the information may be described by generic category, in such forms
   as the court may approve, rather than by identification of the
   specific information of concern to the United States.


http://intelligence.senate.gov/classifiedinformationprocedures.htm

[Classified Information Procedures Act;lie in court, use cover stories]

(b) Notice. - (1) Before any hearing is conducted pursuant to a

  request by the United States under subsection (a), the United
  States shall provide the defendant with notice of the classified
  information that is at issue.  Such notice shall identify the
  specific classified information at issue whenever that information
  previously has been made available to the defendant by the United
  States. When the United States has not previously made the
  information available to the defendant in connection with the case,
  the information may be described by generic category, in such forms
  as the court may approve, rather than by identification of the
  specific information of concern to the United States.