Apple actually made this much harder than it should be by special-casing P256 ECDSA in their build of libressl, which means trying to use it with PKCS#11 breaks

Apple actually made this much harder than it should be by special-casing P256 ECDSA in their build of libressl, which means trying to use it with PKCS#11 breaks: https://mjg59.dreamwidth.org/64968.html https://mjg59.dreamwidth.org/64968.html . The approach Secretive uses (which is what I ended up mimicking) is using the SSH agent protocol rather than PKCS#11, which lets you do the crypto in your own codebase instead.