SCREEN 0, 0
COLOR 14, 1
CLS
LOCATE 3, 10
PRINT "This program will decode (or encode) a word or expression in ROT13"
LOCATE 4, 10
PRINT "ROT13 simply adds or subtracts 13 from rank of letter to be encoded"
LOCATE 5, 10
PRINT "or decoded."
repeat:
LOCATE 7, 10
PRINT "Please enter word or sentence to be decoded (or decoded):"
LOCATE 8, 10
LINE INPUT word$
rot$ = ""
FOR x = 1 TO LEN(word$)
    letter% = ASC(MID$(word$, x, 1))
    SELECT CASE letter%
        CASE IS > 96
            IF letter% < 123 THEN
                cletter% = letter% + 13
                IF cletter% > 122 THEN
                    cletter% = cletter% - 26
                END IF
            ELSE
                cletter% = letter%
            END IF
        CASE IS > 64
            IF letter% < 91 THEN
                cletter% = letter% + 13
                IF cletter% > 90 THEN
                    cletter% = cletter% - 26
                END IF
            ELSE
                cletter% = letter%
            END IF
        CASE ELSE
            cletter% = letter%
    END SELECT
    letter% = cletter%
    rot$ = rot$ + CHR$(letter%)
NEXT
LOCATE 10, 10
PRINT "The decoded or encoded word is: "
LOCATE 11, 10
PRINT rot$
LOCATE 13, 10
PRINT "Decode or encode again?"
DO
    yn$ = INKEY$
LOOP WHILE yn$ <> "y" AND yn$ <> "Y" AND yn$ <> "n" AND yn$ <> "N"
IF yn$ = "y" OR yn$ = "Y" THEN
    CLS
    GOTO repeat
ELSE
    END
END IF