I'm working on a practical application of sharing a static key to better-identify a user, between the recipient (server) and the initiator (client ie. user's machine).
My core question is:
For x25519, can multiple PK's resolve to a single SK, or is there ever only one unique PK-SK pair?
I'm wondering whether it's rational to think of x25519 public-keys as holding an "almost" equal share in securing information? By "almost" equal, I mean that I can always derive the PK from the SK.
If I didn't have the PK or the SK, would I be able to produce a message that matched an SK at better probability than a brute-force guess across (half?) the x25519 key-space?
My original q, slightly re-worded and still bl**dy confusing, sorry!
Without either the PK or SK for an x25519 key-pair, is there a greater probability that I can encrypt a message for an unknown SK, than I would otherwise have being able to determine the SK from the PK (if I had the PK).
Specifics for my application handshake is here: