Review exchange key usage and ensure no risk, or design mitigation for key replay.

[rmhrisk]

One of the many benefits of 25519 is that you can use the same key for signing and encryption without risk.

The move to secp256r1 requires us to introduce an exchange key, which we sign with the identity key creating a cryptographic binding between the two.

This introduces a potential risk of an exchange key replay, we need to review this scenario and ensure that either there is no risk or we must design a mitigation to this