C# Winforms Log In Form using Binary File

I am currently working on a school project that will have your classic log-in/register form. The project requires that we include a binary file to 'securely' store each User's information e.g username, password etc.

I am successfully writing new user's information to a binary file named "users" when they register, and then I am successfully reading it back to a rich text box. (This is simply so I know that everything is being read back).

I am also able to successfully recognise the last registered user when this user is inputted to the Log-In section of the form, but I cannot recognise any other user's information.

My code for the Register (e.g when the registry button is clicked)

private void btnRegister_Click(object sender, EventArgs e)
{
    //Try - Catch Statements
    //Try to set the forename

    try
    {
        newUser.Forename = txtForename.Text;
        errForename.Icon = Properties.Resources.CorrectIcon;
        errForename.SetError(txtForename, "OK");
    }

    // catch the exception error if one is raised and place the message
    //into the ErrorProvider

    catch (NewException exc)
    {
        errForename.SetError(txtForename, exc.MessageM);
    }

    //Try to set the surname
    try
    {
        newUser.Surname = txtSurname.Text;
        errSurname.Icon = Properties.Resources.CorrectIcon;
        errSurname.SetError(txtSurname, "OK");
    }

    // catch the exception error if one is raised and place the message
    //into the ErrorProvider

    catch (NewException exc)
    {
        errSurname.SetError(txtSurname, exc.MessageM);
    }

    //Try to set the password
    try
    {
        newUser.Password = txtPasswordR.Text;
        errPasswordR.Icon = Properties.Resources.CorrectIcon;
        errPasswordR.SetError(txtPasswordR, "OK");
    }

    // catch the exception error if one is raised and place the message
    //into the ErrorProvider

    catch (NewException exc)
    {
        errPasswordR.SetError(txtPasswordR, exc.MessageM);
    }    

    // These lines of code set the User class 

    if (newUser.Password != null )
    { 
        newUser.Forename = txtForename.Text;
        newUser.Surname = txtSurname.Text;
        newUser.Username = txtUsernameR.Text;
        newUser.Password = txtPasswordR.Text;   

        FileStream fileStream = new FileStream("../../Textfiles/users.bin", FileMode.Append);
        BinaryWriter binaryWriter = new BinaryWriter(fileStream);
        {
            binaryWriter.Write(newUser.Forename);
            binaryWriter.Write(newUser.Surname);
            binaryWriter.Write(newUser.Username);
            binaryWriter.Write(newUser.Password);

        }

        binaryWriter.Flush();
        binaryWriter.Close();
    }
}

My Code for Reading from the Bin file

public bool ReadFromBin(string a, string b)
{
    bool okFlag = false;

    FileStream fileStream = File.OpenRead("../../TextFiles/users.bin");
    BinaryReader binaryReader = new BinaryReader(fileStream);

    while (binaryReader.PeekChar() != -1)
    {
        newUser.Forename = binaryReader.ReadString();
        newUser.Surname = binaryReader.ReadString();
        newUser.Username = binaryReader.ReadString();
        newUser.Password = binaryReader.ReadString();


        if ((newUser.Username == a) && (newUser.Password == b))
            okFlag = true;
        else
            okFlag = false;
    }

    binaryReader.Close();
    return okFlag;
}

And finally the very simple code for the Log-in

private void btnLogIn_Click(object sender, EventArgs e)
{
    if(this.ReadFromBin(txtUsername.Text, txtPassword.Text) == false)
    {
        MessageBox.Show("Not registered!");
    }
    else
    {
        MessageBox.Show("Registered!");
        Game frm = new Game(newUser);
        frm.Show();
    }
}

My understanding is that the problem is most likely in the while loop contained within the ReadFromBin method, I am just unsure of how to fix it. Thanks!