How To Encrypt File In ListView (RC4)

I have the files in a listview control, with checkboxes. I would like it to encrypt all files ending with .rc4, that are listed in the listview. But keep the original of file extension
E.g : file.txt --> file_txt.rc4
How can I get it to encrypt multiple items. I already tried and still can't write in...

Here my code
Private Sub BtnEncrypt_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnEncrypt.Click
Dim ByteConverter As Encoding = Encoding.UTF8
Dim Key As Byte() = ByteConverter.GetBytes(TextBox2.Text)
If ListView1.Items.Count <> 0 Then
[Code] .....