Is it possible to run a secure email network using SMTP over Tor?

Question

I have thought about some of the security issues inherent in email.

For example that even if the body is encrypted the headers are plain text so metadata is available.

I was wondering if this could be mitigated by using Tor and SMTP hidden services?

Could you send an email from [email protected] to [email protected] using standard mail programs and hidden services / transparent proxy? It would seem feasible to me, but as I can't see that anyone has suggested it I figure there must be some problem with the idea. What is stopping this from working?

Or is it happening and I have missed it?

Answer 1

SMTP can perform well over a hidden service, in fact there are providers that offer such a thing. SMTP, IMAP and POP, the email protocols are TCP based and since Tor offers a base for TCP connections, every TCP based protocol can operate on top of it. Using SMTP over Tor could anonymize some valuable identity information in email's headers, but email headers can be stripped down in the first place even without using Tor.

Hidden services could be used for email-like applications, for example :

Cable Communication, firstly implemented for Liberte Linux is an email-like application that uses .onion addresses so as users can communicate. Of course Cables doesn't use SMTP protocol at all, which is not that bad since SMTP and many other wide used internet protocols were not designed with anonymity integration in the first place.

Answer 2

Johannes Berg experimented with configuring the Exim MTA to send mail to .onion addresses, and has written a whitepaper on the subject; see http://johannes.sipsolutions.net/Projects/exim-tor-hidden-mail.

Answer 3

Could you send an email from [email protected] to [email protected] using standard mail programs and hidden services / transparent proxy?

Yes it could, but it wouldn't be with a standard Tor. (Unless some extra software was put in the middle of some of the connections somewhere.) Right now Tor's transparent proxy's DNS server returns NOTIMPL for MX queries. This link might be of some use:
https://trac.torproject.org/projects/tor/ticket/10268

Answer 4

Yes it is possible to run a full open source email stack (postfix + dovecot + roundcube + ...) as a Tor hidden service and use it with Tor Browser Bundle (the webmail) and also with torsocks + claws-mail. Additionally, postfix can be tweaked to strip privacy related headers from outgoing emails. We deploy such configurations and do extensive packet sniffing and analysis, there are no leaks.