((a)) In any case in which the United States or an officer or agency of the United States is a party, the Attorney General shall pay all fees of witnesses on the certificate of the United States attorney or assistant United States attorney, and in the proceedings before a United States magistrate judge, on the certificate of such magistrate judge, except that any fees of defense witnesses, other than experts, appearing pursuant to subpoenas issued upon approval of the court, shall be paid by the Attorney General—

((1)) on the certificate of a Federal public defender or assistant Federal public defender, in a criminal case in which the defendant is represented by such Federal public defender or assistant Federal public defender, and

((2)) on the certificate of the clerk of the court upon the affidavit of such witnesses’ attendance given by other counsel appointed pursuant to , in a criminal case in which a defendant is represented by such other counsel.

((b)) In proceedings in forma pauperis for a writ of habeas corpus, and in proceedings in forma pauperis under , the Attorney General shall pay, on the certificate of the district judge, all fees of witnesses for the party authorized to proceed in forma pauperis, except that any fees of witnesses for such party, other than experts, appearing pursuant to subpoenas issued upon approval of the court, shall be paid by the Attorney General—

((1)) on the certificate of a Federal public defender or assistant Federal public defender, in any such proceedings in which a party is represented by such Federal public defender or assistant Federal public defender, and

((2)) on the certificate of the clerk of the court upon the affidavit of such witnesses’ attendance given by other counsel appointed pursuant to , in any such proceedings in which a party is represented by such other counsel.

((c)) Fees and mileage need not be tendered to a witness upon service of a subpoena issued on behalf of the United States or an officer or agency of the United States, upon service of a subpoena issued on behalf of a defendant represented by a Federal public defender, assistant Federal public defender, or other attorney appointed pursuant to , or upon service of a subpoena issued on behalf of a party authorized to proceed in forma pauperis, if the payment of such fees and mileage is to be made by the Attorney General under this section.