[sz3]

Different keys.

For example, you might currently be using a public/private keypair for 4096-bit RSA. That keypair (by definition) only works for the RSA key exchange algorithm. Likewise, an x22519 keypair is for the x25519 key exchange.

A sntrup761x25519 keypair will be its own thing. As an aside, a sntrup761x25519 public key will be two public keys glued together (one for each algorithm). [1] Likewise for the private key.

(one could reuse an existing x25519 keypair for the x25519 component of sntrup761x25519, but it seems like a bad idea)

[1] https://github.com/openssh/openssh-portable/blob/master/kexs...

[ttybird2]

Same keys, servers and clients will keep using their ed25519 keys for authenticating each other, the keys for the key exchange are negotiated on login.

[sz3]

Huh! Thanks for the correction.

That makes a lot of sense -- I have some more reading to do...