On September 13th, the Iranian government began blocking The Onion Router (TOR), a system for evading network censorship. On September 14th, the TOR project changed its code so that it wasn't blocked anymore.
Yesterday morning (in our timezones — that evening, in Iran), Iran added a filter rule to their border routers that recognized Tor traffic and blocked it. Thanks to help from a variety of friends around the world, we quickly discovered how they were blocking it and released a new version of Tor that isn't blocked. Fortunately, the fix is on the relay side: that means once enough relays and bridges upgrade, the many tens of thousands of Tor users in Iran will resume being able to reach the Tor network, without needing to change their software.
How did the filter work technically? Tor tries to make its traffic look like a web browser talking to an https web server, but if you look carefully enough you can tell some differences. In this case, the characteristic of Tor's SSL handshake they looked at was the expiry time for our SSL session certificates: we rotate the session certificates every two hours, whereas normal SSL certificates you get from a certificate authority typically last a year or more. The fix was to simply write a larger expiration time on the certificates, so our certs have more plausible expiry times.