Introducing ProtonMail Bridge, email encryption for Outlook, Thunderbird, and Apple Mail

ProtonMail is officially launching ProtonMail Bridge, which brings easy-to-use email encryption to desktop email clients.

Ever since the day that we first got the idea to create ProtonMail, one of the most enduring challenges has been how to do email security right while simultaneously making encrypted email easy enough to use for normal people. Since our early days working from the CERN cafeteria, we have been working tirelessly to address this specific problem.

In the years since, we have made many great strides towards creating usable encrypted email, first with ProtonMail’s webmail interface and then with our award-winning iOS and Android secure email apps. However, one of our goals has always been to bring easy-to-use encrypted email to desktop. The problem is formidable. Desktop systems encompass multiple operating systems with dozens of popular email clients with their own adherents, and virtually none of them natively speak PGP, the email encryption standard upon which ProtonMail is built.

Around two years ago, we created a small task force to tackle this challenge. Today, we are finally ready to present ProtonMail Bridge.

What is the ProtonMail IMAP/SMTP Bridge?

In a single sentence, ProtonMail Bridge is an application that allows you to use your ProtonMail encrypted email account with your favorite desktop email client such as Thunderbird, Apple Mail, or Outlook, while simultaneously retaining the zero-access encryption and end-to-end encryption that ProtonMail provides. The best part is that this does not require modifying your email client or making changes to your existing workflow. Use email like you have always used it, and the Bridge will automatically encrypt and decrypt messages in the background.

How does the ProtonMail Bridge work?

The Bridge is an app that you download and install locally on your desktop or laptop computer and it runs automatically in the background.

The Bridge essentially acts like a local email server (using the IMAP and SMTP protocols) and interacts with email clients also installed locally on your desktop computer. As a result, all encryption and decryption occur locally and thus the benefits of end-to-end encryption remain. The Bridge communicates with ProtonMail’s encrypted email server via our API, which supports end-to-end encryption, while email clients can communicate directly with the Bridge via standard IMAP and SMTP. In this way, standard email clients which do not natively support end-to-end encryption can support encryption without modification. Another way to think of it is that the ProtonMail Bridge translates end-to-end encrypted email data into a language that any email client can understand, thus “bridging” the gap between ProtonMail’s end-to-end encryption and your standard email client.

Full-text search, multiple accounts, import/export

One of the powerful benefits of using the Bridge and email apps like Thunderbird, AppleMail, and Outlook is being able to use full-body text search within your encrypted emails. The Bridge decrypts messages as they arrive in your computer and delivers them to your desktop email client. These local copies are stored on your computer, so the search features of your desktop client work normally and you can search within your encrypted emails.

Another powerful benefit of the Bridge is being able to have multiple accounts added to an email client. For example, many users will have both a Gmail account and a ProtonMail account. In this scenario, you could simply drag messages between accounts using Thunderbird (for example). This essentially enables you to drag and drop an existing Gmail account into a new ProtonMail account as a way of doing “Account Import” (a dedicated account import and export tool is currently under development). Similarly, for users who want a backup of their ProtonMail data, most native email clients let you mass export your data and download it. You can also have multiple ProtonMail addresses and accounts in a single email client, and move messages between your ProtonMail accounts.

Threat Model

The Bridge preserves end-to-end email encryption, and also zero-access encryption (meaning that even we cannot read your emails). However, the Bridge does not protect your emails from end-point compromise (e.g. compromised laptop). Since the Bridge decrypts data locally, it’s important to ensure that your computer is safe. If someone breaks into your computer while using the Bridge, the unencrypted data could potentially be viewed as well.

During the installation process, the Bridge will auto-generate a “Bridge Password”. This Bridge Password is used to setup and configure your email clients. In this way you don’t need to trust your email client with your secret ProtonMail password.

JavaScript Cryptography and Open Source

Because the Bridge is locally installed, it is like our mobile apps in that it does not do decryption in a browser. Therefore, the Bridge also guards against the threat vector of somebody compromising the connection between you and ProtonMail in order to send you bad JavaScript code, or ProtonMail getting compromised and serving a malicious webpage to users.

Furthermore, after the technical documentation of the ProtonMail Bridge code is done, we will be releasing the source code of the Bridge, so that you can even compile it yourself instead of getting the binaries from us, so there is even less need to trust us. This is an important step in our work to eliminate ProtonMail itself as a threat vector.

Using the ProtonMail Bridge

The Bridge software is easy to set up and use. The setup process consists of:

1. Installing the Bridge app
2. Adding your ProtonMail account to the Bridge
3. Adding your ProtonMail account to your email client (Thunderbird, Apple Mail, Outlook)
4. Configuring your email client’s settings (ports, password, etc).

Currently, the officially supported email clients are Thunderbird, Apple Mail, and Outlook, on both Windows and MacOS (Linux is coming in Spring of 2018). However, in theory, any IMAP email client can work with the Bridge, and in our beta testing, many were shown to work. If you are a paid ProtonMail user, you can immediately get started here:

Finally, we would like to thank the thousands of ProtonMail users who participated in the Bridge Beta over the past year. Your support and feedback was invaluable towards bringing the Bridge to fruition, and we look forward to making ProtonMail even better for the community.

About the Author: The ProtonMail Team

You can read the ProtonMail Bridge press release here.
ProtonMail’s media kit can be found here.
ProtonMail Bridge images can be found here.

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Pierluigi Paganini

(Security Affairs – ProtonMail, privacy)

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