Twitter has acquired a start-up mobile security firm in a bid to tighten up the security of its micro-blogging service over mobile devices.

The company confirmed in a statement it has acquired Whisper Systems, a start-up created last year by security experts Moxie Marlinspike and Stuart Anderson, for an undisclosed sum.

"The Whisper Systems team is joining Twitter starting today," Twitter said in an emailed statement to Reuters on Monday. "As part of our fast-growing engineering team, they will be bringing their technology and security expertise to Twitter's products and services. We're happy to have Moxie and Stuart onboard."

On its website, Whisper describes itself as providing “security and management solutions that transform consumer phones and tablets into enterprise-ready devices.” The firm offers solutions that scramble data, voice conversations and text messages on mobile devices running the Android OS, which is seen as the main target for mobile hackers.

Twitter now boasts over 100 million active users but has had its security credentials called into question in some cases. According to Reuters, some privacy advocates have warned that certain governments have used the service to help monitor dissidents and activists.