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. Debian stable packages Go 1.0.2 which is missing several things obfs4 depends on like SHA256.
• go.crypto (https://code.google.com/p/go.crypto)
• go.net (https://code.google.com/p/go.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)

Thanks

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