Copfag here. To all the concernfags saying that Cohen plead guilty so he can't introduce evidence…
If part of his plea deal involves him providing evidence then he's required to do so or the deal is off.
I do this kind of thing with drug operations all the time. They sign up with me to be an informant and two people hold copies of that agreement. The prosecutor and myself. They get plea deal (or dismissal of misdemeanor charges) as long as they hold up their end of the deal. They fuck me and I shred the paperwork then prosecute as normal.
The informant agreement only becomes public if whatever case the informant gets me (dope dealer, stolen property, guns, murderer, etc) goes to trial because of the suspect's right to confront accuser. If they plea out, my informant's identity remains private because the dope dealer never went to trial and subpoenaed the informant.
So it's entirely possible that stuff has been introduced due to a stipulation in Cohen's plea deal.