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Subject: Re: [PATCH 09/11] nvmet: Implement basic In-Band Authentication
From:   Simo Sorce <simo@redhat.com>
To:     Hannes Reinecke <hare@suse.de>,
        Stephan Mueller <smueller@chronox.de>,
        Christoph Hellwig <hch@lst.de>
Cc:     Sagi Grimberg <sagi@grimberg.me>,
        Keith Busch <keith.busch@wdc.com>,
        linux-nvme@lists.infradead.org,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S . Miller" <davem@davemloft.net>,
        linux-crypto@vger.kernel.org
Date:   Tue, 20 Jul 2021 10:44:58 -0400
In-Reply-To: <4cd673a8-a503-c29a-85f6-3c2703558181@suse.de>
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On Tue, 2021-07-20 at 13:31 +0200, Hannes Reinecke wrote:
> On 7/20/21 12:49 PM, Simo Sorce wrote:
> > On Tue, 2021-07-20 at 12:14 +0200, Hannes Reinecke wrote:
> > > On 7/19/21 1:52 PM, Stephan Mueller wrote:
> > > > Am Montag, dem 19.07.2021 um 13:10 +0200 schrieb Hannes Reinecke:
> > > > > On 7/19/21 12:19 PM, Stephan Mueller wrote:
> > > > > > Am Montag, dem 19.07.2021 um 11:57 +0200 schrieb Hannes Reinecke:
> > > > > > > On 7/19/21 10:51 AM, Stephan Mueller wrote:
> > > [ .. ]
> > > > > > > > 
> > > > > > > > Thank you for clarifying that. It sounds to me that there is no
> > > > > > > > defined protocol (or if there, I would be wondering how the code would have
> > > > > > > > worked
> > > > > > > > with a different implementation). Would it make sense to first specify
> > > > > > > > a protocol for authentication and have it discussed? I personally think
> > > > > > > > it is a bit difficult to fully understand the protocol from the code and
> > > > > > > > discuss protocol-level items based on the code.
> > > > > > > > 
> > > > > > > Oh, the protocol _is_ specified:
> > > > > > > 
> > > > > > >  
> > > > > > > https://nvmexpress.org/wp-content/uploads/NVM-Express-Base-Specification-2_0-2021.06.02-Ratified-5.pdf
> > > > > > > 
> > > > > > > It's just that I have issues translating that spec onto what the kernel
> > > > > > > provides.
> > > > > > 
> > > > > > according to the naming conventions there in figures 447 and following:
> > > > > > 
> > > > > > - x and y: DH private key (kernel calls it secret set with dh_set_secret
> > > > > > or
> > > > > > encoded into param.key)
> > > > > > 
> > > > > 
> > > > > But that's were I got confused; one needs a private key here, but there
> > > > > is no obvious candidate for it. But reading it more closely I guess the
> > > > > private key is just a random number (cf the spec: g^y mod p, where y is
> > > > > a random number selected by the host that shall be at least 256 bits
> > > > > long). So I'll fix it up with the next round.
> > > > 
> > > > Here comes the crux: the kernel has an ECC private key generation function
> > > > ecdh_set_secret triggered with crypto_kpp_set_secret using a NULL key, but it
> > > > has no FFC-DH counterpart.
> > > > 
> > > > That said, generating a random number is the most obvious choice, but not the
> > > > right one.
> > > > 
> > > > The correct one would be following SP800-56A rev 3 and here either section
> > > > 5.6.1.1.3 or 5.6.1.1.4.
> > > > 
> > > Hmm. Okay. But after having read section 5.6.1.1.4, I still do have some
> > > questions.
> > > 
> > > Assume we will be using a bit length of 512 for FFDHE, then we will
> > > trivially pass Step 2 for all supported FFDHE groups (the maximum
> > > symmetric-equivalent strength for ffdhe8192 is 192 bits).
> > 
> > N = 512 is not a good choice, minimum length these days for DH should
> > be 2048 or more.
> > 
> 
> According to RFC7919:
> Peers using ffdhe8192 that want to optimize their key exchange with a
> short exponent (Section 5.2) should choose a secret key of at least
> 400 bits.
> 
> So what is wrong with 512 bits?


RFC7519 is TLS Specific.
I do not know if short-exponents are safe to use in all use cases.

If it is safe, your choice is fine and your arguments will follow, but
then a comment that explains the choice and warns about key checks if
it is changed would be a good idea.

Otherwise the default should be to use N = len(q), which implies the
proper checks need to be applied.

Simo.

> > > From my understanding, the random number generator will fill out all
> > > available bytes in the string (and nothing more), so we trivially
> > > satisfy step 3 and 4.
> > > 
> > > And as q is always larger than the random number, step 6 reduces to
> > > 'if (c > 2^N - 2)',
> > 
> > Where is this coming from ?
> > It seem you assume M = 2^N but M = min(2^N, q)
> > 
> > The point here is to make sure the number X you return is:
> > 0 < X < (q-1)
> > 
> 
> Which is what I've tried to argue. For 512 bits private key and the
> smallest possible FFDHE group (which has 2048 bits, with the top bit
> non-zero) 2^N is always smaller than (q - 1).
> As the other FFHDE groups are using even larger 'q' values, this is true
> for all FFHDE groups.
> 
> > >  ie we just need to check if the random number is a
> > > string of 0xff characters. Which hardly is a random number at all, so
> > > it'll be impossible to get this.
> > > 
> > > Which then would mean that our 'x' is simply the random number + 1,
> > 
> > This is an artifact due to the random number being 0 <= c < 2^N - 1,
> > therefore 1 needs to be added to make sure you never return 0.
> > 
> 
> And my argument here is that all zeros (and all ones) are not a value I
> would expect from our RNG.
> 
> > > which arguably is slightly pointless (one more than a random number is
> > > as random as the number itself), so I do feel justified with just
> > > returning a random number here.
> > > 
> > > Am I wrong with that reasoning?
> > 
> > Looks to me you are not accounting for the fact that N = 512 is too
> > small and a random number falling in the interval (q - 2) < X < 2^N is
> > unsuitable?
> > 
> 
> Only if (q - 2) < 2^N. And my point is that it's not.
> 
> Cheers,
> 
> Hannes

-- 
Simo Sorce
RHEL Crypto Team
Red Hat, Inc