How to plug an external encryption to CMS_SignerInfo signing?

[Dmitry Belyavsky]

Dear Francesco,

I think this link is relevant:
https://github.com/OpenSC/libp11/blob/master/src/eng_front.c

On Sat, Oct 24, 2020 at 1:45 PM Francesco Pretto <ceztko at gmail.com> wrote:

> Hi Dmitry,
>
> thank you for the prompt answer. Are you able to provide me with a
> link to an example of creating such engines that will fit this use
> case? On my searches I was able to find staff like EVP_PKEY_METHOD[1]
> but I wasn't able to use them for my purpose. Not assuming how stuff
> works today, ideally for me such engine should work like this:
> - I prepare an engine that is able to deal with private keys only, and
> just for encryption;
> - I create a fake EVP_PKEY* that will use this engine;
> - I supply this private key to CMS_add1_signer(), together with the
> usual/regular X509 certificate, parsed for example from DER/PEM.
>
> Even if CMS_add1_signer()[2] is currently performing a validation of
> the EVP_PKEY passed, it shouldn't be hard to patch to avoid doing such
> a check, for example by supplying a newly created flag that specifies
> to not do it.
>
> If engines can't be operated this way, then I'm afraid they will still
> require more boilerplate code than really necessary.
>
> Cheers,
> Francesco
>
> [1] https://www.openssl.org/docs/man1.1.1/man3/EVP_PKEY_METHOD.html
> [2]
> https://github.com/openssl/openssl/blob/d1fb6b481b1d70932a1435f83eae10cc68edbe36/crypto/cms/cms_sd.c#L269
>
>
> On Sat, 24 Oct 2020 at 12:12, Dmitry Belyavsky <beldmit at gmail.com> wrote:
> >
> > Dear Francesco,
> >
> > On Sat, Oct 24, 2020 at 1:06 PM Francesco Pretto <ceztko at gmail.com>
> wrote:
> >>
> >> Hello,
> >>
> >> I'm trying to create a CMS context for subsequent export using
> >> CMS_sign(). I add a signer using CMS_add1_signer() that allows me to
> >> specify a X509 certificate and a hash function. I would like the CMS
> >> context to perform hash computation and ANS1 structure filling, but I
> >> want to delegate encryption to an external service, for example an
> >> hardware encryption token (I'm assuming this is a very common use
> >> case). At this point I'm in a stalemate since CMS_add1_signer() asks
> >> me for a private EVP_PKEY that is compatible with the public key
> >> present in the X509 certificate. No other function seems to exist to
> >> create a CMS_SignerInfo by providing an external mechanism for
> >> encryption.
> >>
> >> My hacky solution was to add a signer CMS_add1_signer() supplying the
> >> public key stored in the X509 certificate in the place of the private
> >> one. This passes internal checks of the function and allows me to
> >> subsequently handle (manually) all the ANS1 structure filling and hash
> >> computations. This is barely doable with public openssl API and still
> >> requires a big rip-off of private openssl code (attached as a
> >> standalone C++ class, if it can be useful for someone).
> >>
> >> My question is: is there an easier mechanism to plug a separate
> >> encryption method when creating the CMS_SignerInfo structure and have
> >> openssl do all the other dirty work for me? If so, is it possible to
> >> do with openssl 1.1.0/1.1.1?
> >
> >
> > Engines allow operating by private keys in such a manner.
> > You have to reimplement all the callbacks dealing with private keys.
> Also, it's possible you have to write some wrappers for the functions
> dealing with public keys.
> >
> > For 3.0, the providers should do the same trick, I think.
> >
> > --
> > SY, Dmitry Belyavsky
>


-- 
SY, Dmitry Belyavsky
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