I am creating a public key cryptosystem (just for fun, I don’t roll my own crypto!) That can create identities, which are just Ed25519 + RSA-OAEP key pairs. I want an identity to be able to sign messages and receive encrypted messages. My public identities are large, mainly because of the RSA-OAEP public key, which I am a bit dissatisfied with (I want identities to be personally shared via QR codes, so short length is a plus). This got me thinking … Can I also reuse Ed25519 + X25519 for the asymmetric encoding? If Alice creates a static DH key pair and shares her DH public key (signed, as part of her identity) then Bob can calculate a shared secret and use it for symmetric encryption (sending his short-lived DH public key along with the message). But this, of course, means that Alice’s DH secret is static and will be reused for many senders. Is this a safe thing to do, or should DH key pairs always be ephemeral? (If this is healthy I probably reinvented something, does this schema have a name?) What are the implications of this schema over RSA-OAEP? One difference I see is that Bob can decode the cyphertext … which he produced himself, so I’m not worried about that! Are there any other implications that I have missed? Thanks!
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