After consulting Post lawyers, I developed what we felt was the safest possible sourcing system. If I planned to use someone in a story, I asked them to send me a picture of their government ID, then tried to forget it. I kept notes from reporting conversations in an encrypted drive, never writing down anyone's name. To Google-check facts and identities, I used a private browser with no search history. I retitled every Signal chat by agency - "Transportation Employee," "FDA Reviewer," "EPA Scientist" -