' FB 1.05.0 Win64

Sub Encrypt(s As String, key As Integer)
  Dim c As Integer
  For i As Integer = 0 To Len(s)
    Select Case As Const s[i]
      Case 65 To 90
        c = s[i] + key
        If c > 90 Then c -= 26
        s[i] = c
      Case 97 To 122
        c = s[i] + key
        If c > 122 Then c -= 26
        s[i] = c
    End Select
  Next
End Sub

Sub Decrypt(s As String, key As Integer)
  Dim c As Integer
  For i As Integer = 0 To Len(s)
    Select Case As Const s[i]
      Case 65 To 90
        c = s[i] - key
        If c < 65 Then c += 26
        s[i] = c
      Case 97 To 122
        c = s[i] - key
        If c < 97 Then c += 26
        s[i] = c
    End Select
  Next
End Sub

Dim As String s = "Bright vixens jump; dozy fowl quack."
Print "Plain text : "; s
Encrypt s, 8
Print "Encrypted  : "; s
Decrypt s, 8
Print "Decrypted  : "; s
Sleep