@cwebber
"Perspectives uses notary servers to verify that the same key is seen across the network, making a man-in-the-middle attack much harder since an attacker must insert itself into multiple places. For federation in Matrix, each Home Server acts as a notary. When one Home Server connects to another Home Server that uses a key that it doesn’t recognize, it contacts other Home Servers to ensure that they all see the same key from that Home Server."

https://matrix.org/docs/guides/faq.html#what-is-perspectives