Brian Warner - Magic Wormhole: Simple Secure File Transfer

"magic-wormhole" is a simple tool to move files from one computer to another, like "scp" but without the setup. By telling the recipient just a few secret words, the file is safely encrypted and delivered directly to the correct machine. The talk will explain the security mechanics, the cryptography (NaCl and SPAKE2), and how to use the underlying open-source library in your own applications.

https://us.pycon.org/2016/schedule/presentation/1838/

Transcript

1. Move Things From One Computer to Another, Safely Brian Warner magic-wormhole.io PyCon 2016
2. me == Brian Warner • Buildbot, Foolscap, Versioneer • AllMyData: Tahoe-LAFS • Mozilla: Add-On SDK, Firefox Sync, Persona, Firefox Accounts • Today: Magic Wormhole warner@lothar.com @lotharrr github.com/warner
3. Magic-Wormhole: a File Transfer Program • Securely moves a file from one computer to another • or a directory, or a string • My claim: easier than all other secure tools • Especially for moving to an unrelated computer
4. What It Looks Like pip install magic-wormhole
5. What It Looks Like pip install magic-wormhole
6. What It Looks Like pip install magic-wormhole
7. Solved Problem? • What's wrong with the tools we currently use?
8. "easily". "safely". dictate string to sender dictate string to receiver needs proximity eavesdroppers send email ~30 chars ISPs, CAs, internet upload to FTP/HTTP ~60 chars server, ISPs, CAs, internet dropbox ~60 chars Dropbox, CAs + URL shortener ~20 chars Shortening Service, lucky guessers, Dropbox, CAs USB drive X eww cooties SSH/scp ~740 char pubkey none magic wormhole ~20 chars none
9. How Does It Work? • Rendezvous Message Exchange • PAKE, Key Agreement • IP Address Exchange • Transit Connection • Data Transfer
10. Rendezvous Server typical code: 1-liberty-assume Channel ID
11. PAKE-based Security P assword A uthenticated K ey E xchange 1992: EKE 1997: SRP 2005: SPAKE2 2008: J-PAKE
12. SPAKE2 diagram credit: Dan Boneh Alice Bob pw pw pip install spake2
13. Security of PAKE • Weak Secret + Interaction == Strong Secret • Passive eavesdropper gets zero information • Active MitM gets one guess per protocol run • failed guess == zero information • failed guesses are visible to users
14. Wormhole Codes 2-company-spyglass Channel ID aardvark absurd accrue acme adrift adult afflict ahead aimless Algol allow alone ammo ancient apple artist assume Athens aardvark absurd accrue acme adrift adult afflict ahead aimless Algol allow alone ammo ancient apple artist assume Athens len(words)==256
15. Single Use • Wormhole codes are single-use, forward-secure • Default code: 2 words * (256-word list) == 16 bits • 65536 possible codes, equally likely • User must retry 655 times before attacker has 1% chance of success
16. Laziness Improves Security
17. IP Address Exchange • Find addresses with ifconfig • Listen on TCP ports • Exchange addresses+ports • Try to connect, trade encrypted handshakes • First successful connection wins
18. Data Relay Server
19. Encrypted Transit • Provides encrypted record pipe • Uses NaCl SecretBox (Salsa20/Poly1305) • Keys are HKDF(masterkey, purpose) • Data is hashed (SHA256) during transit • Final ACK confirms the hash
20. Library API w = wormhole(AppID, relay_url) w.set_code("1-peachy-seabird") w.send(b"hello") answer = w.get()
21. Future Work • GUI, pre-packaged installers, browser extension • Negotiate better transports: • WebRTC, ICE/STUN, libutp • Tor Onion Services • Add SPAKE2 to libsodium • Port to other languages: JavaScript, Go, Rust
22. Beyond File Transfer • Use this anywhere you need to deliver a credential • Provisioning new client devices • Pairing client devices to each other • Populating addressbook entries in communication/messaging systems
23. Provisioning Clients Old New Type password into server Type password into client Get Wormhole code from server Type code into client
24. Messaging Apps Old New Alice sends public key to server Bob asks server for Alice's key Alice shows Wormhole code to Bob Bob gets Alice's key from Alice (via wormhole)
25. Add PAKE to your Toolbox • Cryptographic tools disseminate too slowly • We need good examples, compelling use cases, helpful libraries • File transfer is a foot in the door. PAKE is the rest.
26. Magic-Wormhole Move Things From One Computer to Another, Safely Brian Warner https://github.com/warner/magic-wormhole warner@lothar.com Thanks To: Rackspace, Twisted, Glyph, djb, 2255-19 @lotharrr magic-wormhole.io pip install magic-wormhole