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Secret Sharing by Mail

SSBM is a simple system for lazy cospirators who want to share secrets of some sort between a small group of mutually-trusted people, using an existing GPG network of trust, and email as the communication protocol.

The secrets don't really have a structure, they're just simple key/value pairs.

The system works by doing versioned checkins and automatic conflict resolution. The assumption is that the size of the secret database is always going to be small enough to easily fit in an email message.

Installation and Setup

Once you have a proper Go environment set up, this should be enough:

$ go install git.autistici.org/ale/ssbm

The software needs to be installed along with your main GPG secret key, as it will need to sign and decrypt data as yourself. For this reason, it is recommended that you use an agent (unless you want GPG to prompt you for the passphrase every single time you perform an ssbm operation).

You need to find a way to feed certain messages from your INBOX to the ssbm tool (for the receive command).

The program will look for the GnuPG executable as gpg in the current path, but you can override this by setting the GNUPG environment variable.

Step 1: Initialize SSBM

Pick the email address that you will use to receive the updates, and the GPG key ID of your primary public key. Then, run the following command:

$ gpg -a --export ${KEYID} | ssbm init ${EMAIL} ${KEYID}

This will create the ~/.ssbm directory.

Step 2: Setup email forwarding

You need to find a way to forward certain messages from the address you selected above to the ssbm tool. There are many ways to do so, depending on your specific mail setup. If, for instance, you're using Procmail, you can put the following rule in your .procmailrc:

* ^Subject:.*\[SSBM\]
| /usr/bin/ssbm receive

Messages intended for ssbm always have the [SSBM] string in the Subject header. This cannot be modified for the moment, thus limiting usage to a single shared datastore! This is a bug that will be fixed soon.

Step 3 (optional): Import bootstrap data

If you have just been added to the shared datastore (i.e. someone has run add-friend with your email and GPG key), you will have received an encrypted email with [SSBM] in the subject. Verify that the source is trusted and the signature is correct, decrypt the data and pipe it into ssbm bootstrap.


Manage users

Add a user to the list of people who share the secrets:

$ ssbm add-friend ${EMAIL} ${KEYID}

Delete a user from the list:

$ ssbm del-friend ${EMAIL}

Further updates sent from this user will be ignored, and other users will stop sending updates to him.

Get/Set values

Set a secret:

$ ssbm set ${KEY} ${VALUE}

Retrieve its value:

$ ssbm get ${KEY}