Vba excel chiffrement de Vigenère

Sub Bouton1_Cliquer()

Dim txt As String, key As String
txt = Cells(2, 2)
key = Cells(3, 2)

For i = 1 To Len(txt)

    Cells(i + 5, 2) = Mid(txt, i, 1)
    a = Asc(Cells(i + 5, 2)) - 64
    x = x + 1
    If x > Len(key) Then x = 1
    Cells(i + 5, 3).Value = Mid(key, x, 1)
    b = Asc(Cells(i + 5, 3)) - 64
    c = a + b
    If c > 26 Then c = c - 26
    Cells(i + 5, 4).Value = Chr(c + 63)
    
Next i
        
End Sub