Sensitive data about U.S. submarine research has been hacked from an unnamed government contractor by Chinese hackers working for China’s Ministry of State Security or MSS.

Included in the loss were six hundred fourteen gigabytes of data which were stolen, including submarine communications data and information on a secretive project known as Sea Dragon.

The Washington Post, citing anonymous American officials, reports that the data was stored on the contractor’s unclassified network, though officials said the material could be considered classified and was highly sensitive. The compromised data could give China important edges in both technology and tactics at sea, including in its conflict with a half-dozen nations including Japan and Indonesia over territorial claims in the South China Sea.

Little is known about the classified Sea Dragon initiative, but it is an underwater technology that the Defense Department has described as introducing a “disruptive offensive capability,” according to the Post, and which some defense experts regard as a key component in America’s precautions for conflict with China. Submarine weapons are key for China’s military ambitions, but Chinese cyber-incursions in recent years have nabbed designs and data for everything from advanced fighter jets to missile systems.

Former Navy officer and NSA analyst John Schindler, writing for the Observer, highlights the loss of cryptographic information used in submarine communications. Those techniques allow coordination of forces at sea, and their compromise can lend an enemy a huge tactical advantage in any conflict. As a precedent, Schindler cites the Allied compromise of the German Enigma cipher, and the resulting demolition of Nazi U-Boats that hastened the end of World War II—though it’s important to note that the Germans didn’t know the code had been cracked, and the U.S. may be able to react to the recent hack by changing codes and practices to keep messages out of Chinese hands.

The Chinese said that they were not aware of any such operation. Not much is known on Sea Dragon, however, the Pentagon has asked for or spent more than $300 million dollars for the project since late 2015 and said it planned to begin underwater testing by September.

Whether or not this latest hack sets that timetable back remains to be seen.

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