obfs4
obfs4 is a pluggable transport that makes Tor traffic look like random bytes. The default choice when address blocks plus simple protocol detection are in play.
obfs4 is the most widely used pluggable transport and the default offered by Tor Browser when the public network is blocked. It wraps the connection between client and Bridge in an obfuscation layer: the byte stream carries no recognizable structure, so deep packet inspection cannot easily tell it apart from random noise.
What it defeats — and what it does not
obfs4 works against filters that recognize Tor by the pattern of its TLS handshake. It does not help against blocking by address on its own: the bridge behind it must be one the censor has not found yet. That is why obfs4 bridge lines are distributed through limited channels such as BridgeDB rather than published in the open. Active probing — where a censor connects to a suspected bridge and tests how it answers — remains an arms race the transport is continuously adapted to.
What stays the same
The transport sits only on the first hop. Behind it, the circuit is still three relays with the layered encryption of Onion Routing; anonymity is unchanged. obfs4 ships with Tor Browser and needs no extra software — details in Tor Bridges.
Category: Censorship