magic-wormhole/docs/api.md

Magic-Wormhole

This library provides a primitive function to securely transfer small amounts of data between two computers. Both machines must be connected to the internet, but they do not need to have public IP addresses or know how to contact each other ahead of time.

Security and connectivity is provided by means of an "invitation code": a short string that is transcribed from one machine to the other by the users at the keyboard. This works in conjunction with a baked-in "rendezvous server" that relays information from one machine to the other.

Modes

This library will eventually offer multiple modes. For now, only "transcribe mode" is available.

Transcribe mode has two variants. In the "machine-generated" variant, the "initiator" machine creates the invitation code, displays it to the first user, they convey it (somehow) to the second user, who transcribes it into the second ("receiver") machine. In the "human-generated" variant, the two humans come up with the code (possibly without computers), then later transcribe it into both machines.

When the initiator machine generates the invitation code, the initiator contacts the rendezvous server and allocates a "channel ID", which is a small integer. The initiator then displays the invitation code, which is the channel-ID plus a few secret words. The user copies the code to the second machine. The receiver machine connects to the rendezvous server, and uses the invitation code to contact the initiator. They agree upon an encryption key, and exchange a small encrypted+authenticated data message.

When the humans create an invitation code out-of-band, they are responsible for choosing an unused channel-ID (simply picking a random 3-or-more digit number is probably enough), and some random words. The invitation code uses the same format in either variant: channel-ID, a hyphen, and an arbitrary string.

The two machines participating in the wormhole setup are not distinguished: it doesn't matter which one goes first, and both use the same Wormhole class. In the first variant, one side calls get_code() while the other calls set_code(). In the second variant, both sides call set_code(). Note that this is not true for the "Transit" protocol used for bulk data-transfer: the Transit class currently distinguishes "Sender" from "Receiver", so the programs on each side must have some way to decide (ahead of time) which is which.

Each side gets to do one send_data() call and one get_data() call per phase (see below). get_data will wait until the other side has done send_data, so the application developer must be careful to avoid deadlocks (don't get before you send on both sides in the same protocol). When both sides are done, they must call close(), to let the library know that the connection is complete and it can deallocate the channel. If you forget to call close(), the server will not free the channel, and other users will suffer longer invitation codes as a result. To encourage close(), the library will log an error if a Wormhole object is destroyed before being closed.

To make it easier to call close(), the blocking Wormhole objects can be used as a context manager. Just put your code in the body of a with Wormhole(ARGS) as w: statement, and close() will automatically be called when the block exits (either successfully or due to an exception).

Examples

The synchronous+blocking flow looks like this:

from wormhole.blocking.transcribe import Wormhole
from wormhole.public_relay import RENDEZVOUS_RELAY
mydata = b"initiator's data"
with Wormhole(u"appid", RENDEZVOUS_RELAY) as i:
    code = i.get_code()
    print("Invitation Code: %s" % code)
    i.send_data(mydata)
    theirdata = i.get_data()
    print("Their data: %s" % theirdata.decode("ascii"))

import sys
from wormhole.blocking.transcribe import Wormhole
from wormhole.public_relay import RENDEZVOUS_RELAY
mydata = b"receiver's data"
code = sys.argv[1]
with Wormhole(u"appid", RENDEZVOUS_RELAY) as r:
    r.set_code(code)
    r.send_data(mydata)
    theirdata = r.get_data()
    print("Their data: %s" % theirdata.decode("ascii"))

Twisted

The Twisted-friendly flow looks like this:

from twisted.internet import reactor
from wormhole.public_relay import RENDEZVOUS_RELAY
from wormhole.twisted.transcribe import Wormhole
outbound_message = b"outbound data"
w1 = Wormhole(u"appid", RENDEZVOUS_RELAY)
d = w1.get_code()
def _got_code(code):
    print "Invitation Code:", code
    return w1.send_data(outbound_message)
d.addCallback(_got_code)
d.addCallback(lambda _: w1.get_data())
def _got_data(inbound_message):
    print "Inbound message:", inbound_message
d.addCallback(_got_data)
d.addCallback(w1.close)
d.addBoth(lambda _: reactor.stop())
reactor.run()

On the other side, you call set_code() instead of waiting for get_code():

w2 = Wormhole(u"appid", RENDEZVOUS_RELAY)
w2.set_code(code)
d = w2.send_data(my_message)
...

Note that the Twisted-form close() accepts (and returns) an optional argument, so you can use d.addCallback(w.close) instead of d.addCallback(lambda _: w.close()).

Phases

If necessary, more than one message can be exchanged through the relay server. It is not meant as a long-term communication channel, but some protocols work better if they can exchange an initial pair of messages (perhaps offering some set of negotiable capabilities), and then follow up with a second pair (to reveal the results of the negotiation).

To support this, send_data()/get_data() accept a "phase" argument: an arbitrary (unicode) string. It must match the other side: calling send_data(data, phase=u"offer") on one side will deliver that data to get_data(phase=u"offer") on the other.

It is a UsageError to call send_data() or get_data() twice with the same phase name. The relay server may limit the number of phases that may be exchanged, however it will always allow at least two.

Verifier

You can call w.get_verifier() before send_data()/get_data(): this will perform the first half of the PAKE negotiation, then return a verifier object (bytes) which can be converted into a printable representation and manually compared. When the users are convinced that get_verifier() from both sides are the same, call send_data()/get_data() to continue the transfer. If you call send_data()/get_data() before get_verifier(), it will perform the complete transfer without pausing.

The Twisted form of get_verifier() returns a Deferred that fires with the verifier bytes.

Generating the Invitation Code

In most situations, the "sending" or "initiating" side will call i.get_code() to generate the invitation code. This returns a string in the form NNN-code-words. The numeric "NNN" prefix is the "channel id", and is a short integer allocated by talking to the rendezvous server. The rest is a randomly-generated selection from the PGP wordlist, providing a default of 16 bits of entropy. The initiating program should display this code to the user, who should transcribe it to the receiving user, who gives it to the Receiver object by calling r.set_code(). The receiving program can also use input_code_with_completion() to use a readline-based input function: this offers tab completion of allocated channel-ids and known codewords.

Alternatively, the human users can agree upon an invitation code themselves, and provide it to both programs later (with i.set_code() and r.set_code()). They should choose a channel-id that is unlikely to already be in use (3 or more digits are recommended), append a hyphen, and then include randomly-selected words or characters. Dice, coin flips, shuffled cards, or repeated sampling of a high-resolution stopwatch are all useful techniques.

Note that the code is a human-readable string (the python "unicode" type in python2, "str" in python3).

Application Identifier

Applications using this library must provide an "application identifier", a simple string that distinguishes one application from another. To ensure uniqueness, use a domain name. To use multiple apps for a single domain, append a URL-like slash and path, like example.com/app1. This string must be the same on both clients, otherwise they will not see each other. The invitation codes are scoped to the app-id. Note that the app-id must be unicode, not bytes, so on python2 use u"appid".

Distinct app-ids reduce the size of the connection-id numbers. If fewer than ten initiators are active for a given app-id, the connection-id will only need to contain a single digit, even if some other app-id is currently using thousands of concurrent sessions.

Rendezvous Relays

The library depends upon a "rendezvous relay", which is a server (with a public IP address) that delivers small encrypted messages from one client to the other. This must be the same for both clients, and is generally baked-in to the application source code or default config.

This library includes the URL of a public relay run by the author. Application developers can use this one, or they can run their own (see src/wormhole/servers/relay.py) and configure their clients to use it instead. This URL is passed as a unicode string.

Polling and Shutdown

TODO: this is mostly imaginary

The reactor-based (Twisted-style) forms of these objects need to establish TCP connections, re-establish them if they are lost, and sometimes (for transports that don't support long-running connections) poll for new messages. They may also time out eventually. Longer delays mean less network traffic, but higher latency.

These timers should be matched to the expectations, and expected behavior, of your users. In a file-transfer application, where the users are sitting next to each other, it is appropriate to poll very frequently (perhaps every 500ms) for a few minutes, then give up. In an email-like messaging program where the introduction is establishing a long-term relationship, and the program can store any outgoing messages until the connection is established, it is probably better to poll once a minute for the first few minutes, then back off to once an hour, and not give up for several days.

The schedule= constructor argument establishes the polling schedule. It should contain a sorted list of (when, interval) tuples (both floats). At when seconds after the first start() call, the polling interval will be set to interval.

The timeout= argument provides a hard timeout. After this many seconds, the sync will be abandoned, and all callbacks will errback with a TimeoutError.

Both have defaults suitable for face-to-face realtime setup environments.

Serialization

TODO: only the Twisted form supports serialization so far

You may not be able to hold the Wormhole object in memory for the whole sync process: maybe you allow it to wait for several days, but the program will be restarted during that time. To support this, you can persist the state of the object by calling data = w.serialize(), which will return a printable bytestring (the JSON-encoding of a small dictionary). To restore, use the from_serialized(data) classmethod (e.g. w = Wormhole.from_serialized(data)).

There is exactly one point at which you can serialize the wormhole: after establishing the invitation code, but before waiting for get_verifier() or get_data(), or calling send_data(). If you are creating a new invitation code, the correct time is during the callback fired by get_code(). If you are accepting a pre-generated code, the time is just after calling set_code().

To properly checkpoint the process, you should store the first message (returned by start()) next to the serialized wormhole instance, so you can re-send it if necessary.

Bytes, Strings, Unicode, and Python 3

All cryptographically-sensitive parameters are passed as bytes ("str" in python2, "bytes" in python3):

• verifier string
• data in/out
• transit records in/out

Other (human-facing) values are always unicode ("unicode" in python2, "str" in python3):

• wormhole code
• relay URL
• transit URLs
• transit connection hints (e.g. "host:port")
• application identifier
• derived-key "purpose" string: w.derive_key(PURPOSE)

Detailed Example