Would like to store private or public keys for use in other applications similar to how OpenKeychain: Easy PGP works but with S/MIME certificate and key support. Would like to store S/MIME and GPG private keys on Yubikey, have public key stored in a Proton Pass keychain and then be able to send secure emails either with Proton Mail or 3rd party apps such as Thunderbird for Android without storing private keys on device.