' FB 1.05.0 Win64

' uses in place encoding/decoding
Sub rot13(ByRef s As String)
  If s = "" Then Exit Sub
  Dim code As Integer
  For i As Integer = 0 To Len(s) - 1
    Select Case As Const s[i]
      Case 65 To 90  '' A to Z
        code = s[i] + 13
        If code > 90 Then code -= 26
        s[i] = code
      Case 97 To 122 '' a to z
        code = s[i] + 13
        If code > 122 Then code -= 26
        s[i] = code
     End Select
  Next
End Sub

Dim s As String = "nowhere ABJURER"
Print "Before encoding : "; s
rot13(s)
Print "After encoding  : "; s
rot13(s)
Print "After decoding  : "; s
Print
Print "Press any key to quit"
Sleep