Evidence is mounting that a mysterious hacking group claiming to have stolen data from a spying operation linked to the National Security Agency is telling the truth.

Security vendor Kaspersky Labs ZAO, which first identified the NSA-linked operation last year, said files released in the latest hack use an unusual mathematical approach it had seen in that operation’s code. Kaspersky dubbed the operation the Equation Group, and said it appeared to be supporting U.S. interests in cyberspace.

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