There’s a lot of misinformation about how SIGINT unmasking works
Here we go again. The latest twist in President Donald Trump’s never-ending allegation that the Obama White House was spying on Team Trump before inauguration day involves an arcane, highly classified issue about Americans who are mentioned in signals intelligence intercepts.
Trump and his supporters seek to paint any interception of American phone calls—or even discussions about Americans by foreigners—as improper and maybe illegal. That’s not true. Every day, the National Security Agency intercepts lots of calls between foreigners in which Americans are discussed. If they’re important Americans—top politicians, for instance—that intercept may have intelligence value. If it doesn’t, the intercept is deleted and forgotten.
More rarely, the NSA intercepts phone calls in which one of the interlocutors is an American. As long as this operation has been approved per the Foreign Intelligence Surveillance Act—meaning a top-secret Federal court has issued a warrant for this collection—this is perfectly legal SIGINT. Here, too, an intelligence report will be issued in top-secret channels if the NSA determines there’s foreign intelligence value here and somebody, usually the FBI, needs to know what the intercept reveals.
In all cases, the identity of the American or Americans discussed is masked in the top-secret reports issued by NSA. They are referred to as “US Person” or USP for short; if there’s more than one of them, a number is added. Such SIGINT reports look like this fictional excerpt:
Read the rest at The Observer …