With a single encryption upgrade to Facebook’s WhatsApp, a billion users now find themselves confronted with a security technology whose reliability rests on the arcane-sounding end-to-end encryption (e2e) with perfect forward secrecy (PFS).

The firm’s official announcement describes it thus: “The idea is simple: when you send a message, the only person who can read it is the person or group chat that you send that message to. No one can see inside that message. Not cybercriminals. Not hackers. Not oppressive regimes.”

WhatsApp has been using this security in a more limited way since November 2014 when it adopted the Signal protocol from messaging encryption pioneers Open Whisper Systems. So what has changed? Essentially, it is now offering this security to every single user as a mandatory upgrade across all mobile platforms. WhatsApp is not the only firm offering this type of app security - alternatives are available, including directly from Open Whisper Systems itself.

Podcast discussion: WhatsApp's end-to-end encryption

WhatsApp’s end-to-end encryption

Until now, messages between users were sent via WhatsApp’s servers using proprietary technology, which applied and retained the encryption keys used to scramble data. The connection was secure as long as WhatsApp itself didn’t decide to peer at the encrypted content, possibly after being served a warrant to do so by a government or police force. The company could also to do this retrospectively on old messages and files as well as future ones.

The proprietary nature of the code meant that the outside world could not study what WhatsApp was doing for weaknesses or backdoors. Replacing this with the highly regarded open source Signal software from Open Whisper Systems means that the underlying technology is open to code review by anyone.

From now on, assuming communicating users have installed the latest version of WhatsApp (post March 31 – the app warns users to upgrade) private keys will be generated and stored on the user’s device and will no longer be accessible to WhatsApp. In addition, each message or session uses a different private key (called ‘perfect forward secrecy’) which means that no single key gives access to all the data sent by someone in the past or future.

The WhatsApp server does of course store a user’s public key, which is necessary to build a directory of users so that people can contact each other across the service. In PKI encryption, this public key is useless for accessing encrypted content and is merely a way for two users to communicate with one another without the risky need to send each other a private key, for instance a conventional asymmetric key such as a passcode.

None of the above requires WhatsApp users to configure anything – it’s just turned on by default and users who don’t upgrade to e2e security will get warning messages. This security can’t be turned off or downgraded by using an old version at a later stage.

Man-in-the-middle

WhatsApp offers a third security layer designed to stop the possibility of man-in-the -middle attacks in which someone impersonates the recipient of a message without the sender realising. This involves two people comparing a unique identifier, either by scanning a QR code or comparing a 60-digit number (which is not an encryption key, just an ID).

Ideally, the users need to be beside each other and the process has to be regenerated if the app is reinstalled. However, the number can be sent remotely using the ‘share’ button accessible from the app’s chat screen and it is also possible to be notified when a contact’s security code changes.

Conclusion

The integration of Signal across all of WhatsApp’s services marks an important moment for the mass use of secure encryption. At a single stroke, one billion people will start using the sort of security that keeps intelligence services and governments awake at night.

That doesn’t mean the implementation of Signal by WhatsApp might not have weaknesses or that a weakness couldn’t be found in Signal itself in the future. Seemingly secure encryption has had too many difficult moments in the past two years for anyone to make that brave a prediction.

But amidst a gradual erosion of privacy, there is no doubt that WhatsApp’s transition to end-to-end encryption will be remembered a significant tipping of the balance in the opposite direction.

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