La key utilizzata per criptare e decriptare deve essere lunga almeno 8 caratteri e chiaramente deve essere la stessa per lo stesso file, cioe se utilizzi 'abcdefgh' per criptare il file pippo.txt per decriptare lo stesso file dovrai utilizzare la stessa Key.
codice:
Imports System.Security
Imports System.Security.Cryptography


 Private Sub EncryptFile(ByVal sInputFilename As String, _
                   ByVal sOutputFilename As String, _
                   ByVal sKey As String)
        Try
            Dim fsInput As New FileStream(sInputFilename, _
                                        FileMode.Open, FileAccess.Read)
            Dim fsEncrypted As New FileStream(sOutputFilename, _
                                        FileMode.Create, FileAccess.Write)

            Dim DES As New DESCryptoServiceProvider()
            DES.Key = ASCIIEncoding.ASCII.GetBytes(sKey)

            DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey)

            Dim desencrypt As ICryptoTransform = DES.CreateEncryptor()
            Dim cryptostream As New CryptoStream(fsEncrypted, _
                                                desencrypt, _
                                                CryptoStreamMode.Write)

            Dim bytearrayinput(fsInput.Length - 1) As Byte
            fsInput.Read(bytearrayinput, 0, bytearrayinput.Length)
            cryptostream.Write(bytearrayinput, 0, bytearrayinput.Length)
            cryptostream.Close()
            fsInput.Close()
            fsInput.Dispose()
        Catch ex As Exception
            Throw New Exception(ex.Message, ex.InnerException)
        End Try
    End Sub

    Private Sub DecryptFile(ByVal sInputFilename As String, _
        ByVal sOutputFilename As String, _
        ByVal sKey As String)
        Try
            Dim DES As New DESCryptoServiceProvider()
            DES.Key() = ASCIIEncoding.ASCII.GetBytes(sKey)
            DES.IV = ASCIIEncoding.ASCII.GetBytes(sKey)

            Dim fsread As New FileStream(sInputFilename, FileMode.Open, FileAccess.Read)
            Dim desdecrypt As ICryptoTransform = DES.CreateDecryptor()
            Dim cryptostreamDecr As New CryptoStream(fsread, desdecrypt, CryptoStreamMode.Read)
            Dim fsDecrypted As New StreamWriter(sOutputFilename)
            fsDecrypted.Write(New StreamReader(cryptostreamDecr).ReadToEnd)
            fsDecrypted.Flush()
            fsDecrypted.Close()
            fsread.Close()
            fsread.Dispose()
        Catch ex As Exception
            Throw New Exception(ex.Message, ex.InnerException)
        End Try
    End Sub
La Key chiaramente dovrai memorizzarla nel tuo codice ma se ti spiano pure quella???? Ma questi sono altri problemi....