"I certainly can't say that they are clearly impermissible, but I can say that I think that there is at least a Constitutional cloud over them," Harvard Law Professor Laurence Tribe said.
For one thing, they would be difficult to authenticate.
"There would be no way to prove it was issued on a certain date in an official capacity," Tribe said. "If invoked at the time an indictment or prosecution is brought, that would open the possibility for a Constitutional test of whether secret pardons are permitted."