How Mailpile can implement opportunistic PGP email encryption

Posted December 2, 2013 in crypto openpgp security

For those wanting to decentralize the Internet and encrypt all the things, Mailpile is a hot topic.

What’s Mailpile?

Mailpile is a web-based email client (like Thunderbird or Outlook, not to be confused with a service, like Gmail) that you install locally and access by opening http://localhost:33411/ in your browser.

The goal of Mailpile is to give everyone all the nice features they’re used to with Gmail but that you don’t get with a traditional email client, like labels, conversations, and really quick search. You can use Mailpile to check any email address (including your one).

Right now Gmail is overwhelmingly the most popular email provider because it’s such a good product. If you want to escape Google’s centralized one-stop-PRISM-shop email service you have to put up with something that’s more annoying to use. Mailpile promises to make any email service you want to use as usable as Gmail, with the added benefit that Google-bots don’t parse the contents of your email and serve targeted ads, and that you have a choice of who to host your email with (perhaps you want to host it in Iceland, or on a home server). Basically, it might help break Gmail’s deathgrip on the email market, and considerably re-decentralize email.

But what the cypherpunks are most excited about is that Mailpile has the potential to make PGP-encrypted email not suck nearly as much as it does today.

Note: Mailpile is an open source project that’s been funded by a successful Indiegogo campaign. Right now it’s in a pre-alpha state, so it doesn’t work yet. The alpha is scheduled for release in January.

How will PGP be different in Mailpile?

Everyone wants to use PGP email encryption in the Gmail web interface. There have been plenty of attempts at building PGP browser add-ons (Mailvelope seems to be the most promising right now) but so far nothing has really stuck because of some fundamental problems about glueing PGP onto a system that was never designed for it.

Mailpile is being designed for it. When you open an encrypted email in Mailpile the server detects that it’s encrypted and calls out to gpg to decrypt it using gpg-agent. Your operating system pops up a passphrase dialog, so your passphrase never goes through the browser. All OpenPGP operations happen as system calls to gpg, the same way it works with Enigmail. Additionally, since Mailpile will have access to all your local email, it will be possible to do things such as search encrypted email.

This is a chance to build the user interface for OpenPGP correctly and to remove all of the friction from using it. In fact, I believe that Mailpile could pretty effectively make PGP encrypted email encryption happen almost in the background, without the user needing to do much of anything.

How can Mailpile make using PGP suck less?

Mailpile should encourage a PGP trust system similar to how SSH trust works, Trust-On-First-Use (TOFU). Since there’s a very small chance that you’ll be the victim of an active man-in-the-middle attack the very first time you see a public key, you can go ahead and assume that the first key you saw is valid. With, of course, the ability to change this trust setting manually for each key.

This proposal relies heavily on using key servers, and Mailpile should communicate with key servers using Tor. Not using Tor would leak information about who you are and who emails you. The Windows and Mac versions will have a trickier time with a Tor dependency, but it’s certainly possible and I think it’s absolutely worth it.

Everyone should have a PGP key. When you setup an account in Mailpile it should help you create a PGP key if you don’t already have one. If you create a new PGP key and already have old secret keys in your keyring, it should offer to sign your new key with your old ones. Each time you add a new Mailpile account it should offer to add a new user ID to your primary key. Every time your key is changed it should upload your modified key to key servers.

Mailpile should keep a local database of metadata about PGP keys. For each public key it knows about it should record the timestamp from the first time you saw it in your email. Each key should also have a trust settings: either “trusted”, “not trusted”, or “automatic”, completely separate from any data that’s stored in the actual PGP key packets (like user trust or signatures).

Every time you see a PGP-signed email, if the user doesn’t already have the PGP key that signed that email it should search key servers for it and download it. When you add a new account it should also search all of your old email for signatures and try downloading all of those public keys from key servers, too.

All emails should default to being signed, and as many emails as possible should be encrypted by default. When you write a new email and put email addresses in the To, CC, and BCC fields, if you have “trusted” PGP keys for all recipients, the email should default to being encrypted.

The question is: what does “trusted” mean? Since we want to rely on TOFU by default I think it should mean this:

  • If you have signed the key, it should be trusted.
  • If you have manually marked it as “trusted” in the Mailpile interface, it should be trusted.
  • If you have manually marked it as “untrusted” in the Mailpile interface, it should be untrusted.

If you haven’t signed it and you haven’t marked it as “trusted” or “untrusted”, it should use this algorthim (or something similar) to make an intelligent decision about whether this public key should be trustworthy enough to encrypt the message to:

  • If this is the only key that includes a user ID with the recipient’s email address on, it should be trusted.
  • Elif all other keys that include a user ID with the recipient’s email address have signed this key, it should be trusted.
  • Elif this is the first key you’ve seen that includes a user ID with the recipient’s email address on it, it should be trusted.
  • Otherwise, it should be untrusted.

What would this do?

With these defaults, a couple things will happen. You’ll automatically download the public keys of everyone else that you email that uses PGP. When you write emails, they’ll get opportunistically encrypted. If a bunch of different people who regularly email each other all use Mailpile, they don’t have to change any behavior at all, and all their email will automatically get encrypted.

If people try to attack this system by sending you emails that are signed with malicious keys pretending to be other people, you won’t trust those keys by default. You’ll still encrypt to the first key you saw.

If someone creates a new PGP key for their email address and you still have their old one, the first time you email them you’ll encrypt it to the wrong key (unless they’ve been good PGP users and signed their new key with their old one, in which case it will just work). However, you should be able to quickly work out the problem and manually mark the new key as “trusted”. From that point on it should just work.

You can use PGP completely safely, and have very high confidence that most of the public keys you use are valid, without having to participate in or even know what the Web of Trust is. If you do use the Web of Trust, all the keys you sign would be trusted like you expect.

So wait, does this exist?

Nope, it’s just an idea. But I think it could work pretty well.

If other people agree that it’s a good idea, I’d love it if the Mailpile developers decided to implement it as default behavior in Mailpile. If not, I’d love it if at least someone could build a Mailpile plugin that does this.