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Keep your iPhone calls private, whispers Signal

Marlinspike's voice crypto comes to iOS

The crew at Open Whisper Systems has announced Signal, an app offering encrypted voice calls between iPhones.

The open source group has been working on its projects ever since Whisper Systems, co-founded by Moxie Marlinspike, was acquired by Twitter in 2011 – an acquisition that took its Redphone Android project offline, amid criticism it was already being used by Egyptian dissidents.

In December of 2011, Redphone returned as an open source project and Open Whisper Systems was born.

After his post-acquisition stint with Twitter, Marlinspike joined back in the open source development effort, and its TextSecure SMS encryption project got baked into CyanogenMod in December 2013.

As with a number of encryption apps, Signal's focus is on usability to the ordinary user. As the organisation explains, it “uses your existing number, doesn’t require a password, and leverages privacy-preserving contact discovery to immediately display which of your contacts are reachable with Signal.”

At registration, Signal checks the user's address book to identify other Signal users, and its integrated with Redphone, allowing encrypted calls between iPhone and Android users.

Signal uses the ZRTP protocol (authored by Phil Zimmerman et al) for key exchange, with AES 128 bit encryption for the conversations. When two users are trying to call each other, the app shows a pair of confirmation words on the screen for the parties to read to each other, as confirmation of identity and man-in-the-middle protection.

Marlinspike's hope is that unlike Zimmerman's paid-for Silent Circle software, he'll be able to keep Redphone and Signal free. He told Wired that the project has received grants from from the Shuttleworth Foundation and the US government's Open Technology Fund, and Open Whisper Systems also accepts donations.

A chunk of that has to be spent on local servers, so that Signal (and Redphone) traffic doesn't have to trombone off to the US en route. ®

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