Generates a Signed Pre-Key: X25519 keypair + identity-key
signature over its public key.
Why the signature?
The initiator downloads SPK from an untrusted server. Without
the signature, a malicious server could substitute its own SPK
and position itself as a man-in-the-middle. The signature
(by the long-lived identity key) lets the initiator verify
that the SPK is authentic.
Generates a Signed Pre-Key: X25519 keypair + identity-key signature over its public key.
Why the signature? The initiator downloads SPK from an untrusted server. Without the signature, a malicious server could substitute its own SPK and position itself as a man-in-the-middle. The signature (by the long-lived identity key) lets the initiator verify that the SPK is authentic.