNG is
    > a design flaw that has existed probably because when trusted keys were
    > introduced TPM was more niche than it is today.
    >
    For non-TPM keys, possibly. Assuming the kernel RNG indeed adds
    (or at least does not weaken) entropy. And assuming I _can_ trust
    the kernel RNG implementation. Question is: why would I trust that
    more than the TPM implementation? Sure, I could look at the code,
    but would I truly and fully understand it? (so maybe _I_ would,
    but would Joe Random User?)

    > Please remember that a trusted key is not a TPM key. The reality
    > distortion field is strong here it seems.
    >
    Agree. But you should not mess with the possibility to generate
    keys based on _just_ the TPM RNG _where that is required_ (and
    perhaps _only_ where that is required, if possible)

    > /Jarkko

    Regards,
    Pascal van Leeuwen
    Silicon IP Architect, Multi-Protocol Engines @ Verimatrix
    www.insidesecure.com

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