[noise] Another anti-DoS Approach: MAC the message with the recipient's public key

[Jason A. Donenfeld]

A new thread for this idea:

By adding a MAC (with keyed-blake2) to each handshake message, using
the recipient's public key as the key, we ensure that the only parties
who can cause DH calculations to occur are those who know to whom
they're talking. If the public keys are kept secret, we have a
worthwhile mitigation.