Security Experts:

iPhone App "Signal" Enables Free Encrypted Phone Calls

Open Whisper Systems, the creators of the RedPhone secure calling app for Android, announced on Tuesday the availability of Signal, an iPhone application that lets users make encrypted voice calls worldwide for free.

According to the open source software group, Signal is specifically created for mobile devices and it's fully compatible with RedPhone. The new application uses existing phone numbers and it's designed to display only contacts that area reachable through Signal.

Users don't need any passwords when utilizing the app. Instead, both the caller and the receiver are presented with a pair of words. If the words match on both ends, the connection is secure; if not, someone is possibly eavesdropping on the conversation. 

In order to ensure that communications are protected, Signal uses ZRTP, a protocol invented by Phil Zimmermann, who is also the creator of Pretty Good Privacy (PGP). In fact, Zimmermann also co-founded Silent Circle, a company providing encrypted communications services.

While Silent Circle and Open Whisper Systems develop similar solutions, the former's services are not free. Signal is not only free, but it's also open source, which allows anyone to review the code and contribute to making it better.

This summer, Open Whisper Systems plans on integrating support for text messages into the iPhone version of Signal, a feature that will be compatible with the TextSecure app for Android. In the future, TextSecure and RedPhone will be combined into a single Signal app for Android. The goal is to make Signal a unified secure communications platform for iPhone, Android and the browser, so the group is also working on an extension for Web browsers.

Open Whisper Systems was launched after Twitter acquired Whisper Systems, a company co-founded by security researcher Moxie Marlinspike, and decided to stop work on the RedPhone project. Both RedPhone and TextSecure were released as free and open source software shortly after the Twitter acquisition and their development continued under Open Whisper Systems.

Marlinspike, who has been actively involved in the project, told Wired that in addition to donations they've also received funding from the Open Technology Fund, which has also funded Tor and Cryptocat.

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.