100 INPUT ""; T$

110 LET K% = RND(1) * 25 + 1
120 PRINT "ENCODED WITH ";
130 GOSUB 200ENCODED

140 LET K% = 26 - K%
150 PRINT "DECODED WITH ";
160 GOSUB 200DECODED

170 END

REM ENCODED/DECODED
200 PRINT "CAESAR " K%;
210 LET K$(1) = " (ROT-13)"
220 PRINT K$(K% = 13)
230 GOSUB 300CAESAR
240 PRINT T$
250 RETURN

REM CAESAR T$ K%
300 FOR I = 1 TO LEN(T$)
310    LET C$ = MID$(T$, I, 1)
320    GOSUB 400ENCODE
330    LET L = I - 1
340    LET T$(0) = MID$(T$, 1, L)
350    LET L = I + 1
360    LET T$ = C$ + MID$(T$, L)
370    LET T$ = T$(0) + T$
380 NEXT I
390 RETURN

REM ENCODE C$ K%
400 LET C = ASC(C$)
410 LET L = (C > 95) * 32
420 LET C = C - L
430 IF C < 65 THEN RETURN
440 IF C > 90 THEN RETURN
450 LET C = C + K%
460 IF C > 90 THEN C = C - 26
470 LET C$ = CHR$(C + L)
480 RETURN