D'oh! Basic flaw in WhatsApp could allow attackers to decrypt messages

WhatsApp uses the same RC4 encryption key for incoming and outgoing streams, a Dutch developer said

The popular mobile messaging application WhatsApp Messenger has a major design flaw in its cryptographic implementation that could allow attackers to decrypt intercepted messages, according to a Dutch developer.

The problem is that the same key is used to encrypt both outgoing and incoming streams between the client and the WhatsApp server, said Thijs Alkemade, a computer science and mathematics student at Utrecht University in the Netherlands and lead developer of the open-source Adium instant messaging client for Mac OS X.

"RC4 is a PRNG [pseudo-random number generator] that generates a stream of bytes, which are xored [a crypto operation] with the plaintext that is to be encrypted. By xoring the ciphertext with the same stream, the plaintext is recovered," Alkemade said Tuesday in a blog post that describes the issue in detail.

Because of this, if two messages are encrypted with the same key and an attacker can intercept them, like on an open wireless network, he can analyze them to cancel out the key and eventually recover the original plaintext information.

Reusing the key in this manner is a basic crypto implementation error that the WhatsApp developers should have been aware of, Alkemade said Wednesday. It's a mistake made by the Soviets in the 1950s and by Microsoft in its VPN software in 1995, he said.

Alkemade released proof-of-concept exploit code for the vulnerability, but initially tested it on the WhatsPoke open-source library, not on the official WhatsApp client. Since then he has confirmed that the issue exists in the WhatsApp clients for Nokia Series 40 and Android devices.

"I don't think the situation will be different with the iOS client," he said.

WhatsApp also uses the same RC4 encryption key for HMAC (hash-based message authentication code) operations to authenticate messages.

This allows an attacker to intercept a message sent by a user to the server and resend it back to the user as if it came from the WhatsApp server, but this is not something that can be easily exploited, Alkemade said.

The Dutch developer didn't attempt to contact WhatsApp before disclosing the issue publicly. "I thought that it's important for people to know that WhatsApp is not secure and I didn't expect them to fix it rapidly," he said.

WhatsApp did not immediately respond to a request for comment on Wednesday.

Fixing this doesn't require rethinking the entire encryption implementation, Alkemade said. If they add a method to generate different keys for encryption in both directions, as well as for message authentication, then the problem is solved, he said.

According to Alkemade, users for now should assume that anyone who can intercept their WhatsApp connections can also decrypt their messages and should consider their previous WhatsApp conversations compromised.

Until the issue is fixed the only thing that users can do to protect themselves is to stop using the application, Alkemade said.

Join the newsletter!

Or

Sign up to gain exclusive access to email subscriptions, event invitations, competitions, giveaways, and much more.

Membership is free, and your security and privacy remain protected. View our privacy policy before signing up.

Error: Please check your email address.

Tags privacymobilemobile securitymobile applicationsWhatsApp

Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Lucian Constantin

IDG News Service
Show Comments

Cool Tech

Toys for Boys

Family Friendly

Stocking Stuffer

SmartLens - Clip on Phone Camera Lens Set of 3

Learn more >

Christmas Gift Guide

Click for more ›

Brand Post

Most Popular Reviews

Latest Articles

Resources

PCW Evaluation Team

Aysha Strobbe

Microsoft Office 365/HP Spectre x360

Microsoft Office continues to make a student’s life that little bit easier by offering reliable, easy to use, time-saving functionality, while continuing to develop new features that further enhance what is already a formidable collection of applications

Michael Hargreaves

Microsoft Office 365/Dell XPS 15 2-in-1

I’d recommend a Dell XPS 15 2-in-1 and the new Windows 10 to anyone who needs to get serious work done (before you kick back on your couch with your favourite Netflix show.)

Maryellen Rose George

Brother PT-P750W

It’s useful for office tasks as well as pragmatic labelling of equipment and storage – just don’t get too excited and label everything in sight!

Cathy Giles

Brother MFC-L8900CDW

The Brother MFC-L8900CDW is an absolute stand out. I struggle to fault it.

Luke Hill

MSI GT75 TITAN

I need power and lots of it. As a Front End Web developer anything less just won’t cut it which is why the MSI GT75 is an outstanding laptop for me. It’s a sleek and futuristic looking, high quality, beast that has a touch of sci-fi flare about it.

Emily Tyson

MSI GE63 Raider

If you’re looking to invest in your next work horse laptop for work or home use, you can’t go wrong with the MSI GE63.

Featured Content

Product Launch Showcase

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?