http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/191-mitm-bridge-detection-resistance.html
Specifically, the bridge would "tag" the Serial Number (or any
other covert field) of her certificate with the (potentially
truncated) HMAC of her link public key, using the shared secret of
proposal 190 as the key: HMAC(shared_secret, link_public_key).