Visual Basic.NET - Ayuda con comparacion de campos con encriptacion MD5

// Funcion para Encriptar

        public static string Encrypt(string Text, string Key)

        {

            const int keysize = 256;

            const string initVector = "tu89geji340t89u2";

            byte[] initVectorBytes = Encoding.UTF8.GetBytes(initVector);

            byte[] plainTextBytes = Encoding.UTF8.GetBytes(Text);

            PasswordDeriveBytes password = new PasswordDeriveBytes(Key, null);

            byte[] keyBytes = password.GetBytes(keysize / 8);

            RijndaelManaged symmetricKey = new RijndaelManaged();

            symmetricKey.Mode = CipherMode.CBC;

            ICryptoTransform encryptor = symmetricKey.CreateEncryptor(keyBytes, initVectorBytes);

            MemoryStream memoryStream = new MemoryStream();

            CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write);

            cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);

            cryptoStream.FlushFinalBlock();

            byte[] Encrypted = memoryStream.ToArray();

            memoryStream.Close();

            cryptoStream.Close();

            return Convert.ToBase64String(Encrypted);

        }

        // Funcion para Encriptar

        public static string Decrypt(string EncryptedText, string Key)

        {

            const int keysize = 256;

            const string initVector = "tu89geji340t89u2";

            byte[] initVectorBytes = Encoding.ASCII.GetBytes(initVector);

            byte[] DeEncryptedText = Convert.FromBase64String(EncryptedText);

            PasswordDeriveBytes password = new PasswordDeriveBytes(Key, null);

            byte[] keyBytes = password.GetBytes(keysize / 8);

            RijndaelManaged symmetricKey = new RijndaelManaged();

            symmetricKey.Mode = CipherMode.CBC;

            ICryptoTransform decryptor = symmetricKey.CreateDecryptor(keyBytes, initVectorBytes);

            MemoryStream memoryStream = new MemoryStream(DeEncryptedText);

            CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read);

            byte[] plainTextBytes = new byte[DeEncryptedText.Length];

            int decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);

            memoryStream.Close();

            cryptoStream.Close();

            return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);

        }

        // para usarlas...   seria algo asi

        usuario = txtUsuario.text;

        password = txtPassword.text;

        password = Encrypt( password, "mi_llave" );

        // y suponiendo que tienes una clase ligada a una tabla de usuarios que debes consultar

        var usuario = from u db.Usuarios

                     where u.usuario == usuario && u.password  == password

                        select u;

        if ( usuario == null )

        {

             alert.Show( "error en el nombre de usuario o contraseña" );

        }

If txtUsername.Text <> "" And txtContraseña.Text <> "" Then

    consulta = "SELECT id, usuario, password FROM usuarios WHERE usuario='" & txtUsername.Text & "' and password='MD5(" & txtContraseña.Text & ")'"

    adaptador = New MySqlDataAdapter(consulta, conexion)

    datos = New DataSet

    adaptador.Fill(datos, "usuarios")

    lista = datos.Tables("usuarios").Rows.Count

End If

If lista <> 0 Then

    MsgBox("Bienvenido")

Else

    MsgBox("Intentelo de nuevo")

End If

consulta = "SELECT id, usuario, password FROM usuarios WHERE usuario='" & txtUsername.Text & "' and password='MD5(" & txtContraseña.Text & ")'"

consulta = "SELECT id, usuario, password FROM usuarios WHERE usuario='" & txtUsername.Text & "' and password=MD5('" & txtContraseña.Text & "')"