Const MIN_ASC = 65

Function X2D(x As String) As String

  Dim s As String, zf As String, i As Integer

  s = ""

  For i = 1 To Len(x)

    zf = Mid(x, i, 1)

    If zf >= "a" And zf <= "z" Then

      s=s+

    Else

      s = s + zf

    End If

 Next i

 X2D = s

End Function

Private Sub Command1_Click()

Dim M As String, K As String, C As String, flag As Boolean

Dim i As Integer, t As Integer, ch As Integer, pos As Integer, j As Integer

M = Text1.Text       '明文

K = Text2.Text       '密钥

t = Len(K)

K = X2D(K)

C = ""

For i = 1 To Len(M)

  ch = Asc(Mid(M, i, 1))

  flag = False

  If ch >= 97 And ch <= 122 Then ch = ch - 32: flag = True

  pos=                  '密钥的字符位置值

  If pos = 0 Then pos = t

  j = Asc(Mid(K, pos, 1))      '密钥字符的ASCII值

  ch = ch - MIN_ASC

  j = j - MIN_ASC

  ch=

  ch = ch + MIN_ASC

  If flag Then ch = ch + 32     '设置字母的大小写形式

  C = C & Chr(ch)

Next i

Text3.Text = C

End Sub