Dependency Injection container added. SecurityService work stated, not finished. Minor UI improvements. BROKEN

QRBee.Core/Data/ClientToMerchantResponse.cs

@@ -36,7 +36,9 @@
         /// Convert ClientToMerchantResponse to string to be used as QR Code source (along with client signature)
         /// </summary>
         /// <returns> Converted string</returns>
-        public string AsQRCodeString() => $"{ClientId}|{TimeStampUTC:O}|{MerchantRequest.AsQRCodeString()}";
+        public string AsQRCodeString() => $"{AsDataForSignature()}|{ClientSignature}";
+
+        public string AsDataForSignature() => $"{ClientId}|{TimeStampUTC:O}|{MerchantRequest.AsQRCodeString()}";
 
         /// <summary>
         /// Convert from string
@@ -56,7 +58,8 @@
             {
                 MerchantRequest = MerchantToClientRequest.FromString(string.Join("|", s.Skip(2))),
                 ClientId = s[0],
-                TimeStampUTC = DateTime.ParseExact(s[1], "O", null)
+                TimeStampUTC = DateTime.ParseExact(s[1], "O", null),
+                ClientSignature = s[3]
             };
 
             return res;

QRBee.Core/Data/MerchantToClientRequest.cs

@@ -44,7 +44,9 @@ namespace QRBee.Core.Data
         /// Convert MerchantToClientRequest to string to be used as QR Code source (along with merchant signature)
         /// </summary>
         /// <returns>String conversion</returns>
-        public string AsQRCodeString() => $"{MerchantTransactionId}|{MerchantId}|{Name}|{Amount.ToString("0.00", CultureInfo.InvariantCulture)}|{TimeStampUTC:O}";
+        public string AsQRCodeString() => $"{AsDataForSignature()}|{MerchantSignature}";
+
+        public string AsDataForSignature() => $"{MerchantId}|{MerchantTransactionId}|{Name}|{Amount.ToString("0.00", CultureInfo.InvariantCulture)}|{TimeStampUTC:O}";
 
         /// <summary>
         /// Convert from string
@@ -55,9 +57,9 @@ namespace QRBee.Core.Data
         public static MerchantToClientRequest FromString(string input)
         {
             var s = input.Split('|');
-            if (s.Length != 5)
+            if (s.Length != 6)
             {
-                throw new ApplicationException("Expected 5 elements");
+                throw new ApplicationException("Expected 6 elements");
             }
 
             var res = new MerchantToClientRequest
@@ -66,7 +68,8 @@ namespace QRBee.Core.Data
                 MerchantTransactionId = s[1],
                 Name = s[2],
                 Amount = Convert.ToDecimal(s[3], CultureInfo.InvariantCulture),
-                TimeStampUTC = DateTime.ParseExact(s[4],"O",null)
+                TimeStampUTC = DateTime.ParseExact(s[4],"O",null),
+                MerchantSignature = s[5]
             };
 
 

QRBee.Core/Security/IPrivateKeyHandler.cs

@@ -17,14 +17,14 @@ namespace QRBee.Core.Security
         /// Generate new private key and store it
         /// </summary>
         /// <param name="subjectName"></param>
-        /// <returns>Certificate request to be sent to CA</returns>
+        /// <returns>Certificate request to be sent to CA in PEM format</returns>
-        byte [] GeneratePrivateKey(string? subjectName = null);
+        string GeneratePrivateKey(string? subjectName = null);
 
         /// <summary>
         /// Re-create certificate request if CA response was not received in time.
         /// </summary>
-        /// <returns></returns>
+        /// <returns>Certificate request to be sent to CA in PEM format</returns>
-        byte[] CreateCertificateRequest();
+        string CreateCertificateRequest();
 
         /// <summary>
         /// Attach CA-generated public key certificate to the private key

QRBee.Core/Security/PrivateKeyHandlerBase.cs

@@ -0,0 +1,39 @@
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+
+namespace QRBee.Core.Security
+{
+    /// <summary>
+    /// Private key handler for API server
+    /// </summary>
+    public class PrivateKeyHandlerBase : IPrivateKeyHandler
+    {
+
+        protected string CommonName { get; set; }
+        protected string CertificatePassword { get; set; }
+        public bool Exists()
+        {
+            throw new NotImplementedException();
+        }
+
+        public string GeneratePrivateKey(string? subjectName = null)
+        {
+            throw new NotImplementedException();
+        }
+
+        public string CreateCertificateRequest()
+        {
+            throw new NotImplementedException();
+        }
+
+        public void AttachCertificate(X509Certificate2 cert)
+        {
+            throw new NotImplementedException();
+        }
+
+        public X509Certificate2 LoadPrivateKey()
+        {
+            throw new NotImplementedException();
+        }
+    }
+}

QRBee/QRBee.Android/MainActivity.cs

@@ -6,11 +6,15 @@ using Android.Runtime;
 using Android.OS;
 using Android.Support.V4.Content;
 using AndroidX.Core.App;
+using Microsoft.Extensions.DependencyInjection;
 using Plugin.Fingerprint;
+using QRBee.Core.Security;
+using QRBee.Droid.Services;
+using QRBee.Services;
 
 namespace QRBee.Droid
 {
-    [Activity(Label = "QRBee", Icon = "@mipmap/icon", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize )]
+    [Activity(Label = "QRBee", Icon = "@mipmap/icon", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize | ConfigChanges.Orientation, ScreenOrientation = ScreenOrientation.Portrait)]
     public class MainActivity : global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity
     {
         protected override void OnCreate(Bundle savedInstanceState)
@@ -23,7 +27,7 @@ namespace QRBee.Droid
             CrossFingerprint.SetCurrentActivityResolver(()=>Xamarin.Essentials.Platform.CurrentActivity);
 
             global::Xamarin.Forms.Forms.Init(this, savedInstanceState);
-            LoadApplication(new App());
+            LoadApplication(new App(AddServices));
             ZXing.Mobile.MobileBarcodeScanner.Initialize(Application);
 
             if (ContextCompat.CheckSelfPermission(this, Manifest.Permission.Camera) == (int) Permission.Granted)
@@ -37,11 +41,22 @@ namespace QRBee.Droid
             }
 
         }
+
         public override void OnRequestPermissionsResult(int requestCode, string[] permissions, [GeneratedEnum] Android.Content.PM.Permission[] grantResults)
         {
             Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);
             ZXing.Net.Mobile.Android.PermissionsHandler.OnRequestPermissionsResult(requestCode, permissions, grantResults);
             base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
         }
+
+        private static void AddServices(IServiceCollection services)
+        {
+            services
+                .AddSingleton<ISecurityService,AndroidSecurityService>()
+                .AddSingleton<ILocalSettings, LocalSettings>()
+                .AddSingleton<IQRScanner, QRScannerService>()
+                ;
+        }
+
     }
 }

QRBee/QRBee.Android/QRBee.Android.csproj

@@ -59,6 +59,9 @@
     <Reference Include="System.Xml" />
   </ItemGroup>
   <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection">
+      <Version>6.0.0</Version>
+    </PackageReference>
     <PackageReference Include="Plugin.Fingerprint">
       <Version>2.1.4</Version>
     </PackageReference>
@@ -80,7 +83,7 @@
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Services\LocalSettings.cs" />
     <Compile Include="Services\QRScannerService.cs" />
-    <Compile Include="Services\SecurityService.cs" />
+    <Compile Include="Services\AndroidSecurityService.cs" />
   </ItemGroup>
   <ItemGroup>
     <None Include="Resources\AboutResources.txt" />

QRBee/QRBee.Android/Resources/values/colors.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
     <color name="launcher_background">#FFFFFF</color>
-    <color name="colorPrimary">#3F51B5</color>
+    <color name="colorPrimary">#FFC928</color>
-    <color name="colorPrimaryDark">#303F9F</color>
+    <color name="colorPrimaryDark">#FFC928</color>
-    <color name="colorAccent">#FF4081</color>
+    <color name="colorAccent">#FFC928</color>
 </resources>

QRBee/QRBee.Android/Services/AndroidSecurityService.cs

@@ -0,0 +1,56 @@
+using System;
+using System.Security.Cryptography.X509Certificates;
+using System.Text;
+using QRBee.Core.Security;
+
+namespace QRBee.Droid.Services
+{
+    internal class AndroidSecurityService : SecurityServiceBase
+    {
+
+        public AndroidSecurityService(IPrivateKeyHandler privateKeyHandler)
+        : base(privateKeyHandler)
+        {
+        }
+
+        /// <inheritdoc/>
+        public override X509Certificate2 CreateCertificate(string subjectName, byte[] rsaPublicKey)
+        {
+            throw new ApplicationException("Client never issues certificates");
+        }
+
+        private const string CertHeader = "-----BEGIN CERTIFICATE-----";
+        private const string CertFooter = "-----END CERTIFICATE-----";
+
+        /// <inheritdoc/>
+        public override X509Certificate2 Deserialize(string pemData)
+        {
+            var start = pemData.IndexOf(CertHeader);
+            if (start == -1)
+                throw new ApplicationException("Invalid certificate format");
+            start = start + CertHeader.Length;
+
+            var end = pemData.IndexOf(CertFooter);
+            if (end == -1)
+                throw new ApplicationException("Invalid certificate format");
+
+            var base64 = pemData.Substring(start, end - start); // contains new lines, but it does not matter
+            var data = Convert.FromBase64String(base64);
+            
+            return new X509Certificate2(data);
+        }
+
+        /// <inheritdoc/>
+        public override string Serialize(X509Certificate2 cert)
+        {
+            // https://stackoverflow.com/questions/43928064/export-private-public-keys-from-x509-certificate-to-pem
+            var builder = new StringBuilder();
+            builder.AppendLine(CertHeader);
+            builder.AppendLine(Convert.ToBase64String(cert.RawData, Base64FormattingOptions.InsertLineBreaks));
+            builder.AppendLine(CertFooter);
+
+            return builder.ToString();
+        }
+    }
+
+}

QRBee/QRBee.Android/Services/SecurityService.cs

@@ -1,40 +0,0 @@
-using System;
-using System.Security.Cryptography;
-using System.Security.Cryptography.X509Certificates;
-using System.Text.RegularExpressions;
-using QRBee.Core.Security;
-
-namespace QRBee.Api.Services
-{
-    internal class SecurityService : SecurityServiceBase
-    {
-
-        public SecurityService(IPrivateKeyHandler privateKeyHandler)
-        : base(privateKeyHandler)
-        {
-        }
-
-        /// <inheritdoc/>
-        public override X509Certificate2 CreateCertificate(string subjectName, byte[] rsaPublicKey)
-        {
-            throw new ApplicationException("Client never issues certificates");
-        }
-
-        /// <inheritdoc/>
-        public override X509Certificate2 Deserialize(string pemData)
-        {
-            throw new NotImplementedException();
-            //return X509Certificate2.CreateFromPem(pemData);
-        }
-
-        /// <inheritdoc/>
-        public override string Serialize(X509Certificate2 cert)
-        {
-            throw new NotImplementedException();
-            // https://stackoverflow.com/questions/43928064/export-private-public-keys-from-x509-certificate-to-pem
-            //var pem = PemEncoding.Write("CERTIFICATE", cert.RawData);
-            //return new string(pem);
-        }
-    }
-
-}

QRBee/QRBee/App.xaml

@@ -7,7 +7,8 @@
     -->
     <Application.Resources>
         <ResourceDictionary>
-            <Color x:Key="Primary">#2196F3</Color>
+            <!--2296F3 -->
+            <Color x:Key="Primary">#FFC928</Color>
             <Style TargetType="Button">
                 <Setter Property="TextColor" Value="White"></Setter>
                 <Setter Property="VisualStateManager.VisualStateGroups">

QRBee/QRBee/App.xaml.cs

@@ -1,21 +1,52 @@
 using QRBee.Services;
 using QRBee.Views;
 using System;
+using Microsoft.Extensions.DependencyInjection;
+using QRBee.Core.Security;
+using QRBee.ViewModels;
 using Xamarin.Forms;
-using Xamarin.Forms.Xaml;
 
 namespace QRBee
 {
     public partial class App : Application
     {
 
-        public App()
+        public App(Action<IServiceCollection> addPlatformServices = null)
         {
             InitializeComponent();
 
+            SetupServices(addPlatformServices);
+
             DependencyService.Register<MockDataStore>();
             MainPage = new AppShell();
         }
+        protected static IServiceProvider ServiceProvider { get; set; }
+
+        void SetupServices(Action<IServiceCollection> addPlatformServices = null)
+        {
+            var services = new ServiceCollection();
+
+            // Add platform specific services
+            addPlatformServices?.Invoke(services);
+
+            // TODO: Add core services here
+            services
+                .AddSingleton<IPrivateKeyHandler, ClientPrivateKeyHandler>()
+                ;
+
+            // Add ViewModels
+            services
+                .AddTransient<MerchantPageViewModel>()
+                .AddTransient<ClientPageViewModel>()
+                .AddTransient<RegisterViewModel>()
+                .AddTransient<LoginViewModel>()
+                ;
+
+
+            ServiceProvider = services.BuildServiceProvider();
+        }
+
+        public static BaseViewModel GetViewModel<TViewModel>() where TViewModel : BaseViewModel => ServiceProvider.GetService<TViewModel>();
 
         protected override void OnStart()
         {

QRBee/QRBee/AppShell.xaml

@@ -30,8 +30,6 @@
     </Shell.Resources>
 
     <TabBar>
-        <!-- <ShellContent Title="About" Icon="icon_about.png" Route="AboutPage" ContentTemplate="{DataTemplate local:AboutPage}" /> -->
-        <!-- <ShellContent Title="Browse" Icon="icon_feed.png" ContentTemplate="{DataTemplate local:ItemsPage}" /> -->
         <ShellContent Title="LoginPage" ContentTemplate="{DataTemplate local:LoginPage}"/>
     </TabBar>
 

QRBee/QRBee/QRBee.csproj

@@ -14,6 +14,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="6.0.0" />
     <PackageReference Include="Plugin.Fingerprint" Version="2.1.4" />
     <PackageReference Include="Xamarin.Forms" Version="5.0.0.2244" />  
     <PackageReference Include="Xamarin.Essentials" Version="1.7.0" />  

QRBee/QRBee/Services/ClientPrivateKeyHandler.cs

@@ -0,0 +1,23 @@
+using QRBee.Core.Security;
+
+namespace QRBee.Services
+{
+    /// <summary>
+    /// Private key handler for Client side
+    /// </summary>
+    public class ClientPrivateKeyHandler : PrivateKeyHandlerBase
+    {
+
+        private const string VeryBadAndInsecureCertificatePassword = "Rî‹T=›'ÄζgÚrʯю™pudF";
+
+        public ClientPrivateKeyHandler(ILocalSettings settings)
+        {
+            CertificatePassword = VeryBadAndInsecureCertificatePassword;
+
+            var clientSettings = settings.LoadSettings();
+            CommonName = clientSettings?.ClientId;
+        }
+
+    }
+}
+

QRBee/QRBee/ViewModels/ClientPageViewModel.cs

@@ -1,5 +1,7 @@
 using System;
+using System.Text;
 using QRBee.Core.Data;
+using QRBee.Core.Security;
 using QRBee.Services;
 using QRBee.Views;
 using Xamarin.Forms;
@@ -8,17 +10,26 @@ namespace QRBee.ViewModels
 {
     internal class ClientPageViewModel: BaseViewModel
     {
-        public bool _isVisible;
+        private readonly IQRScanner _scanner;
+        private readonly ISecurityService _securityService;
+        public bool _isAcceptDenyButtonVisible;
+        public bool _isQrVisible;
+        public bool _isScanButtonVisible;
+
+
         public string _amount;
         private string _qrCode;
         private MerchantToClientRequest _merchantToClientRequest;
-        private readonly ClientPage _clientPage;
 
-        public ClientPageViewModel(Views.ClientPage clientPage)
+        public ClientPageViewModel(IQRScanner scanner, ISecurityService securityService)
         {
+            _scanner = scanner;
+            _securityService = securityService;
             ScanCommand = new Command(OnScanButtonClicked);
-            GenerateQrCommand = new Command(OnGenerateQrClicked);
+            AcceptQrCommand = new Command(OnAcceptQrCommand);
-            _clientPage = clientPage;
+            DenyQrCommand = new Command(OnDenyQrCommand);
+
+            IsScanButtonVisible = true;
         }
 
         public Command ScanCommand
@@ -26,23 +37,33 @@ namespace QRBee.ViewModels
             get;
         }
 
-        public Command GenerateQrCommand 
+        public Command AcceptQrCommand 
         { 
             get; 
         }
 
+        public Command DenyQrCommand
+        {
+            get;
+        }
+
+
         private async void OnScanButtonClicked(object sender)
         {
+            QrCode = null;
+            IsQrVisible = false;
+            IsAcceptDenyButtonVisible = false;
+
             try
             {
-                var scanner = DependencyService.Get<IQRScanner>();
+                var result = await _scanner.ScanQR();
-                var result = await scanner.ScanQR();
                 if (result == null) 
                     return;
 
                 _merchantToClientRequest = MerchantToClientRequest.FromString(result);
                 Amount = $"{_merchantToClientRequest.Amount:N2}";
-                IsVisible = true;
+                IsAcceptDenyButtonVisible = true;
+                IsScanButtonVisible = false;
             }
             catch (Exception)
             {
@@ -64,17 +85,45 @@ namespace QRBee.ViewModels
             }
         }
 
-        public bool IsVisible
+        public bool IsAcceptDenyButtonVisible
         {
-            get => _isVisible;
+            get => _isAcceptDenyButtonVisible;
             set
             {
-                if (value == _isVisible)
+                if (value == _isAcceptDenyButtonVisible)
                 {
                     return;
                 }
-                _isVisible = value;
+                _isAcceptDenyButtonVisible = value;
-                OnPropertyChanged(nameof(IsVisible));
+                OnPropertyChanged(nameof(IsAcceptDenyButtonVisible));
+            }
+        }
+
+        public bool IsQrVisible
+        {
+            get => _isQrVisible;
+            set
+            {
+                if (value == _isQrVisible)
+                {
+                    return;
+                }
+                _isQrVisible = value;
+                OnPropertyChanged(nameof(IsQrVisible));
+            }
+        }
+
+        public bool IsScanButtonVisible
+        {
+            get => _isScanButtonVisible;
+            set
+            {
+                if (value == _isScanButtonVisible)
+                {
+                    return;
+                }
+                _isScanButtonVisible = value;
+                OnPropertyChanged(nameof(IsScanButtonVisible));
             }
         }
 
@@ -83,7 +132,6 @@ namespace QRBee.ViewModels
             get => _qrCode;
             set
             {
-                // _qrCode = $"{Amount}/{Name}";
                 if (_qrCode == value)
                     return;
 
@@ -96,12 +144,11 @@ namespace QRBee.ViewModels
         /// Reaction on GenerateQR button clicked
         /// </summary>
         /// <param name="obj"></param>
-        public async void OnGenerateQrClicked(object obj)
+        public async void OnAcceptQrCommand(object obj)
         {
 
-            var answer = await _clientPage.DisplayAlert("Confirmation", "Would you like to accept the offer?", "Yes", "No");
+            var answer = await Application.Current.MainPage.DisplayAlert("Confirmation", "Would you like to accept the offer?", "Yes", "No");
-            if (!answer) 
+            if (!answer) return;
-                return;
 
             var response = new ClientToMerchantResponse
             {
@@ -111,10 +158,23 @@ namespace QRBee.ViewModels
                 MerchantRequest = _merchantToClientRequest
 
             };
-            // TODO Create merchant signature.
+            // TODO Create client signature.
+            var clientSignature = _securityService.Sign(Encoding.UTF8.GetBytes(response.AsDataForSignature()));
+            response.ClientSignature = Convert.ToBase64String(clientSignature);
+
             QrCode = response.AsQRCodeString();
+            IsQrVisible = true;
+            IsAcceptDenyButtonVisible = false;
+            IsScanButtonVisible = true;
+        }
 
-
+        public void OnDenyQrCommand(object obj)
+        {
+            QrCode = null;
+            IsQrVisible = false;
+            IsAcceptDenyButtonVisible = false;
+            IsScanButtonVisible = true;
+            Amount = "";
         }
 
     }

QRBee/QRBee/ViewModels/MerchantPageViewModel.cs

@@ -1,7 +1,9 @@
 using System;
 using System.Linq;
 using System.Net.Http;
+using System.Text;
 using QRBee.Core.Data;
+using QRBee.Core.Security;
 using QRBee.Services;
 using Xamarin.Forms;
 
@@ -9,6 +11,9 @@ namespace QRBee.ViewModels
 {
     internal class MerchantPageViewModel : BaseViewModel
     {
+        private readonly IQRScanner _scanner;
+        private readonly ILocalSettings _settings;
+        private readonly ISecurityService _securityService;
         private bool _isVisible;
         private decimal _amount;
         private string _qrCode;
@@ -16,8 +21,11 @@ namespace QRBee.ViewModels
         public Command GenerateQrCommand { get; }
         public Command ScanCommand{ get; }
 
-        public MerchantPageViewModel()
+        public MerchantPageViewModel(IQRScanner scanner, ILocalSettings settings, ISecurityService securityService)
         {
+            _scanner = scanner;
+            _settings = settings;
+            _securityService = securityService;
             ScanCommand = new Command(OnScanButtonClicked);
             GenerateQrCommand = new Command(OnGenerateQrClicked);
             var localSettings = DependencyService.Resolve<ILocalSettings>();
@@ -28,15 +36,13 @@ namespace QRBee.ViewModels
         {
             try
             {
-                var scanner = DependencyService.Get<IQRScanner>();
+                var result = await _scanner.ScanQR();
-                var result = await scanner.ScanQR();
                 if (result == null) 
                     return;
 
                 var client = new HttpClient(GetInsecureHandler());
-                var localSettings = DependencyService.Resolve<ILocalSettings>();
 
-                var service = new Core.Client.Client(localSettings.QRBeeApiUrl, client);
+                var service = new Core.Client.Client(_settings.QRBeeApiUrl, client);
                 var paymentRequest = PaymentRequest.FromString(result);
 
                 //QrCode = null;
@@ -132,7 +138,10 @@ namespace QRBee.ViewModels
                     Amount = Amount,
                     TimeStampUTC = DateTime.UtcNow
                 };
-                // TODO Create merchant signature.
+
+                var merchantSignature = _securityService.Sign(Encoding.UTF8.GetBytes(trans.AsDataForSignature()));
+                trans.MerchantSignature = Convert.ToBase64String(merchantSignature);
+
                 QrCode = trans.AsQRCodeString();
                 IsVisible = true;
             }

QRBee/QRBee/ViewModels/RegisterViewModel.cs

@@ -12,13 +12,14 @@ namespace QRBee.ViewModels
 {
     internal class RegisterViewModel: BaseViewModel
     {
+        private readonly ILocalSettings _settings;
         private string _password1;
         private string _password2;
-        public RegisterViewModel()
+        public RegisterViewModel(ILocalSettings localSettings)
         {
+            _settings = localSettings;
             RegisterCommand = new Command(OnRegisterClicked);
 
-            var localSettings = DependencyService.Resolve<ILocalSettings>();
             var settings = localSettings.LoadSettings();
 
             Name           = settings.Name;
@@ -96,14 +97,13 @@ namespace QRBee.ViewModels
         {
             //TODO when to dispose the client?
             var client = new HttpClient(GetInsecureHandler());
-            var localSettings = DependencyService.Resolve<ILocalSettings>();
 
-            var service = new Core.Client.Client(localSettings.QRBeeApiUrl,client);
+            var service = new Core.Client.Client(_settings.QRBeeApiUrl,client);
 
             try
             {
                 //TODO Check if ClientId already in LocalSettings. If Yes update data in database
-                var settings = localSettings.LoadSettings();
+                var settings = _settings.LoadSettings();
 
                 //save local settings
                 settings.CardHolderName = CardHolderName;
@@ -117,7 +117,7 @@ namespace QRBee.ViewModels
                 settings.Name           = Name;
                 settings.PIN = Pin;
 
-                await localSettings.SaveSettings(settings);
+                await _settings.SaveSettings(settings);
 
                 var request = new RegistrationRequest
                 {
@@ -132,9 +132,9 @@ namespace QRBee.ViewModels
                     var response = await service.RegisterAsync(request);
 
                     // Save ClientId to LocalSettings
-                    settings = localSettings.LoadSettings();
+                    settings = _settings.LoadSettings();
                     settings.ClientId = response.ClientId;
-                    await localSettings.SaveSettings(settings);
+                    await _settings.SaveSettings(settings);
 
                     var page = Application.Current.MainPage.Navigation.NavigationStack.LastOrDefault();
                     await page.DisplayAlert("Success", "You have been registered successfully", "Ok");

QRBee/QRBee/Views/ClientPage.xaml

@@ -6,11 +6,9 @@
              xmlns:common="clr-namespace:ZXing.Common;assembly=zxing.portable"
              x:DataType="viewmodels:ClientPageViewModel"
              x:Class="QRBee.Views.ClientPage">
+
     <ContentPage.Content>
         <StackLayout HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
-            <!-- <Label Text="Hello this is a test page." -->
-            <!--     VerticalOptions="CenterAndExpand"  -->
-            <!--     HorizontalOptions="CenterAndExpand" /> -->
             <StackLayout Orientation="Vertical" VerticalOptions="FillAndExpand">
                 <StackLayout Orientation="Vertical">
                     <Label VerticalOptions="FillAndExpand" Text="Amount:"/>
@@ -21,7 +19,8 @@
                     BarcodeFormat="QR_CODE"
                     BarcodeValue="{Binding QrCode}"
                     HorizontalOptions="FillAndExpand"
-                    VerticalOptions="FillAndExpand">
+                    VerticalOptions="FillAndExpand"
+                    IsVisible="{Binding IsQrVisible}">
                     <forms:ZXingBarcodeImageView.BarcodeOptions>
                         <common:EncodingOptions Width="300" Height="300" />
                     </forms:ZXingBarcodeImageView.BarcodeOptions>
@@ -31,10 +30,10 @@
             
             <StackLayout Orientation="Vertical" VerticalOptions="End" Margin="0,0,0,10">
                 <StackLayout Orientation="Horizontal">
-                    <Button  Text="Accept" HorizontalOptions="FillAndExpand" BackgroundColor="DarkGreen" IsVisible="{Binding IsVisible}" Command="{Binding GenerateQrCommand}"/>
+                    <Button  Text="Accept" HorizontalOptions="FillAndExpand" BackgroundColor="DarkGreen" IsVisible="{Binding IsAcceptDenyButtonVisible}" Command="{Binding AcceptQrCommand}"/>
-                    <Button  Text="Deny" HorizontalOptions="FillAndExpand" BackgroundColor="DarkRed" IsVisible="{Binding IsVisible}"/>
+                    <Button  Text="Deny" HorizontalOptions="FillAndExpand" BackgroundColor="DarkRed" IsVisible="{Binding IsAcceptDenyButtonVisible}" Command="{Binding DenyQrCommand}"/>
                 </StackLayout>
-                <Button  Text="Scan me" HorizontalOptions="FillAndExpand" BackgroundColor="Aqua" TextColor="Red" Command="{Binding ScanCommand}"/>
+                <Button  Text="Scan merchant QR Code" HorizontalOptions="FillAndExpand" BackgroundColor="Aqua" TextColor="Red" Command="{Binding ScanCommand}" IsVisible="{Binding IsScanButtonVisible}"/>
             </StackLayout>
 
         </StackLayout>

QRBee/QRBee/Views/ClientPage.xaml.cs

@@ -11,8 +11,8 @@ namespace QRBee.Views
     {
         public ClientPage()
         {
+            BindingContext = App.GetViewModel<ClientPageViewModel>();
             InitializeComponent();
-            this.BindingContext = new ClientPageViewModel(this);
         }
     }
 }

QRBee/QRBee/Views/MainPage.xaml

@@ -8,6 +8,4 @@
         <views:ClientPage Title="Client"/>
         <views:MerchantPage Title="Merchant"/>
   </TabbedPage.Children>
-
-  <!-- <ContentPage Title="Merchant" /> -->
 </TabbedPage>

QRBee/QRBee/Views/MerchantPage.xaml.cs

@@ -8,7 +8,7 @@ namespace QRBee.Views
     {
         public MerchantPage()
         {
-            BindingContext = new MerchantPageViewModel();
+            BindingContext = App.GetViewModel<MerchantPageViewModel>();
             InitializeComponent();
         }
     }

QRBee/QRBee/Views/RegisterPage.xaml.cs

@@ -15,7 +15,7 @@ namespace QRBee.Views
         public RegisterPage()
         {
             InitializeComponent();
-            this.BindingContext = new RegisterViewModel();
+            BindingContext = App.GetViewModel<RegisterViewModel>();
         }
     }
 }

QRBeeApi/Services/SecurityService.cs

@@ -1,5 +1,7 @@
-using System.Security.Cryptography;
+using System;
+using System.Security.Cryptography;
 using System.Security.Cryptography.X509Certificates;
+using System.Text;
 using System.Text.RegularExpressions;
 using QRBee.Core.Security;
 
@@ -16,46 +18,70 @@ namespace QRBee.Api.Services
         /// <inheritdoc/>
         public override X509Certificate2 CreateCertificate(string subjectName, byte[] rsaPublicKey)
         {
-            if (!IsValidSubjectName(subjectName))
+            throw new ApplicationException("Client never issues certificates");
-                throw new CryptographicException("Invalid subject name");
-
-            // https://stackoverflow.com/questions/60930065/generate-and-sign-certificate-in-different-machines-c-sharp
-
-            using var publicKey = RSA.Create();
-
-            publicKey.ImportSubjectPublicKeyInfo(rsaPublicKey, out var nBytes);
-            //TODO: check that nBytes is within allowed range
-
-            var request = new CertificateRequest("CN=" + subjectName, publicKey, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1);
-
-            request.CertificateExtensions.Add(new X509BasicConstraintsExtension(false, false, 0, false));
-            request.CertificateExtensions.Add(new X509KeyUsageExtension(X509KeyUsageFlags.DigitalSignature | X509KeyUsageFlags.DataEncipherment | X509KeyUsageFlags.NonRepudiation, false));
-            request.CertificateExtensions.Add(new X509SubjectKeyIdentifierExtension(request.PublicKey, false));
-
-            // create a new certificate
-            using var caPrivateKey = PrivateKeyHandler.LoadPrivateKey();
-            var certificate = request.Create(
-                caPrivateKey,
-                DateTimeOffset.UtcNow.AddSeconds(-1), // user can use it now
-                DateTimeOffset.UtcNow.AddDays(30), // user need to login every 30 days
-                Guid.NewGuid().ToByteArray());
-
-            return certificate;
-
         }
 
+        /// <summary>
+        /// Generate Client certificate request (i.e. without KeyCertSign usage extension)
+        /// </summary>
+        /// <param name="distinguishedName"></param>
+        /// <param name="rsa"></param>
+        /// <returns></returns>
+        private static CertificateRequest CreateRequest(X500DistinguishedName distinguishedName, RSA rsa)
+        {
+            var request = new CertificateRequest(
+                distinguishedName,
+                rsa,
+                HashAlgorithmName.SHA256,
+                RSASignaturePadding.Pkcs1
+            );
+
+            request.CertificateExtensions.Add(
+                new X509KeyUsageExtension(
+                    X509KeyUsageFlags.DataEncipherment
+                    | X509KeyUsageFlags.KeyEncipherment
+                    | X509KeyUsageFlags.DigitalSignature,
+                    //| X509KeyUsageFlags.KeyCertSign,
+                    false));
+
+
+            // request.CertificateExtensions.Add(
+            //     new X509EnhancedKeyUsageExtension(
+            //         new OidCollection { new Oid("1.3.6.1.5.5.7.3.1") }, false));
+            return request;
+        }
+
+        private const string CertHeader = "-----BEGIN CERTIFICATE-----";
+        private const string CertFooter = "-----END CERTIFICATE-----";
+
         /// <inheritdoc/>
         public override X509Certificate2 Deserialize(string pemData)
         {
-            return X509Certificate2.CreateFromPem(pemData);
+            var start = pemData.IndexOf(CertHeader);
+            if (start == -1)
+                throw new ApplicationException("Invalid certificate format");
+            start = start + CertHeader.Length;
+
+            var end = pemData.IndexOf(CertFooter);
+            if (end == -1)
+                throw new ApplicationException("Invalid certificate format");
+
+            var base64 = pemData.Substring(start, end - start); // contains new lines, but it does not matter
+            var data = Convert.FromBase64String(base64);
+            
+            return new X509Certificate2(data);
         }
 
         /// <inheritdoc/>
         public override string Serialize(X509Certificate2 cert)
         {
             // https://stackoverflow.com/questions/43928064/export-private-public-keys-from-x509-certificate-to-pem
-            var pem = PemEncoding.Write("CERTIFICATE", cert.RawData);
+            var builder = new StringBuilder();
-            return new string(pem);
+            builder.AppendLine(CertHeader);
+            builder.AppendLine(Convert.ToBase64String(cert.RawData, Base64FormattingOptions.InsertLineBreaks));
+            builder.AppendLine(CertFooter);
+
+            return builder.ToString();
         }
     }
 

QRBeeApi/Services/ServerPrivateKeyHandler.cs

@@ -0,0 +1,21 @@
+using QRBee.Core.Security;
+
+namespace QRBee.Api.Services
+{
+    /// <summary>
+    /// Private key handler for API server
+    /// </summary>
+    public class ServerPrivateKeyHandler : PrivateKeyHandlerBase
+    {
+        private const string CACommonName = "QRBee-CA";
+        
+        private const string VeryBadAndInsecureCertificatePassword = "U…Š)+œ¶€=ø‘ c¬Í↨ð´áY/ÿ☼æX";
+
+        public ServerPrivateKeyHandler()
+        {
+            CertificatePassword = VeryBadAndInsecureCertificatePassword;
+            CommonName = CACommonName;
+        }
+
+    }
+}