Investigate whether equivalent public keys can damage the security of handshake mode
Some ECDH public keys are equivalent, in the sense that multiplying them by the same scalar produces the same point. If an attacker sends us a handshake public key that's equivalent to a contact's handshake public key, we'll fail to detect that it's the same key (see #1565) and derive the same handshake mode transport keys. The attacker won't be able to derive the keys, but we'll use the same keys for handshaking with the contact and trying to handshake with the attacker.
This shouldn't affect confidentiality, integrity or authenticity, as we use a unique random nonce with the stream header key, but it could affect protocol obfuscation by using the same tags for sending streams to the contact and the attacker.
Work out whether this attack is possible, and if so, whether we can detect and prevent it.
Subtask of #1232 (closed).