Re: [PATCH 09/11] nvmet: Implement basic In-Band Authentication

[Stephan Mueller <smueller@chronox.de>]

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:
> > > > Am Montag, dem 19.07.2021 um 10:15 +0200 schrieb Hannes Reinecke:
> > > > > On 7/18/21 2:56 PM, Stephan Müller wrote:
> > > > > > Am Sonntag, 18. Juli 2021, 14:37:34 CEST schrieb Hannes Reinecke:
> > > > 
> > > > > > > The key is also used when using the ffdhe algorithm.
> > > > > > > Note: I _think_ that I need to use this key for the ffdhe
> > > > > > > algorithm,
> > > > > > > because the implementation I came up with is essentially plain
> > > > > > > DH
> > > > > > > with
> > > > > > > pre-defined 'p', 'q' and 'g' values. But the DH implementation
> > > > > > > also
> > > > > > > requires a 'key', and for that I'm using this key here.
> > > > > > > 
> > > > > > > It might be that I'm completely off, and don't need to use a key
> > > > > > > for
> > > > > > > our
> > > > > > > DH implementation. In that case you are correct.
> > > > > > > (And that's why I said I'll need a review of the FFDHE
> > > > > > > implementation).
> > > > > > > But for now I'll need the key for FFDHE.
> > > > > > 
> > > > > > Do I understand you correctly that the dhchap_key is used as the
> > > > > > input
> > > > > > to
> > > > > > the 
> > > > > > DH - i.e. it is the remote public key then? It looks strange that
> > > > > > this
> > > > > > is
> > > > > > used 
> > > > > > for DH but then it is changed here by hashing it together with
> > > > > > something
> > > > > > else 
> > > > > > to form a new dhchap_key. Maybe that is what the protocol says.
> > > > > > But it
> > > > > > sounds 
> > > > > > strange to me, especially when you think that dhchap_key would be,
> > > > > > say,
> > > > > > 2048 
> > > > > > bits if it is truly the remote public key and then after the
> > > > > > hashing
> > > > > > it is
> > > > > > 256 
> > > > > > this dhchap_key cannot be used for FFC-DH.
> > > > > > 
> > > > > > Or are you using the dhchap_key for two different purposes?
> > > > > > 
> > > > > > It seems I miss something here.
> > > > > > 
> > > > > No, not entirely. It's me who buggered it up.
> > > > > I got carried away by the fact that there is a
> > > > > crypto_dh_encode_key()
> > > > > function, and thought I need to use it here.
> > > > 
> > > > 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.

Ciao
Stephan
> 
> > - g^x mod p  / g^y mod p: DH public keys from either end that is
> > communicated
> > over the wire (corresponding to the the DH private keys of x and y) - to
> > set
> > it, you initialize a dh request and set the public key to it with
> > kpp_request_set_input. After performing the
> > crypto_kpp_compute_shared_secret
> > you receive the shared secret
> > 
> > - g^xy mod p: DH shared secret - this is the one that is to be used for
> > the
> > subsequent hashing /HMAC operations as this is the one that is identical
> > on
> > both, the host and the controller.
> > 
> Thanks. Will be checking the code if I do it correctly.
> 
> Cheers,
> 
> Hannes