Must involve national security matters.
One thing is certain, any person being prosecuted has a right to discovery of even classified information if it would aid his defense. If the government doesn’t want to disclose it, they have to drop the charges.
Anything threatening Mueller's destruction of Trump is a "national security matter"
Under CIPA, the government can offer unclassified summaries or substitutes for classified information at discovery. They can only do this with permission of the court and if the “defendant has substantially the same ability to make his defense.”
https://www.law.cornell.edu/uscode/html/uscode18a/usc_sec_18a_02000004——000-.html