Within a few hours of the Guardian report, the claim was debunked by other outlets, including this detailed piece in Gizmodo. While it is now clear that the backdoor is actually not a backdoor but a natural functionality of the messaging app, here’s what you need to know and what you can do to protect yourself from potential security mishaps.
What’s the deal?
WhatsApp’s secure messaging, used by dissidents and reporters across the world, uses the acclaimed Signal end-to-end encryption (E2EE) protocol, which encrypts sent messages in a way that can be exclusively undone by the recipient, ensuring that no one else will be able to decipher the messages.
However, WhatsApp has implemented a modification that can force a sender to re-encrypt an unsent message with a new key and resend it. This, according to researchers, is a backdoor that could practically enable a third party (such as a three-letter agency) to intercept and read messages without the recipient—and possibly the sender—becoming aware.
Why is it not a backdoor?
WhatsApp claims that the alleged backdoor is in fact a practical function meant to prevent undelivered messages from being lost in transit. Keys are bound to devices so when a user changes their device, or even reinstalls the app, the old keys become obsolete.
That’s why WhatsApp uses this feature to make sure that unsent messages are re-encrypted and delivered to the recipient with the new keys.
Open Whisper Systems, the company that has developed the Signal protocol, also dismissed Guardian’s claims and called WhatsApp’s handling of keys not a backdoor but “how cryptography works.”
The fact that the renegotiation is performed in a non-blocking manner makes it a bit controversial, but OWS justifies it as a reasonable sacrifice to keep the user experience seamless and also a possible block against man-in-the-middle attacks.
How easy is it to exploit?
According to Alec Muffett, a security researcher interviewed by Gizmodo on the issue, exploiting this feature for surveillance or eavesdropping purposes would require significant collaboration with WhatsApp, and this is something the company is unlikely to do (although Facebook, WhatsApp’s parent company, allegedly cooperated with the NSA in its mass-surveillance program PRISM).
As an advertising platform, Facebook does have a hunger for user data and content (as do other companies such as Google). But even The Guardian in a follow-up piece acknowledges that the burden to exploit the feature for profit and advertising purposes would be too high to be cost-effective.
Furthermore, tech-savvy users can discover any effort to misuse the feature, which could be bad for the company’s reputation.
What can you do to protect yourself?
WhatsApp has a security notification feature that can warn you if a recipient’s security keys have been modified when sending messages. This can warn you in case a potential exploit of the key renegotiation feature is in process.
To enable security notifications, take the following steps:
- Launch WhatsApp
- Go to Settings
- Open the Account menu and tap Security
- Enable the “Show security notifications” option
After enabling the feature, you’ll be warned whenever the recipient changes keys.
While this won’t prevent a malicious party from exploiting the feature, it’ll at least raise the alarm, after which you should contact the recipient through other means and confirm that they have taken an action that has caused the regeneration of keys.
I trust Open Whisper System’s word that WhatsApp is good to go. But since WhatsApp decided to betray user privacy once by sharing information with Facebook, I would recommend to err on the side of caution and enable security notifications to protect yourself from any possible exploits of the feature.
However, I would still recommend Signal as the most secure messaging app (by the way, Signal doesn’t automatically deliver unsent messages when keys are changed).
As for The Guardian, this is not the first time that the publication has posted viral stories of questionable integrity, so I would check twice before believing anything I read there.