plaintext$ = "Pack my box with five dozen liquor jugs"
      PRINT plaintext$

      key% = RND(25)
      cyphertext$ = FNcaesar(plaintext$, key%)
      PRINT cyphertext$

      decyphered$ = FNcaesar(cyphertext$, 26-key%)
      PRINT decyphered$

      END

      DEF FNcaesar(text$, key%)
      LOCAL I%, C%
      FOR I% = 1 TO LEN(text$)
        C% = ASC(MID$(text$,I%))
        IF (C% AND &1F) >= 1 AND (C% AND &1F) <= 26 THEN
          C% = (C% AND &E0) OR (((C% AND &1F) + key% - 1) MOD 26 + 1)
          MID$(text$, I%, 1) = CHR$(C%)
        ENDIF
      NEXT
      = text$