Twitter has acquired Whisper Systems, a tiny security start-up that makes software for Android smartphones and other mobile devices. The price for the deal was not disclosed.

Whisper's co-founders are highly respected online security experts–likely a part of the company's appeal for Twitter, which does not have the best reputation for security, Wired observed. One of the Whisper's co-founders, a developer and hacker who goes by the name Moxie Marlinspike, is an expert in SSL (secure sockets layer) encryption and Twitter "could use his skills to lock down its services and make life harder for phishers."

"The Whisper Systems team is joining Twitter starting today. 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 [Anderson] onboard," Twitter said in an e-mail statement.

Whisper's products will "live on," the San Francisco start-up said in a blog post, but they temporarily will be unavailable while they're integrated with Twitter. Among the start-up's offerings: RedPhone, which provides call encryption between Android users; Flashback, which provides encrypted backup for Android; and TextSecure, which offers dual-encrypted SMS messaging.

In March, after a Federal Trade Commission investigation into two security breaches, Twitter finalized a settlement over charges that the micro-blogging service failed to safeguard user information adequately. (The company agreed to establish a privacy program.) In September, hackers accessed NBC News' Twitter account and posted a message saying terrorists had launched an attack at Ground Zero.

According to Twitter, its iPhone app is the third most popular way to access the site, after Twitter.com and mobile.twitter.com.

In an interview earlier this year, Marlinspike observed that attacks on phones are getting easier and cheaper. Once upon a time it cost $750,000 to attack a cellular network, but now an attack can be done with hardware that costs less than $20. "Right now, we are kind of at the point with cellular wireless where we were with [Wi-Fi] 10 years ago," Marlinspike told Technology Review

Whisper blogged of the Twitter deal: "We started Whisper Systems with the goal of improving security and privacy for mobile devices. We were attracted to this not only because we saw it as an opportunity to reinvent the security solutions that never really worked in the PC environment to begin with, but also because the stakes are much higher—due to the nature of mobile devices themselves—and we didn't like the way that things were looking."

In September, Twitter acquired social information analysis start-up Julpan for an undisclosed sum. (With the purchase, it got the talent of Ori Allon, aka the Guy Who Helped Perfect Google Search, as TechCrunch called him.)

Other Twitter purchases this year include TweetDeck (bought for more than $40 million) and social analytics start-up Backtype (price undisclosed.)