I'm trying to implement a digital envelope system, where you use (fast) symmetric crypto to encrypt a message with a randomly generated key, then use (slow) asymmetric crypto to encrypt just that key. Then you can send both safely, knowing only the intended recipient can decrypt the key and therefore the message.
I found plenty of libraries for symmetric crypto, but not many for asymmetric crypto. The one I found, Crypto by Rhys Weatherley, supports only very specific use cases, like Diffie-Hellman and digital signatures.
Is there an Arduino library I can use to just encrypt or decrypt data with an asymmetric key?