IDENTIFICATION DIVISION.
FUNCTION-ID. rot-13.

DATA DIVISION.
LOCAL-STORAGE SECTION.
01  STR-LENGTH   CONSTANT AS 100.
01  normal-lower CONSTANT AS "abcdefghijklmnopqrstuvwxyz".
01  rot13-lower  CONSTANT AS "nopqrstuvwxyzabcdefghijklm".
01  normal-upper CONSTANT AS "ABCDEFGHIJKLMNOPQRSTUVWXYZ".
01  rot13-upper  CONSTANT AS "NOPQRSTUVWXYZABCDEFGHIJKLM".
LINKAGE SECTION.
77  in-str       PICTURE IS X(STR-LENGTH).
77  out-str      PICTURE IS X(STR-LENGTH).

PROCEDURE DIVISION USING in-str, RETURNING out-str.
    MOVE in-str TO out-str
    INSPECT out-str CONVERTING normal-lower TO rot13-lower
    INSPECT out-str CONVERTING normal-upper TO rot13-upper
    GOBACK.

END FUNCTION rot-13.