During a recent cabinet meeting, President Donald Trump’s then national security adviser, Mike Waltz, must have been bored. Apparently unaware of the photographer behind him, he was caught clandestinely checking his Signal messages under the table.Only he wasn’t using the official Signal app, which is widely considered to be the gold standard of encrypted messaging apps. He was actually using a clone of Signal called TeleMessage Signal, or TM SGNL. This app, made by TeleMessage (which was recent...