PROGRAM CAESAR

!$INCLUDE="PC.LIB"

PROCEDURE CAESAR(TEXT$,KY%->CY$)
    LOCAL I%,C%
    FOR I%=1 TO LEN(TEXT$) DO
      C%=ASC(MID$(TEXT$,I%))
      IF (C% AND $1F)>=1 AND (C% AND $1F)<=26 THEN
        C%=(C% AND $E0) OR (((C% AND $1F)+KY%-1) MOD 26+1)
        CHANGE(TEXT$,I%,CHR$(C%)->TEXT$)
      END IF
    END FOR
    CY$=TEXT$
END PROCEDURE

BEGIN
    RANDOMIZE(TIMER)
    PLAINTEXT$="Pack my box with five dozen liquor jugs"
    PRINT(PLAINTEXT$)

    KY%=1+INT(25*RND(1))  ! generates random between 1 and 25
    CAESAR(PLAINTEXT$,KY%->CYPHERTEXT$)
    PRINT(CYPHERTEXT$)

    CAESAR(CYPHERTEXT$,26-KY%->DECYPHERED$)
    PRINT(DECYPHERED$)

END PROGRAM