gpgme - raw RSA operation using GPG public/private keys?

[gnupg-user at niob.at]

Am 12.07.2017 um 01:55 schrieb Daniel Kahn Gillmor:
> On Fri 2017-07-07 18:01:03 +0200, gnupg-user at niob.at wrote:
>> I am looking for a "simple" way to use a GPG public/private RSA key to
>> do "raw" RSA operations. I have the impression, that gpgme only deals
>> with "real" OpenPGP data structures, but this does not fit my use case.
>> This is for an application that is currently based on openssl crypto.
> you're right -- gpgm is only for higher-level protocol operations,
> whether they're OpenPGP or CMS (cryptographic message syntax).  it
> doesn't offer low-level crypto primitives.
>
> if you want low-level crypto primitives that are GPL-compatible, you can
> use libhogweed (from the nettle project) or libgcrypt.
Thanks a lot for the answer. So the next question is: How? That is: I
could not find any libgcrypt functions taking a gpg key obtainable
through gpgme.

But that is the key problem (haha): I *could* (by hand) parse a secret
key exported using gpg (or, if possible, through gpgme) and use the RSA
parameters to build up the key structure required for either libgcrypt
(or openssl). But that would make it impossible to deal with e.g. gpg
agents.

So to rephrase the question: How would I proceed to do raw RSA
operations using libcrypt for gpg keys stored in a standard key ring? Or
is this functionality not exposed directly in any library? Would it be
best to look at how gpg itself does this? Any pointers (source files,
docs, examples, etc.?)

> Modern GnuPG uses libgcrypt for its crypto primitives, fwiw.
I want to be modern as well... :-)
>        --dkg

peter