By Joseph Menn

SAN FRANCISCO (Reuters) - Russian government-backed hackers who penetrated high-profile U.S. government and defense industry computers this year used a method combining Twitter with data hidden in seemingly benign photographs, according to experts studying the campaign.

In a public report Wednesday, researchers at security company FireEye Inc <FEYE.O> said the group used the unusual tandem as a means of communicating with previously infected computers. FireEye has briefed law enforcement on what it found.

The technique, uncovered during a FireEye investigation at an unnamed victim organization, shows how government-backed hackers can shift tactics on the fly after they are discovered.

“It’s striking how many layers of obfuscation that the group adopts,” said FireEye Strategic Analysis Manager Jennifer Weedon. “These groups are innovating and becoming more creative.”

The machines were given an algorithm for checking a different Twitter account every day. If a human agent registered that account and tweeted a certain message, instructions for a series of actions by the computer would be activated.

The tweeted information included a website address, a number and a handful of letters. The computer would go to the website and look for a photo of at least the size indicated by the number, while the letters were part of a key for decoding the instructions in a message hidden within the data used to display the picture on the website.

Weedon said the communication method might have been a failsafe in case other channels were discovered and cut. Vikram Thakur, a senior manager at Symantec Corp <SYMC.O>, said his team had also found Twitter controls combined with hidden data in photos, a technique known as steganography.

FireEye identified the campaign as the work of a group it has been internally calling APT29, for advanced persistent threat. In April, it said another Russian-government supported group, APT28, had used a previously unknown flaws in Adobe Systems Inc.’s Flash software to infect high-value targets.

Other security firms use different names for the same or allied groups. Symantec recently reported another data-stealing tool used in tandem with the steganography, which it calls Seaduke. Thakur said both tools were employed by the group it knows as the Duke family.

Thakur said another tool in that kit is CozyDuke, which Russian firm Kaspersky Lab says is associated with recent breaches at the State Department and the White House.

(Reporting by Joseph Menn; Editing by Cynthia Osterman)