Feature request: Enable signing/verifying using PKCS#11

[udf2457]

I would be useful if the lib could be expanded to include PKCS#11 (so for example, people using Yubikeys, smart cards or traditional HSMs could work with securesystemslib).

This request originates from an old go-tuf request, but since they have securesystemslib as a dependency I guess it ultimately falls to you guys to (hopefully !) implement.

[adityasaky]

I think adding that would certainly be very valuable, for both TUF and in-toto. The blocker I foresee is developer bandwidth to implement this. I can, however, promise review time. Do you think you could submit a PR?