If Stefan Halper is the “secret” informant, then I don’t know why they don’t just admit it. He’s not a secret. There is no national security concern in keeping his name out of it.
Because Valerie Plame. She was no secret either. No one wants to get the Scooter Libby treatment. And they don't even need to assign a Special Council, as Mueller is already available for investigations of unlimited scope...