Regardless of what the government knows and how it knows it, the decision of whether to make attribution evidence public is another matter. When Sony was attacked, many security experts—myself included—were skeptical of both the government’s attribution claims and the flimsy evidence associated with it. I only became convinced when The New York Times ran a story about the government’s attribution, which talked about both secret evidence inside the NSA and human intelligence assets inside North Korea. In contrast, when the Office of Personnel Management was breached in 2015, the US government decided not to accuse China publicly, either because it didn’t want to escalate the political situation or because it didn’t want to reveal any secret evidence.
It’s one thing for the government to know who attacked it. It’s quite another for it to convince the public who attacked it. As attribution increasingly relies on secret evidence—as it did with North Korea’s attack of Sony in 2014 and almost certainly does regarding Russia and the previous election—the government is going to have to face the choice of making previously secret evidence public and burning sources and methods, or keeping it secret and facing perfectly reasonable skepticism.