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WhatsApp’s end-to-end encryption has a security vulnerability? Company says no

WhatsApp’s end-to-end encryption has a security flaw, which could allow backdoor entry and let third-parties read messages, claims a report in The Guardian. However, both WhatsApp and Open Whisper Systems, which has provided the end-to-end encryption protocol for the Facebook-owned app, have denied there’s any truth to this report.
WhatsApp later issued a statement, according to ArsTechnica, which reads, “WhatsApp does not give governments a “backdoor” into its systems and would fight any government request to create a backdoor. The design decision referenced in the Guardian story prevents millions of messages from being lost, and WhatsApp offers people security notifications to alert them to potential security risks. WhatsApp published a technical white paper on its encryption design and has been transparent about the government requests it receives, publishing data about those requests in the Facebook Government Requests Report.
So what is the Guardian report saying? The report is based on research done by Tobias Boelter, who is a cryptography and security researcher at the University of California, Berkeley. He claims he had alerted WhatsApp about the same in April 2016, but was told the company knew about the issue. Essentially according to Boelter and the Guardian report, the flaw lies in how WhatsApp deals with messages, which are sent when the receiver’s security code has changed.
For those who are familiar with WhatsApp’s end-to-end encryption, it generates a unique set of security keys, (there’s a QR code and a string of 60 numbers), and you can match it with a recipient in order to make sure this is working. However, when a user reinstalls WhatsApp, changes device, or switches SIM, the WhatApp security code changes. If you’ve activated notifications for this, you’ll get a message saying the security code has changed for a particular sender.
Now, according to Guardian, in case of WhatsApp messages that don’t get delivered because the security code has changed, the app doesn’t force users to match codes, but instead regenerates the encryption keys, and the re-encrypts the message with new keys and sends them.The  report claims, “This re-encryption and rebroadcasting effectively allows WhatsApp to intercept and read users’ messages,” and according to Boelter, WhatsApp can simply grant access to a government due to a change in keys.
The report points out that unlike WhatsApp, Signal which also uses the same protocol, but it doesn’t let undelivered messages go through if there’s a change in security keys of the recipient. Only when a user has verified the keys will the message be delivered.
However, both WhatsApp and Open Whisper Systems are critical of the the Guardian report. Open Whisper Systems’ Moxie Marlinspike, who is developer of the encryption protocol used by both Signal and WhatsApp, has written a detailed blogpost defending WhatsApp.
According to the blogpost, what Guardian is calling a design flaw which is the issue of key change, is in fact what ends up happening very often. The post notes, “The fact that WhatsApp handles key changes is not a “backdoor,” it is how cryptography works. Any attempt to intercept messages in transmit by the server is detectable by the sender, just like with Signal, PGP, or any other end-to-end encrypted communication system.”
The blogpost also notes even if WhatsApp tries to do a man in the middle (MITM) attack, there’s a risk that it will be caught especially by those users who are regularly verifying these keys. The post further explains,”The WhatsApp clients have been carefully designed so that they will not re-encrypt messages that have already been delivered. Once the sending client displays a “double check mark,” it can no longer be asked to re-send that message. This prevents anyone who compromises the server from being able to selectively target previously delivered messages for re-encryption.
It also says given WhatsApp’s user base, they are justified in going with what it calls a “non-blocking,” approach to deliver the message, as it gives a simple user experience. The blogpost also criticises the Guardian report for not reaching out to them for a comment given Open Whisper Systems created the encryption protocol used by WhatsApp.
Even though we are the creators of the encryption protocol supposedly “backdoored” by WhatsApp, we were not asked for comment. We believe that it is important to honestly and accurately evaluate the choices that organizations like WhatsApp or Facebook make. There are many things to criticize Facebook for; running a product that deployed end-to-end encryption by default for over a billion people is not one of them,” adds the blogpost.

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