Running an encrypted mailing list is surprisingly tricky. One of the first challenges is that you need to decide what the threat model is. Are you worried about someone compromising the list server? One of the subscribers stealing the list of subscriber email addresses? You can't just "turn on encryption", you have to think about what you're trying to defend against.

I decided to use schleuder. Here's how I set it up.

Requirements

What I decided to create was a mailing list where people could subscribe and receive emails encrypted to them from the list itself. In order to post, they need to send an email encrypted to the list' public key and signed using the private key of a subscriber.

What the list then does is decrypt the email and encrypts it individually for each subscriber. This protects the emails while in transit, but is vulnerable to the list server itself being compromised since every list email transits through there at some point in plain text.

Installing the schleuder package

The first thing to know about installing schleuder on Debian or Ubuntu is that at the moment it unfortunately depends on ruby 1.8. This means that you can only install it on Debian wheezy or Ubuntu precise: trusty and jessie won't work (until schleuder is ported to a more recent version of ruby).

If you're running wheezy, you're fine, but if you're running precise, I recommend adding my ppa to your /etc/apt/sources.list to get a version of schleuder that actually lets you create a new list without throwing an error.

Then, simply install this package:

apt-get install schleuder

Postfix configuration

The next step is to configure your mail server (I use postfix) to handle the schleuder lists.

This may be obvious but if you're like me and you're repurposing a server which hasn't had to accept incoming emails, make sure that postfix is set to the following in /etc/postfix/main.cf:

inet_interfaces = all

Then follow the instructions from /usr/share/doc/schleuder/README.Debian and finally add the following line (thanks to the wiki instructions) to /etc/postfix/main.cf:

local_recipient_maps = proxy:unix:passwd.byname $alias_maps $transport_maps

Creating a new list

Once everything is set up, creating a new list is pretty easy. Simply run schleuder-newlist [email protected] and follow the instructions.

After creating your list, remember to update /etc/postfix/transports and run postmap /etc/postfix/transports.

Then you can test it by sending an email to [email protected]. You should receive the list's public key.

Adding list members

Once your list is created, the list admin is the only subscriber. To add more people, you can send an admin email to the list or follow these instructions to do it manually:

  1. Get the person's GPG key: gpg --recv-key KEYID
  2. Verify that the key is trusted: gpg --fingerprint KEYID
  3. Add the person to the list's /var/lib/schleuder/HOSTNAME/LISTNAME/members.conf: 
    - email: [email protected]
  key_fingerprint: 8C470B2A0B31568E110D432516281F2E007C98D1
  4. Export the public key: gpg --export -a KEYID

  5. Paste the exported key into the list's keyring:

    sudo -u schleuder gpg --homedir /var/lib/schleuder/HOSTNAME/LISTNAME/ --import

Signing the list key

In order to give assurance to list members that the list key is legitimate, it should be signed by the list admin.

  1. Export the key:

    sudo -u schleuder gpg --homedir /var/lib/schleuder/HOSTNAME/LISTNAME/ -a --export LISTNAE@HOSTNAME > ~/LISTNAME.asc
  2. Copy the key onto your own machine using scp and import it: gpg --import < LISTNAME.asc
  3. Sign the key locally by editing the key gpg --edit-key KEYID and using the sign command.
  4. Export your signature: gpg -a --export KEYID > LISTNAME.asc
  5. Copy the signed key back onto the server using scp.

  6. Import your signature into the schleuder public keyring:

    sudo -u schleuder gpg --homedir /var/lib/schleuder/HOSTNAME/LISTNAME/ --import < ~/LISTNAME.asc

After that, anybody requesting the list key will get your signature as well.