Wednesday, November 22, 2017

To be an encrypted ninja or not to be...

Its a debate whether or not foreign powers hacked into Hillary Clinton's private email servers. There is consensus however that the private email server was hacked though, and this is precisely how emails can easily get leaked. To solve this sort of problem you can either have Hilary and Hilary's friend's become ninjas at cryptography, have cryptography tools become mainstream and transparent, or have a middle ground solution somehow. For this year's 2017 Hackweek at SUSE and Aaron Swartz day I have worked on a middle ground solution as a proof of concept using GPG, forcing all incoming emails to you to be encrypted, even if you use gmail or yahoo to store your emails. On this post I will explain the motivation for such work, and document how to accomplish this for yourself, should you want to implement this for yourself.

Motivation

Emails sent to you when you are using popular email servers such as gmail or yahoo get encrypted only on the wire, as they make their way onto email servers hosted by the companies that provide these services. The emails are however stored unencrypted. Likewise for typical private email servers. You are at left at the whims of the security best practices of these companies, and even if you did have your private email server, to get things done right requires substantial work. In fact, even if you used a good company to store your email, you may still face issues with ensuring your email privacy remains outside of the control of intelligence agencies which may argue they should be able to read everyone's email.

Reasons for wanting your emails stored with good cryptography vary but here are a few reasons:

  • You're a politician
  • You're a therapist
  • You're a journalist
  • You're a human rights advocate
  • You just give a damn about privacy


For most people's day to day, the below diagram simplifies and reveals how email transactions work, Exhibit-A:


One solution to this is to have everyone, for example, use encryption tools when crafting and sending emails, Exhibit-B:


This is a bit unrealistic, however for some folks this is possible, for instance if you're a journalist working with very sensitive material. If you fall into one of the categories below you might not be able to get to this point:

  • You're a human rights watch group worker dealing with folks who can't easily become ninjas... 
  • Your're a therapist, who obviously deals with folks who don't even care about what a crypto ninja is
  • You're a politician and just want to encrypt everything
  • You want to open up your email on a certain date and use an escrow to stash your PGP key, such key becomes public after certain date
  • You want to ask company admins to setup a secure and sensible way to forward some company emails to a public mail server safely (say, a way to get work email on public servers)
  • You just care about cryptography
  • You cannot trust your email provider's data store at all
  • You don't want your data to be scraped by the company hosting it

Making cryptography more easily accessible is a much better approach. Such good efforts exists, one example I found was FlowCrypt which lets you uses Public Key Cryptography, however that does mean trying to trust a private key on the plugin store locally. Another effort, which doesn't use Public Key Cryptography is SecureGmail by streak, you encrypt emails using a one way cipher. Both and similar solutions still require some effort or deploying some sort of software on the sender's side.

What I've worked on means as a ninja, or if you have a ninja friend, you get the benefit of having your emails stored on your preferred email server encrypted, provided you can trust a particular middle service provider I'll describe how to set up, and you can get it secured. You end up with the following, Exhibit-C:

To accomplish this we need a middle end system which does the actual encryption for you using your public key. Email providers such as Google, Yahoo, and others won't do this for us today, and they have some reasons not to. By scraping your email they get the ability to provide search facilities, they get to scrape emails as they might legally see fit, and advertise for you. This is how they make money off of storing our emails for free. Using a middle layer to encrypt your email is reflected in the following diagram, Exhibit-D:

One must admin that this shifts trust to a particular server admin who sets this server up,  and to trust the setup to parse and bounce emails to your preferred email server properly. Your emails are still at risk but they are not stored on the middle server if done propery, they are just being piped through. Also, with unencrypted emails even your old emails are at risk, once an email server is compromised all your emails stored on that server are at risk. With a super simple service such as the one I am describing, it would be fairly easy to monitor against attacks and only protect one thing: receive encrypted emails via TLS, encrypt them write away without writing them to disk, and immediately bounce them. Nothing unencrypted lands on disk or storage.

How do I get this?

If you're curious to try it for a few tests cases, you trust me for such tests cases, shoot me an email and I can set you up with an account on my proof of concept email system, encrypted.ninja. I can give you an account on such system, and if you get an email sent to that email address all emails will be immediately bounced back to you, encrypted with your PGP key.

I would not recommend you to use this setup just as-is though, it'd be best to have spam detection be done on your behalf, otherwise it may be possible your email provided's spam detection tool won't pick up spam, and you end up getting tons of spam.

As such, this is just a proof of concept at this point.

How do I replicate your setup?

Even though this uses PGP keys to encrypt data, you'll need to set up an email server with proper TLS certificates for encryption for communication between senders and bouncing emails to email servers. Fortunately letsencrypt can give you a free certificate, it must be renewed (easy to do). The same SSL certificate you get for them for your apache setup can be used for email as well. So first thing you should do is get a DNS name, then get a simple website up with an SSL certificate from letsencrypt.

If you have control over the email server you may not want to give an full shell login account to all users, but just an email alias. I used postfix for my email server, as its easy to setup, and has some hooks we'll use later. So get yourself postfix installed and setup, no need to setup TLS for your first setup. Just get it receiving emails locally first. Once you have that setup, setup the same SSL certificate you used for your apache setup for your postfix configuration. The following is my setup, roughly.


You'll then need to edit /etc/postfix/master.cf and add the following phphook like, and replace your smtp line with the one below as well:

pgphook unix - n n - - pipe flags=F user=www-data argv=/opt/bin/mail2pgp.sh ${sender} ${size} ${recipient}
smtp inet n - - - - smtpd -o content_filter=pgphook:dummy


Then setup virtual aliases, /etc/postfix/address.txt looks like this:

mcgrof@encrypted.ninja FILTER pgphook:dummy

Add more entries per email address you want to add. After updating it you must run:

postmap /etc/postfix/address.txt

Then its all a matter or just one script and one procmailrc file, and ensuring the script, its gpg directory, and keyring are all owned by the user the email server runs as. That's it.

I stashed the script, procmailrc and gpg directory and keyring for the email server in:

/opt/mail2pgp/
mkdir /opt/mail2pgp/.gnupg
chmod o-rwx /opt/mail2pgp/.gnupg
chmod g-rwx /opt/mail2pgp/.gnupg
sudo chown -R www-data /opt/mail2pgp/

To create a keyring with keys, or update them later with new keys as you update the alias file, and script provided later:

gpg --search-keys hexkeyid
gpg --export --output keyring.gpg
cp keyring.gpg /opt/mail2pgp/keyring.gpg

The script:


You'll also need a MIME preamble, and postfix:


And finally, the procailrc file:


That's it. In fact, you can use the MIME  preamble and postfix and procmailrc file as a template on a system you *don't* have root on to bounce encrypted emails out to you in a much more secure way as well.

Now I'll surely see someone try to hack this server :) and I'm sure they will ;)