Relay/RelayClient/MainPage.xaml.cs

using RelayClient.Crypto;
using RelayClient.Models;
using WebSocketSharp;
using System.Text.Json;

namespace RelayClient;

public partial class MainPage : ContentPage
{
    private readonly string _username;
    private readonly WebSocket wsc;
    private string? _serverPublicKey;

    public MainPage(string username)
    {
        InitializeComponent();

        _username = username;
        UserLabel.Text = $"Logged in as: {_username}";
        
        if (!KeyStorage.HasKeys(_username))
        {
            var keys = E2EeHelper.GenerateRsaKeyPair();
            KeyStorage.SavePrivateKey(_username, keys.privateKey);
            KeyStorage.SavePublicKey(_username, keys.publicKey);
        }

        wsc = new WebSocket("ws://localhost:1337/");

        wsc.OnMessage += WscOnMessage;
        wsc.Connect();
        
        var publicKey = KeyStorage.LoadPublicKey(_username);
        wsc.Send($"REGISTER_KEY|{_username}|{publicKey}");
        wsc.Send("GET_SERVER_KEY");
        wsc.Send($"GET_HISTORY|{_username}");
    }

    private void SendButton_OnClicked(object? sender, EventArgs e)
    {
        SendMessage();
    }

    private void MessageEntry_OnCompleted(object? sender, EventArgs e)
    {
        SendMessage();
    }

    private void SendMessage()
    {
        var text = MessageEntry.Text?.Trim();

        if (string.IsNullOrWhiteSpace(text))
            return;

        if (string.IsNullOrWhiteSpace(_serverPublicKey))
        {
            Console.WriteLine("Server public key not loaded yet.");
            return;
        }

        var encrypted = E2EeHelper.EncryptForRecipient(text, _serverPublicKey);

        var payload = new SocketEncryptedMessage
        {
            Type = "client_encrypted_chat",
            SenderUsername = _username,
            CipherText = encrypted.CipherText,
            Nonce = encrypted.Nonce,
            Tag = encrypted.Tag,
            EncryptedKey = encrypted.EncryptedKey
        };

        var json = JsonSerializer.Serialize(payload);
        wsc.Send(json);

        Console.WriteLine($"[{_username}] sent encrypted message.");

        MessageEntry.Text = string.Empty;
        MessageEntry.Focus();
    }

    private void WscOnMessage(object? sender, MessageEventArgs e)
    {
        if (e.Data.StartsWith("SERVER:REGISTERED_KEY:"))
        {
            Console.WriteLine(e.Data);
            return;
        }

        Console.WriteLine($"[{_username}] RAW WS DATA: {e.Data}");

        try
        {
            using var doc = JsonDocument.Parse(e.Data);
            var root = doc.RootElement;

            if (!root.TryGetProperty("Type", out var typeElement))
                return;

            var type = typeElement.GetString();

            if (type == "server_public_key")
            {
                var serverKeyMessage = JsonSerializer.Deserialize<ServerPublicKeyMessage>(e.Data);
                if (serverKeyMessage is not null)
                {
                    _serverPublicKey = serverKeyMessage.PublicKey;
                    Console.WriteLine($"[{_username}] loaded server public key.");
                }

                return;
            }

            if (type != "encrypted_chat")
                return;

            var payload = JsonSerializer.Deserialize<SocketEncryptedMessage>(e.Data);
            if (payload is null)
                return;

            if (payload.RecipientUsername != _username)
                return;

            Console.WriteLine($"[{_username}] received encrypted payload for {payload.RecipientUsername}");

            var privateKey = KeyStorage.LoadPrivateKey(_username);

            var decryptedText = E2EeHelper.DecryptForRecipient(
                new EncryptedPayload
                {
                    CipherText = payload.CipherText,
                    Nonce = payload.Nonce,
                    Tag = payload.Tag,
                    EncryptedKey = payload.EncryptedKey
                },
                privateKey
            );

            Console.WriteLine($"[{_username}] decrypted message from {payload.SenderUsername}: {decryptedText}");

            var message = new ChatMessage
            {
                SenderUsername = payload.SenderUsername,
                Text = decryptedText,
                Timestamp = DateTime.Now
            };

            MainThread.BeginInvokeOnMainThread(async () =>
            {
                bool isOwnMessage = message.SenderUsername == _username;

                var bubble = new Border
                {
                    StrokeThickness = 1,
                    Padding = 10,
                    Margin = isOwnMessage
                        ? new Thickness(40, 0, 0, 0)
                        : new Thickness(0, 0, 40, 0),
                    HorizontalOptions = isOwnMessage
                        ? LayoutOptions.End
                        : LayoutOptions.Start,
                    Content = new VerticalStackLayout
                    {
                        Spacing = 2,
                        Children =
                        {
                            new Label
                            {
                                Text = message.SenderUsername,
                                FontAttributes = FontAttributes.Bold,
                                FontSize = 12
                            },
                            new Label
                            {
                                Text = message.Text,
                                FontSize = 14
                            },
                            new Label
                            {
                                Text = message.Timestamp.ToString("h:mm tt"),
                                FontSize = 10
                            }
                        }
                    }
                };

                MessagesLayout.Children.Add(bubble);
                await MessagesScrollView.ScrollToAsync(MessagesLayout, ScrollToPosition.End, true);
            });
        }
        catch (Exception ex)
        {
            Console.WriteLine($"[{_username}] failed to process websocket message: {ex.Message}");
        }
    }

    protected override void OnDisappearing()
    {
        wsc.OnMessage -= WscOnMessage;
        wsc.Close();
        base.OnDisappearing();
    }
}