Magic Wormhole - Get Things From One Computer To Another, Safely

• PyCon 2016 presentation: Slides, Video

Example
Sender:

% wormhole send README.md
Sending 7924 byte file named 'README.md'
On the other computer, please run: wormhole receive
Wormhole code is: 7-crossover-clockwork

Sending (<-10.0.1.43:58988)..
100%|=========================| 7.92K/7.92K [00:00<00:00, 6.02MB/s]
File sent.. waiting for confirmation
Confirmation received. Transfer complete.

Receiver:

% wormhole receive
Enter receive wormhole code: 7-crossover-clockwork
Receiving file (7924 bytes) into: README.md
ok? (y/n): y
Receiving (->tcp:10.0.1.43:58986)..
100%|===========================| 7.92K/7.92K [00:00<00:00, 120KB/s]
Received file written to README.md

Installation
$ pip install magic-wormhole

OS X
On OS X, you may need to install pip and run $ xcode-select --install to get GCC.
Or with homebrew:
$ brew install magic-wormhole

Linux
On Debian 9 and Ubuntu 17.04+ with apt:
$ sudo apt install magic-wormhole
On previous versions of the Debian/Ubuntu systems, or if you want to install the latest version, you may first need:
$ apt-get install python-pip build-essential python-dev libffi-dev libssl-dev
On Fedora:
$ dnf install python-pip python-devel libffi-devel openssl-devel gcc-c++ libtool redhat-rpm-config.
Note: If you get errors like fatal error: sodium.h: No such file or directory on Linux, either use SODIUM_INSTALL=bundled pip install magic-wormhole, or try installing the libsodium-dev / libsodium-devel package. These work around a bug in pynacl which gets confused when the libsodium runtime is installed (e.g. libsodium13) but not the development package.

Windows
On Windows, python2 may work better than python3. On older systems, $ pip install --upgrade pip may be necessary to get a version that can compile all the dependencies.

Motivation

• Moving a file to a friend's machine, when the humans can speak to each other (directly) but the computers cannot
• Delivering a properly-random password to a new user via the phone
• Supplying an SSH public key for future login use

Copying files onto a USB stick requires physical proximity, and is uncomfortable for transferring long-term secrets because flash memory is hard to erase. Copying files with ssh/scp is fine, but requires previous arrangements and an account on the target machine, and how do you bootstrap the account? Copying files through email first requires transcribing an email address in the opposite direction, and is even worse for secrets, because email is unencrypted. Copying files through encrypted email requires bootstrapping a GPG key as well as an email address. Copying files through Dropbox is not secure against the Dropbox server and results in a large URL that must be transcribed. Using a URL shortener adds an extra step, reveals the full URL to the shortening service, and leaves a short URL that can be guessed by outsiders.
Many common use cases start with a human-mediated communication channel, such as IRC, IM, email, a phone call, or a face-to-face conversation. Some of these are basically secret, or are "secret enough" to last until the code is delivered and used. If this does not feel strong enough, users can turn on additional verification that doesn't depend upon the secrecy of the channel.
The notion of a "magic wormhole" comes from the image of two distant wizards speaking the same enchanted phrase at the same time, and causing a mystical connection to pop into existence between them. The wizards then throw books into the wormhole and they fall out the other side. Transferring files securely should be that easy.

Design
The wormhole tool uses PAKE "Password-Authenticated Key Exchange", a family of cryptographic algorithms that uses a short low-entropy password to establish a strong high-entropy shared key. This key can then be used to encrypt data. wormhole uses the SPAKE2 algorithm, due to Abdalla and Pointcheval1.
PAKE effectively trades off interaction against offline attacks. The only way for a network attacker to learn the shared key is to perform a man-in-the-middle attack during the initial connection attempt, and to correctly guess the code being used by both sides. Their chance of doing this is inversely proportional to the entropy of the wormhole code. The default is to use a 16-bit code (use --code-length= to change this), so for each use of the tool, an attacker gets a 1-in-65536 chance of success. As such, users can expect to see many error messages before the attacker has a reasonable chance of success.

Timing
The program does not have any built-in timeouts, however it is expected that both clients will be run within an hour or so of each other. This makes the tool most useful for people who are having a real-time conversation already, and want to graduate to a secure connection. Both clients must be left running until the transfer has finished.

Relays
The wormhole library requires a "Rendezvous Server": a simple WebSocket-based relay that delivers messages from one client to another. This allows the wormhole codes to omit IP addresses and port numbers. The URL of a public server is baked into the library for use as a default, and will be freely available until volume or abuse makes it infeasible to support. Applications which desire more reliability can easily run their own relay and configure their clients to use it instead. Code for the Rendezvous Server is included in the library.
The file-transfer commands also use a "Transit Relay", which is another simple server that glues together two inbound TCP connections and transfers data on each to the other. The wormhole send file mode shares the IP addresses of each client with the other (inside the encrypted message), and both clients first attempt to connect directly. If this fails, they fall back to using the transit relay. As before, the host/port of a public server is baked into the library, and should be sufficient to handle moderate traffic.
The protocol includes provisions to deliver notices and error messages to clients: if either relay must be shut down, these channels will be used to provide information about alternatives.

CLI tool

• wormhole send [args] --text TEXT
• wormhole send [args] FILENAME
• wormhole send [args] DIRNAME
• wormhole receive [args]

Both commands accept additional arguments to influence their behavior:

• --code-length WORDS: use more or fewer than 2 words for the code
• --verify : print (and ask user to compare) extra verification string

Library
The wormhole module makes it possible for other applications to use these code-protected channels. This includes Twisted support, and (in the future) will include blocking/synchronous support too. See docs/api.md for details.
The file-transfer tools use a second module named wormhole.transit, which provides an encrypted record-pipe. It knows how to use the Transit Relay as well as direct connections, and attempts them all in parallel. TransitSender and TransitReceiver are distinct, although once the connection is established, data can flow in either direction. All data is encrypted (using nacl/libsodium "secretbox") using a key derived from the PAKE phase. See src/wormhole/cli/cmd_send.py for examples.

Development
To set up Magic Wormhole for development, you will first need to install virtualenv.
Once you've done that, cd into the root of the repository and run:

virtualenv venv
source venv/bin/activate
pip install --upgrade pip setuptools

Now your virtualenv has been activated. You'll want to re-run source venv/bin/activate for every new terminal session you open.
To install Magic Wormhole and its development dependencies into your virtualenv, run:

pip install -e .[dev]

Running Tests
Within your virtualenv, the command-line program trial will run the test suite:

trial wormhole

This tests the entire wormhole package. If you want to run only the tests for a specific module, or even just a specific test, you can specify it instead via Python's standard dotted import notation, e.g.:

trial wormhole.test.test_cli.PregeneratedCode.test_file_tor

Developers can also just clone the source tree and run tox to run the unit tests on all supported (and installed) versions of python: 2.7, 3.4, 3.5, and 3.6.

Troubleshooting
Every so often, you might get a traceback with the following kind of error:

pkg_resources.DistributionNotFound: The 'magic-wormhole==0.9.1-268.g66e0d86.dirty' distribution was not found and is required by the application

If this happens, run pip install -e .[dev] again.