obfs4 - The obfourscator

Yawning Angel (yawning at torproject dot org)

What?

This is a look-like nothing obfuscation protocol that incorporates ideas and concepts from Philipp Winter's ScrambleSuit protocol. The obfs naming was chosen primarily because it was shorter, in terms of protocol ancestery obfs4 is much closer to ScrambleSuit than obfs2/obfs3.

The notable differences between ScrambleSuit and obfs4:

• The handshake always does a full key exchange (no such thing as a Session Ticket Handshake).
• The handshake uses the Tor Project's ntor handshake with public keys obfuscated via the Elligator 2 mapping.
• The link layer encryption uses NaCl secret boxes (Poly1305/XSalsa20).

As an added bonus, obfs4proxy also supports acting as an obfs2/3 client and bridge to ease the transition to the new protocol.

Why not extend ScrambleSuit?

It's my protocol and I'll obfuscate if I want to.

Since a lot of the changes are to the handshaking process, it didn't make sense to extend ScrambleSuit as writing a server implementation that supported both handshake variants without being obscenely slow is non-trivial.

Dependencies

Build time library dependencies are handled by go get automatically but are listed for clarity.

• Go 1.2.0 or later. Prior versions of Go (Eg: 1.0.2) are missing certain important parts of the runtime library like a SHA256 implementation.
• go.crypto (https://golang.org/x/crypto)
• go.net (https://golang.org/x/net)
• ed25519/extra25519 (https://github.com/agl/ed25519/extra25519)
• SipHash-2-4 (https://github.com/dchest/siphash)
• goptlib (https://git.torproject.org/pluggable-transports/goptlib.git)

Installation

To build: go get git.torproject.org/pluggable-transports/obfs4.git/obfs4proxy

To install: Copy $GOPATH/bin/obfs4proxy to a permanent location (Eg: /usr/local/bin)

Client side torrc configuration:

ClientTransportPlugin obfs4 exec /usr/local/bin/obfs4proxy

Bridge side torrc configuration:

# Act as a bridge relay.
BridgeRelay 1

# Enable the Extended ORPort
ExtORPort auto

# Use obfs4proxy to provide the obfs4 protocol.
ServerTransportPlugin obfs4 exec /usr/local/bin/obfs4proxy

# (Optional) Listen on the specified address/port for obfs4 connections as
# opposed to picking a port automatically.
#ServerTransportListenAddr obfs4 0.0.0.0:443

Tips and tricks

• On modern Linux systems it is possible to have obfs4proxy bind to reserved ports (<=1024) even when not running as root by granting the CAP_NET_BIND_SERVICE capability with setcap:

  # setcap 'cap_net_bind_service=+ep' /usr/local/bin/obfs4proxy

• obfs4proxy can also act as an obfs2 and obfs3 client or server. Adjust the ClientTransportPlugin and ServerTransportPlugin lines in the torrc as appropriate.

• obfs4proxy can also act as a ScrambleSuit client. Adjust the ClientTransportPlugin line in the torrc as appropriate.

• The autogenerated obfs4 bridge parameters are placed in DataDir/pt_state/obfs4_state.json. To ease deployment, the client side bridge line is written to DataDir/pt_state/obfs4_bridgeline.txt.

Thanks

• David Fifield for goptlib.
• Adam Langley for his Elligator implementation.
• Philipp Winter for the ScrambleSuit protocol which provided much of the design.