What is a Tor Bridge, and how does a Bridge Authority make bridges hard to block?

A Bridge is an unlisted Tor entry node used to bypass censorship; the Bridge Authority hands out bridge addresses through several channels so a censor can't grab them all at once.

Bridges (Brücken): special Tor entry nodes that are not published in the Directory Servers' public lists. Because their IPs aren't publicly known, a censor can't simply block them along with the rest of the public relays. In countries like China or Iran, where Tor is blocked, bridges provide a way in.

Bridge Authority: distributes bridge addresses through three pools / multiple channels — e-mail, HTTPS, and Moat — to make access easier. Spreading distribution across channels prevents censors from collecting all bridge addresses in one go.

Tip: A normal relay is on the public menu (easy to block); a bridge is an off-menu secret handed out a few at a time.

Go deeper:

• Tor bridges (Tor Project support) — unlisted bridges and how addresses are distributed to resist blocking.