yubicrypt

An easy to use public key encryption program for your YubiKey.

Supported YubiKey algorithms:

For encryption with slot 9d:

RSA: 2048, 3072, 4096 bits

For signing with slot 9c:

ECC: P-256, P-384, Ed25519

Simply use Yubico Authenticator to create your signing and encryption certificate and export your encryption certificate, to give it to your friends. You don't have to export your signing certificate, because it is not needed for signature verification.