http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/191-mitm-bridge-detection-resistance.html
Usually, the bridge operator
gives the hash digest to her bridge users along with the rest of
the bridge credentials, like the bridge's address and port.
3. Security implications
Bridge clients who have pinned a bridge to a certificate
fingerprint will be able to detect an MITMing adversary in time.