S/MIME 4.0 as described in RFC8551 specifies:
- MUST support EdDSA with curve25519 using PureEdDSA mode
- MUST support at least one of the following algorithms: ECDSA with curve P-256 and SHA-256, or EdDSA with curve25519 using PureEdDSA mode.
Receiving and sending agents:
- MUST support ECDH ephemeral-static mode for X25519 using HKDF-256
How do I create a key and a CSR using OpenSSL to create a S/MIME certificate that will use ed25519/curve25519/EdDSA ideally with SHA-512?
Existing guides mostly show how to create a RSA key and a CSR.