OK. How this prevents from man-in-the-middle attack on public keys sent over untrusted network? Can you point me to the description of the protocol aimed agains

OK. How this prevents from man-in-the-middle attack on public keys sent over untrusted network? Can you point me to the description of the protocol aimed against man-in-the-middle attack during long term public keys' distribution? Let's say we have a server with a public key S and a client with a public key C. How can they prevent man-in-the-middle attack when exchanging their public keys over untrusted network? I couldn't find any relevant details regarding this question on the official DNSCurve site http://dnscurve.org/ http://dnscurve.org/ .