Magic Wormhole Invites

Magic Wormhole

magic wormhole is a server and a client which together use Password Authenticated Key Exchange (PAKE) to use a short code to establish a secure channel between two computers. These codes are one-time use and an attacker gets at most one “guess”, thus allowing low-entropy codes to be used.

Invites and Joins

Inside Tahoe-LAFS we are using a channel created using magic wormhole to exchange configuration and the secret fURL of the Introducer with new clients. In the future, we would like to make the Magic Folder (Magic Folder HOWTO) invites and joins work this way as well.

This is a two-part process. Alice runs a grid and wishes to have her friend Bob use it as a client. She runs tahoe invite bob which will print out a short “wormhole code” like 2-unicorn-quiver. You may also include some options for total, happy and needed shares if you like.

Alice then transmits this one-time secret code to Bob. Alice must keep her command running until Bob has done his step as it is waiting until a secure channel is established before sending the data.

Bob then runs tahoe create-client --join <secret code> with any other options he likes. This will “use up” the code establishing a secure session with Alice’s computer. If an attacker tries to guess the code, they get only once chance to do so (and then Bob’s side will fail). Once Bob’s computer has connected to Alice’s computer, the two computers performs the protocol described below, resulting in some JSON with the Introducer fURL, nickname and any other options being sent to Bob’s computer. The tahoe create-client command then uses these options to set up Bob’s client.

Tahoe-LAFS Secret Exchange

The protocol that the Alice (the one doing the invite) and Bob (the one being invited) sides perform once a magic wormhole secure channel has been established goes as follows:

Alice and Bob both immediately send an “abilities” message as JSON. For Alice this is {"abilities": {"server-v1": {}}}. For Bob, this is {"abilities": {"client-v1": {}}}.

After receiving the message from the other side and confirming the expected protocol, Alice transmits the configuration JSON:

    "needed": 3,
    "total": 10,
    "happy": 7,
    "nickname": "bob",
    "introducer": "pb://"

Both sides then disconnect.

As you can see, there is room for future revisions of the protocol but as of yet none have been sketched out.