Signal is a new, open-source software application for the iPhone that was released this week. It allows iPhone users to easily make calls — over Wi-Fi and cellular data networks — that are end-to-end encrypted for complete privacy.
The app, which uses the popular open-source ZRTP protocol for voice communication, uses the users' cell phone number as a login with SMS text verification. There is no password or login required, and the app pulls contact information from the iPhone's own address book for a more seamless experience.
By default, only users who have already downloaded Signal will show up in the contacts list. However, placing a call to others will send a text message that prompts them to download the free app.
To assure that the call is secure, users are shown a pair of code words on their iPhone's screen at the start of a Signal call. If the words are the same to both users, it means a secure connection has been implemented. Were the conversation intercepted by a third-party, the words would not match.
Signal is able to make calls to other iPhones using this app, plus Android devices running RedPhone, a similar app for Android devices.
Currently, the app can only make voice calls, but the company says it's planning a secure text communication component later this year that will be compatible with TextSecure, a secure texting app on the Android platform.
Both TextSecure and RedPhone are made by Open Whisper Systems, the same company that makes Signal, and it said that both apps will be combined into a single Signal app on Android as well. There is also a browser extension being developed.
During testing, the voice quality of Signal was similar to a normal phone call, though with the added benefits (and negatives) of other VoIP calling services. All calls are free, even when made between international destinations.
For business customers looking to ensure that their calls are kept private, particularly when traveling overseas, Signal could be a great solution. The (relative) ease of setup, along with automatic and password-free logins make this a fairly seamless solution for the security-minded user.
Signal uses Apple's iOS push notification system to notify the user of incoming phone calls, which saves battery life and keeps the app from needing to run constantly in the background.
Of course, there is never certainty when working with allegedly secure communications platforms, but the open-source Signal is open to investigation by security specialists.
How do you make secure calls on your iOS device? Let us know in the discussion thread below.
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Jordan Golson is an Apple Columnist for TechRepublic. He also writes about technology and automobiles for WIRED and MacRumors. He has worked for Apple Retail twice and has been writing about technology since 2007.