100 PROGRAM "CaesarCi.bas"
110 STRING M$*254
120 INPUT PROMPT "String: ":M$
130 DO
140   INPUT PROMPT "Key (1-25): ":KEY
150 LOOP UNTIL KEY>0 AND KEY<26
160 PRINT "Original message:  ";M$
170 CALL ENCRYPT(M$,KEY)
180 PRINT "Encrypted message: ";M$
190 CALL DECRYPT(M$,KEY)
200 PRINT "Decrypted message: ";M$
210 DEF ENCRYPT(REF M$,KEY)
220   STRING T$*254
230   LET T$=""
240   FOR I=1 TO LEN(M$)
250     SELECT CASE M$(I)
260     CASE "A" TO "Z"
270       LET T$=T$&CHR$(65+MOD(ORD(M$(I))-65+KEY,26))
280     CASE "a" TO "z"
290       LET T$=T$&CHR$(97+MOD(ORD(M$(I))-97+KEY,26))
300     CASE ELSE
310       LET T$=T$&M$(I)
320     END SELECT
330   NEXT
340   LET M$=T$
350 END DEF
360 DEF DECRYPT(REF M$,KEY)
370   STRING T$*254
380   LET T$=""
390   FOR I=1 TO LEN(M$)
400     SELECT CASE M$(I)
410     CASE "A" TO "Z"
420       LET T$=T$&CHR$(65+MOD(ORD(M$(I))-39-KEY,26))
430     CASE "a" TO "z"
440       LET T$=T$&CHR$(97+MOD(ORD(M$(I))-71-KEY,26))
450     CASE ELSE
460       LET T$=T$&M$(I)
470     END SELECT
480   NEXT
490   LET M$=T$
500 END DEF