Rule 31

(A) As used in this rule, 'official information' means information acquired in confidence by a public officer or employee in the course of his duty and not open or officially disclosed to the public prior to the time the claim of privilege is made.(B) A witness has a privilege to refuse to disclose any matter which constitutes official information, and evidence of such matter is not admissible if the court determines that said matter is official information and disclosure of it is forbidden by law, or that disclosure of such information in the proceeding would be prejudicial to the interest of the government which the witness represents.