'
' Caesar cypher
'
OPENW 1 ! Creates a window for handling input/output
CLEARW 1
INPUT "string to encrypt ";text$
INPUT "encryption key    ";key%
encrypted$=@encrypt$(UPPER$(text$),key%)
PRINT "Encrypted: ";encrypted$
PRINT "Decrypted: ";@decrypt$(encrypted$,key%)
'
PRINT "(Press any key to end program.)"
~INP(2)
CLOSEW 1
'
FUNCTION encrypt$(text$,key%)
  LOCAL result$,i%,c%
  result$=""
  FOR i%=1 TO LEN(text$)
    c%=ASC(MID$(text$,i%))
    IF c%<ASC("A") OR c%>ASC("Z") ! don't encrypt non A-Z
      result$=result$+CHR$(c%)
    ELSE
      c%=c%+key%
      IF c%>ASC("Z")
        c%=c%-26
      ENDIF
      result$=result$+CHR$(c%)
    ENDIF
  NEXT i%
  RETURN result$
ENDFUNC
'
FUNCTION decrypt$(text$,key%)
  RETURN @encrypt$(text$,26-key%)
ENDFUNC