RE: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()

[Pascal Van Leeuwen]

> -----Original Message-----
> From: linux-crypto-owner@xxxxxxxxxxxxxxx <linux-crypto-owner@xxxxxxxxxxxxxxx> On Behalf Of
> Jarkko Sakkinen
> Sent: Wednesday, October 9, 2019 1:54 AM
> To: Ken Goldman <kgold@xxxxxxxxxxxxx>
> Cc: Safford, David (GE Global Research, US) <david.safford@xxxxxx>; Mimi Zohar
> <zohar@xxxxxxxxxxxxx>; linux-integrity@xxxxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx; open
> list:ASYMMETRIC KEYS <keyrings@xxxxxxxxxxxxxxx>; open list:CRYPTO API <linux-
> crypto@xxxxxxxxxxxxxxx>; open list <linux-kernel@xxxxxxxxxxxxxxx>
> Subject: Re: [PATCH] KEYS: asym_tpm: Switch to get_random_bytes()
> 
> 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.
> 
So having an implementation reviewed by a capable third party of 
your choosing (as that's how certification usually works) is less
trustworthy than having random individuals hacking away at it?
(and trust me, _most_ people are not going to review that by 
themselves - very few people on this planet are capable to do so)

> 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).
> 
Life is not that black and white.

If certification is _not_ a requirement you can use the kernel random
number generator, especially if you don't trust, say, the TPM one.
If you _do_ require certification - and in many industries this is
_mandatory_, you simply _must_ follow the certification rules (which
may impose restrictions how the random number generation is done),
and this should not be made impossible for such _existing_ use cases.

Having said all that, generating _true_ entropy (and, importantly,
ensuring it cannot be manipulated) is a very complicated subject and
considering how all encryption security ultimately  depends on the
quality of the random numbers used for key material, I would not 
trust any implementation that has not been certified or otherwise 
carefully scrutinized by people _proven_ to have the expertise.

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