A San Francisco startup specializing in security for smartphones and tablets powered by Android software said it had been bought by Twitter.
Whisper Systems did not disclose the terms of the deal.
"We started Whisper Systems with the goal of improving security and privacy for mobile devices," the startup said in a blog post at its website.
"Now that we're joining Twitter, we're looking forward to bringing our technology and our expertise into Twitter's products and services," it added.
Whisper Systems products include encryption, software firewalls, and secure text messaging for mobile devices running on Google's Android platform.