The right to communicate with your friends and family in private, without anyone snooping, is a right every citicen have in a liberal democracy. But this right is under serious attack these days.
A while back it occurred to me that one way to make the dragnet surveillance conducted by NSA, GCHQ, FRA and others (and confirmed by the whisleblower Snowden) more expensive for Internet email, is to deliver all email using SMTP via Tor. Such SMTP option would be a nice addition to the FreedomBox project if we could send email between FreedomBox machines without leaking metadata about the emails to the people peeking on the wire. I proposed this on the FreedomBox project mailing list in October and got a lot of useful feedback and suggestions. It also became obvious to me that this was not a novel idea, as the same idea was tested and documented by Johannes Berg as early as 2006, and both the Mailpile and the Cables systems propose a similar method / protocol to pass emails between users.
To implement such system one need to set up a Tor hidden service providing the SMTP protocol on port 25, and use email addresses looking like email@example.com. With such addresses the connections to port 25 on hidden-service-name.onion using Tor will go to the correct SMTP server. To do this, one need to configure the Tor daemon to provide the hidden service and the mail server to accept emails for this .onion domain. To learn more about Exim configuration in Debian and test the design provided by Johannes Berg in his FAQ, I set out yesterday to create a Debian package for making it trivial to set up such SMTP over Tor service based on Debian. Getting it to work were fairly easy, and the source code for the Debian package is available from github. I plan to move it into Debian if further testing prove this to be a useful approach.
If you want to test this, set up a blank Debian machine without any mail system installed (or run apt-get purge exim4-config to get rid of exim4). Install tor, clone the git repository mentioned above, build the deb and install it on the machine. Next, run /usr/lib/exim4-smtorp/setup-exim-hidden-service and follow the instructions to get the service up and running. Restart tor and exim when it is done, and test mail delivery using swaks like this:
torsocks swaks --server dutlqrrmjhtfa3vp.onion \ --to firstname.lastname@example.org
This will test the SMTP delivery using tor. Replace the email address with your own address to test your server. :)
The setup procedure is still to complex, and I hope it can be made easier and more automatic. Especially the tor setup need more work. Also, the package include a tor-smtp tool written in C, but its task should probably be rewritten in some script language to make the deb architecture independent. It would probably also make the code easier to review. The tor-smtp tool currently need to listen on a socket for exim to talk to it and is started using xinetd. It would be better if no daemon and no socket is needed. I suspect it is possible to get exim to run a command line tool for delivery instead of talking to a socket, and hope to figure out how in a future version of this system.
Until I wipe my test machine, I can be reached using the email@example.com mail address, deliverable over SMTorP. :)