encryption

I am try to encrypt a text for the first time
I have the code  below but when i try to run it with input string

"MessageBox.Show(Encrypt("string is that you need to encrypt", "abc123¿"))"

i get the message
"Specified key is not a valid size for this algorithm." when it reach line 8
 

1.Public Function Encrypt(ByVal strText As String, ByVal strEncrKey As String) As String
2.       Dim IV() As Byte = {&H12, &H34, &H56, &H78, &H90, &HAB, &HCD, &HEF}
3.        Try
4.            Dim bykey() As Byte = System.Text.Encoding.UTF8.GetBytes(Left(strEncrKey, 8))
5.            Dim InputByteArray() As Byte = System.Text.Encoding.UTF8.GetBytes(strText)
6.            Dim des As New DESCryptoServiceProvider
7.            Dim ms As New MemoryStream
8.            Dim cs As New CryptoStream(ms, des.CreateEncryptor(bykey, IV), CryptoStreamMode.Write)
            cs.Write(InputByteArray, 0, InputByteArray.Length)
            cs.FlushFinalBlock()
            Return Convert.ToBase64String(ms.ToArray())
        Catch ex As Exception
            Return ex.Message
        End Try
    End Function

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