Whether investigating charges of torture by the C.I.A., rolling up an organized crime network or prosecuting crooked government officials, John H. Durham, the veteran federal prosecutor named by Attorney General William P. Barr to investigate the origins of the Russia inquiry, burnished his reputation for impartiality over the years by keeping his mouth closed about his work. At the height of the Boston mob prosecution that made his name, he not only rebuffed a local newspaper’s interview request, but he also told his office not to release his résumé or photo. That wall of silence cracked this month when Mr....