let readPassword () =
    let rec readMask pw =
        let k = System.Console.ReadKey()
        match k.Key with
        | System.ConsoleKey.Enter -> pw
        | System.ConsoleKey.Escape -> pw
        | System.ConsoleKey.Backspace ->
            match pw with
            | [] -> readMask []
            | _::t ->
                System.Console.Write " \b"
                readMask t
        | _ ->
            System.Console.Write "\b*"
            readMask (k.KeyChar::pw)
    let password = readMask [] |> Seq.rev |> System.String.Concat
    System.Console.WriteLine ()
    password