Re: [PATCH v4 06/15] crypto: dh - introduce common code for built-in safe-prime group support

[Hannes Reinecke]

On 2/21/22 13:10, Nicolai Stange wrote:
> Recent work on NVME in-band authentication support ([1]) needs to invoke
> the "dh" KPP with the FFDHE safe-prime group parameters as specified in
> RFC 7919 and generate ephemeral keys suitable for the respective group. By
> coincidence, the requirements from NIST SP800-56Arev3,
> sec. 5.5.2 ("Assurance of Domain-Parameter Validity") basically boil down
> to disallowing any group parameters not among the approved safe-prime
> groups specified in either RFC 7919 or RFC 3526 in FIPS mode. Furthermore,
> SP800-56Arev3 specifies the respective security strength for each of the
> approved safe-prime groups, which has a direct impact on the minimum key
> lengths.
>
> In this light, it's desirable to introduce built-in support for the
> RFC 7919 safe-prime groups to the kernel's DH implementation, provide a
> SP800-56Arev3 conforming key generation primitive for those and render
> non-approved group parameters unusable in FIPS mode on the way.
>
> As suggested ([2]) in the course of discussion to previous iterations of
> this patchset, the built-in support for ffdhe groups would be best made
> available in the form of templates wrapping the existing "dh"
> implementation, one for each group specified by RFC 7919: ffdhe2048(dh),
> ffdhe3072(dh), ffdhe4096(dh), ffdhe6144(dh) and ffdhe8192(dh). As these
> templates differ only in the safe-prime constants they'd configure the
> inner "dh" transforms with, they can share almost all of their
> "dh"-wrapping template implementation code.
>
> Introduce this common code to dh_generic. The actual dump of the RFC 7919
> safe-prime constants will be deferred to the next patch in order to
> facilitate review. The ephemeral key generation primitive mentioned above
> likewise deserves a patch on its own, as does the mechanism by which
> unapproved groups are rendered unusable in FIPS mode.
>
> Define a struct dh_safe_prime container for specifying the individual
> templates' associated safe-prime group constants. All ffdheXYZ(dh) template
> instances will store a pointer to such a dh_safe_prime in their context
> areas each. Implement the common __dh_safe_prime_create() template
> instantiation helper. The intention is that the individual ffdheXYZ(dh)
> crypto_templates' ->create() implementations will simply forward any calls
> to __dh_safe_prime_create(), passing a suitable dh_safe_prime in addition
> to the received ->create() arguments. __dh_safe_prime_create() would then
> create and register a kpp_instance as appropriate, storing the given
> dh_safe_prime pointer alongside a crypto_kpp_spawn for the inner "dh"
> kpp_alg in the context area.
>
> As the ffdheXYZ(dh) kpp_instances are supposed to act as proxies to the
> inner "dh" kpp_alg, make each of their associated crypto_kpp transforms to
> in turn own an inner "dh" transform, a pointer to which gets stored in the
> context area. Setup and teardown are getting handled from the outer
> ->init_tfm() and ->exit_tfm() respectively.
>
> In order to achieve the overall goal and let the ffdheXYZ(dh) kpp_instances
> configure the inner "dh" transforms with the respective group parameters,
> make their common ->set_secret(), the new dh_safe_prime_set_secret(), fill
> in the P and G values before forwarding the call to the inner "dh"'s
> ->set_secret(). Note that the outer ->set_secret() can obtain the P value
> associated with the given ffdheXYZ(dh) kpp_instance by means of the
> dh_safe_prime referenced from the latter's context. The value of G OTOH
> always equals constant 2 for the safe-prime groups.
>
> Finally, make the remaining two kpp_alg primitives both operating on
> kpp_requests, i.e. ->generate_public_key() and ->compute_shared_secret(),
> to merely forward any request to the inner "dh" implementation. However, a
> kpp_request instance received from the outside cannot get simply passed
> on as-is, because its associated transform (crypto_kpp_reqtfm()) will have
> been set to the outer ffdheXYZ(dh) one. In order to handle this, reserve
> some space in the outer ffdheXYZ(dh) kpp_requests' context areas for in
> turn storing an inner kpp_request suitable for "dh" each. Make the outer
> ->generate_public_key() and ->compute_shared_secret() respectively to setup
> this inner kpp_request by means of the new dh_safe_prime_prepare_dh_req()
> helper before handing it over to the "dh" implementation for further
> processing. dh_safe_prime_prepare_dh_req() basically copies the outer
> kpp_request received from the outside over to the inner one, but installs
> the inner transform and its own ->complete() proxy callback therein. This
> completion callback, the new dh_safe_prime_complete_req(), doesn't do
> anything beyond completing the outer request. Note that there exist some
> examples in crypto/, which would simply install the completion handler
> from the outer request at the inner one in similar setups, e.g. seqiv.
> However, this would mean that the user-provided completion handler won't
> get called with the address of the outer kpp_request initially submitted
> and the handler might not be prepared for this. Users could certainly work
> around this by setting the callback ->data properly, but IMO it's cleaner
> this way. Furthermore, it might make sense to extend
> dh_safe_prime_complete_req() in the future and move e.g. those
> post-computation FIPS checks from the generic "dh" implementation to the
> ffdheXYZ(dh) templates.
>
> [1] https://lore.kernel.org/r/20211202152358.60116-1-hare@xxxxxxx
> [2] https://lore.kernel.org/r/20211217055227.GA20698@xxxxxxxxxxxxxxxxxxx
>
> Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
> ---
>   crypto/dh.c | 208 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 208 insertions(+)
Reviewed-by: Hannes Reinecke <hare@xxxxxxx>

Cheers,

Hannes
--
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@xxxxxxx			                  +49 911 74053 688
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