Why is there a trade-off in WhatsApp between security and usability, what is the Signal protocol, what does it mean for my privacy and are there any alternatives?

The design of WhatsApp trades off some security for usability, a decision unlikely to affect most users. But people whose threat models require very secure messaging should examine the trade-off carefully.

What is WhatsApp?

WhatsApp is a free messaging and calling service that uses the internet to deliver communications. It is used by over 1 billion people across the world including in countries with oppressive regimes. It was bought by Facebook in 2014 for $22bn (£18bn) and implemented end-to-end encryption in April 2016.

What is end-to-end encryption?

End-to-end encryption (E2EE) ensures that a conversation can only be read by the sender and recipient and not intercepted by a middleman. WhatsApp uses the acclaimed Signal encryption protocol that relies on the exchanging of unique security keys that are verified between users to guarantee communications cannot be intercepted by a middleman.



Anything said between users with E2EE is guaranteed to be private during transit, unless there’s a vulnerability in the implementation of that encryption.

What has WhatsApp done?

The Signal encryption protocol has no known weaknesses if it is implemented correctly. WhatsApp’s implementation generates new encryption keys for offline users who come back online with a new device. Messages that were in transit to them when they were offline get re-encrypted and resent automatically to the recipient. The sender has no opportunity to stop the messages being resent, and learns of the key change only if the sender has the notification activated in their settings.



Why is it like it is?

WhatsApp says it used this implementation of the Signal protocol to aid usability. If it is not in place, messages sent to an offline user, who then changes their smartphone or has to re-install WhatsApp and in doing so generates new security keys for themselves, would remain undelivered once the user comes back online.

A WhatsApp spokesperson told the Guardian: “In many parts of the world, people frequently change devices and Sim cards. In these situations, we want to make sure people’s messages are delivered, not lost in transit.”

How dangerous is this?

Reassuringly, security experts say the trade-off cannot be exploited for the mass surveillance of WhatsApp users – individuals need to be targeted – and because it can only be exploited to access messages that have yet to be marked as delivered it probably cannot be used to access messaging histories that have been confirmed as delivered.

However, for people choosing a service which offers end-to-end encryption in order to ensure that their messages are secure, some security experts have warned that the trade-off could pose a risk. Undelivered messages could be accessed, including in situations that do not require WhatsApp’s involvement: for example, if an activist is arrested, the app does not prevent messages sent to the activist, but not yet delivered, from being read by anyone who puts the Sim card into a different phone. Even if both the users had turned on WhatsApp’s strongest security settings, the sender would not be notified that the swap had occurred until after their messages were re-sent. The basic principles of this attack are easily demonstrated.

The consensus among experts is that, as WhatsApp is presently understood - and remembering that Facebook’s servers are a blackbox to outsiders, so trust is required - systematic targeted surveillance would be very difficult. Challenges involving timing, concealment and targeting would be formidable, they say, even for a major private or public actor with access to Facebook’s servers. Electronic Frontiers Foundation has analysed the issue. Risk to the security of undelivered messages of activists if, say, their phones were confiscated and a key change were orchestrated, was greatly reduced by the roll-out of two-step verification.

What are the alternatives?

There are several alternatives to WhatsApp that use encryption to secure communications against interception. The most recommended for its focus on security is Signal, which was developed by Open Whisper Systems, is the namesake of the Signal E2EE protocol, and is used by the NSA whistleblower Edward Snowden. However some security experts say it is less user friendly than WhatsApp and highlight that using it could mark a user out as a person of interest to authorities. Other messaging services such as Apple’s iMessage have end-to-end encryption but do not implement fingerprinting – so users have no way of verifying encryption keys themselves. Facebook’s Messenger and Google’s Allo don’t have end-to-end encryption by default, though forms of it can be enabled. Other older methods of messaging such as SMS don’t have any encryption at all.

Should I stop using WhatsApp?

There is no easy answer. All decisions around security involve trade offs, particularly between usability and confidentiality. For people worried about mass surveillance or the monitoring of messaging for advertising or commercial purposes, the vulnerability in WhatsApps implementation may not be too concerning. For individuals for whom an individual message could put them in danger, the vulnerability could be critical, so seek advice. The EFF has expressed reservations about WhatsApp and warned users to “take extra caution when deciding whether and when to communicate using WhatsApp”.

• This article was amended on 17, 20 and 25 January 2017 to correct and clarify, and on 28 June 2017 following a review and amendment to the news article which prompted this article.