100 PROGRAM "Rot13.bas"
110 DO
120   LINE INPUT PROMPT "Line: ":LINE$
130   PRINT ROT13$(LINE$)
140 LOOP UNTIL LINE$=""
150 DEF ROT13$(TEXT$)
160   LET RESULT$=""
170   FOR I=1 TO LEN(TEXT$)
180     LET CH$=TEXT$(I)
190     SELECT CASE CH$
200     CASE "A" TO "M","a" TO "m"
210       LET CH$=CHR$(ORD(CH$)+13)
220     CASE "N" TO "Z","n" TO "z"
230       LET CH$=CHR$(ORD(CH$)-13)
240     CASE ELSE
250     END SELECT
260     LET RESULT$=RESULT$&CH$
270   NEXT
280   LET ROT13$=RESULT$
290 END DEF