SecurityService implemented. Not working due to Android limitations (CertificateRequest).

QRBee.Core/Data/RegistrationRequest.cs

@@ -1,4 +1,6 @@
-namespace QRBee.Core.Data
+using QRBee.Core.Security;
+
+namespace QRBee.Core.Data
 {
     public record RegistrationRequest
     {
@@ -20,7 +22,7 @@
             set;
         }
 
-        public string CertificateRequest
+        public ReadableCertificateRequest CertificateRequest
         {
             get;
             set;

QRBee.Core/Security/IPrivateKeyHandler.cs

@@ -18,13 +18,13 @@ namespace QRBee.Core.Security
         /// </summary>
         /// <param name="subjectName"></param>
         /// <returns>Certificate request to be sent to CA in PEM format</returns>
-        string GeneratePrivateKey(string? subjectName = null);
+        ReadableCertificateRequest GeneratePrivateKey(string? subjectName = null);
 
         /// <summary>
         /// Re-create certificate request if CA response was not received in time.
         /// </summary>
         /// <returns>Certificate request to be sent to CA in PEM format</returns>
-        string CreateCertificateRequest();
+        ReadableCertificateRequest CreateCertificateRequest();
 
         /// <summary>
         /// Attach CA-generated public key certificate to the private key

QRBee.Core/Security/PrivateKeyHandlerBase.cs

@@ -1,39 +0,0 @@
-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.Core/Security/ReadableCertificateRequest.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace QRBee.Core.Security
+{
+    public class ReadableCertificateRequest
+    {
+        private byte[] _rsaPublicKey;
+        private byte[] _signature;
+
+        public string SubjectName { get; set; }
+
+        public string RsaPublicKey
+        {
+            get => _rsaPublicKey != null ? Convert.ToBase64String(_rsaPublicKey): null;
+            set => _rsaPublicKey = value != null ? Convert.FromBase64String(value) : null;
+        }
+
+        public string Signature
+        {
+            get => _signature != null ? Convert.ToBase64String(_signature): null;
+            set => _signature = value != null ? Convert.FromBase64String(value) : null;
+        }
+
+        public string AsDataForSignature() => $"{SubjectName}|{RsaPublicKey}";
+
+    }
+}

QRBee/QRBee.Android/MainActivity.cs

@@ -55,6 +55,7 @@ namespace QRBee.Droid
                 .AddSingleton<ISecurityService,AndroidSecurityService>()
                 .AddSingleton<ILocalSettings, LocalSettings>()
                 .AddSingleton<IQRScanner, QRScannerService>()
+                .AddSingleton<IPrivateKeyHandler, AndroidPrivateKeyHandler>()
                 ;
         }
 

QRBee/QRBee.Android/QRBee.Android.csproj

@@ -82,6 +82,7 @@
     <Compile Include="Resources\Resource.designer.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Services\LocalSettings.cs" />
+    <Compile Include="Services\AndroidPrivateKeyHandler.cs" />
     <Compile Include="Services\QRScannerService.cs" />
     <Compile Include="Services\AndroidSecurityService.cs" />
   </ItemGroup>

QRBee/QRBee.Android/Services/AndroidPrivateKeyHandler.cs

@@ -0,0 +1,162 @@
+using System;
+using System.IO;
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+using System.Text;
+using Javax.Xml.Transform;
+using QRBee.Core.Security;
+
+namespace QRBee.Droid.Services
+{
+    /// <summary>
+    /// Private key handler for API server
+    /// </summary>
+    public class AndroidPrivateKeyHandler : IPrivateKeyHandler
+    {
+        private X509Certificate2? _certificate;
+        private readonly object   _syncObject = new object();
+
+        private const string FileName             = "private_key.p12";
+        protected string CommonName { get; set; }
+        private const int RSABits                 = 2048;
+        private const int CertificateValidityDays = 3650;
+      
+        protected string CertificatePassword { get; set; }
+
+        private string PrivateKeyFileName => $"{System.Environment.SpecialFolder.LocalApplicationData}/{FileName}";
+
+        /// <inheritdoc/>
+        public bool Exists()
+            => File.Exists(PrivateKeyFileName);
+
+        /// <inheritdoc/>
+        public ReadableCertificateRequest GeneratePrivateKey(string? subjectName)
+        {
+            // locking used to make sure that only one thread generating a private key
+            lock (_syncObject)
+            {
+                var pk = CreateSelfSignedClientCertificate(subjectName ?? CommonName);
+                var pkcs12data = pk.Export(X509ContentType.Pfx, CertificatePassword);
+                File.WriteAllBytes(PrivateKeyFileName, pkcs12data);
+
+                _certificate?.Dispose();
+                _certificate = new X509Certificate2(pkcs12data, CertificatePassword);
+            }
+
+            return CreateCertificateRequest();
+        }
+
+        /// <inheritdoc/>
+        public ReadableCertificateRequest CreateCertificateRequest()
+        {
+            var pk = LoadPrivateKey();
+            var rsa = pk.GetRSAPublicKey();
+
+            var request = new ReadableCertificateRequest
+            {
+                RsaPublicKey = Convert.ToBase64String(rsa.ExportRSAPublicKey()),
+                SubjectName = pk.SubjectName.Name
+            };
+            var data = Encoding.UTF8.GetBytes(request.AsDataForSignature());
+
+            //We can't use SecurityService here because it uses this class. This creates cyclic dependency.
+            var signature = rsa?.SignData(data, 0, data.Length, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1) ?? throw new CryptographicException("No private key found");
+            request.Signature = Convert.ToBase64String(signature);
+            return request;
+        }
+
+        /// <summary>
+        /// Generate EXPORTABLE certificate
+        /// </summary>
+        /// <param name="subjectName"></param>
+        /// <returns></returns>
+        private X509Certificate2 CreateSelfSignedClientCertificate(string subjectName)
+        {
+            // https://stackoverflow.com/questions/42786986/how-to-create-a-valid-self-signed-x509certificate2-programmatically-not-loadin
+
+            var distinguishedName = new X500DistinguishedName($"CN={subjectName}");
+
+            using var rsa = RSA.Create(RSABits);
+            var request = CreateRequest(distinguishedName, rsa);
+
+            var certificate = request.CreateSelfSigned(
+                new DateTimeOffset(DateTime.UtcNow.AddDays(-1)),
+                new DateTimeOffset(DateTime.UtcNow.AddDays(CertificateValidityDays))
+                );
+
+            return certificate;
+        }
+
+        /// <summary>
+        /// Generate CA certificate request (i.e. with KeyCertSign usage extension)
+        /// </summary>
+        /// <param name="distinguishedName"></param>
+        /// <param name="rsa"></param>
+        /// <returns></returns>
+        private static CertificateRequest CreateRequest(X500DistinguishedName distinguishedName, RSA rsa)
+        {
+            //TODO not supported on Android
+            var request = new CertificateRequest(
+                distinguishedName,
+                rsa,
+                HashAlgorithmName.SHA256,
+                RSASignaturePadding.Pkcs1
+                );
+
+            request.CertificateExtensions.Add(
+                new X509KeyUsageExtension(
+                      X509KeyUsageFlags.DataEncipherment
+                    | X509KeyUsageFlags.KeyEncipherment
+                    | X509KeyUsageFlags.DigitalSignature,
+            false));
+
+            return request;
+        }
+
+        /// <inheritdoc/>
+        public X509Certificate2 LoadPrivateKey()
+        {
+            if (_certificate != null)
+                return _certificate;
+
+            // double locking 
+            lock ( _syncObject )
+            {
+                if (_certificate != null)
+                    return _certificate;
+
+                if (!Exists())
+                    throw new CryptographicException("PrivateKey does not exist");
+
+                _certificate = new X509Certificate2(PrivateKeyFileName, CertificatePassword);
+                return _certificate;
+            }
+        }
+
+        /// <inheritdoc/>
+        public void AttachCertificate(X509Certificate2 cert)
+        {
+            // heavily modified version of:
+            // https://stackoverflow.com/questions/18462064/associate-a-private-key-with-the-x509certificate2-class-in-net
+
+            // we can't use LoadPrivateKey here as it creating non-exportable key
+            var pk = new X509Certificate2(PrivateKeyFileName, CertificatePassword, X509KeyStorageFlags.Exportable);
+            using var rsa = pk.GetRSAPrivateKey();
+            if (rsa == null)
+                throw new CryptographicException("Can't get PrivateKey");
+
+            var newPk = cert.CopyWithPrivateKey(rsa);
+
+            var pkcs12data = newPk.Export(X509ContentType.Pfx, CertificatePassword);
+            File.WriteAllBytes(PrivateKeyFileName, pkcs12data);
+
+            lock ( _syncObject )
+            {
+                _certificate?.Dispose();
+                _certificate = null;
+                // it will be loaded on the next access
+            }
+        }
+
+    }
+}

QRBee/QRBee/App.xaml.cs

@@ -30,9 +30,7 @@ namespace QRBee
             addPlatformServices?.Invoke(services);
 
             // TODO: Add core services here
-            services
+            
-                .AddSingleton<IPrivateKeyHandler, ClientPrivateKeyHandler>()
-                ;
 
             // Add ViewModels
             services

QRBee/QRBee/Services/ClientPrivateKeyHandler.cs

@@ -1,23 +0,0 @@
-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/RegisterViewModel.cs

@@ -4,6 +4,7 @@ using System.Net.Http;
 using System.Text.RegularExpressions;
 using QRBee.Core;
 using QRBee.Core.Data;
+using QRBee.Core.Security;
 using QRBee.Services;
 using QRBee.Views;
 using Xamarin.Forms;
@@ -13,15 +14,18 @@ namespace QRBee.ViewModels
     internal class RegisterViewModel: BaseViewModel
     {
         private readonly ILocalSettings _settings;
+        private readonly IPrivateKeyHandler _privateKeyHandler;
+        private readonly ISecurityService _securityService;
         private string _password1;
         private string _password2;
-        public RegisterViewModel(ILocalSettings localSettings)
+        public RegisterViewModel(ILocalSettings localSettings, IPrivateKeyHandler privateKeyHandler, ISecurityService securityService)
         {
             _settings = localSettings;
+            _privateKeyHandler = privateKeyHandler;
+            _securityService = securityService;
             RegisterCommand = new Command(OnRegisterClicked);
 
             var settings = localSettings.LoadSettings();
-
             Name           = settings.Name;
             Email          = settings.Email;
             DateOfBirth    = settings.DateOfBirth;
@@ -115,15 +119,21 @@ namespace QRBee.ViewModels
                 settings.IssueNo        = IssueNo;
                 settings.ValidFrom      = ValidFrom;
                 settings.Name           = Name;
-                settings.PIN = Pin;
+                settings.PIN            = Pin;
 
                 await _settings.SaveSettings(settings);
 
+                if (!_privateKeyHandler.Exists())
+                {
+                    _privateKeyHandler.GeneratePrivateKey(settings.Name);
+                }
+
                 var request = new RegistrationRequest
                 {
                     DateOfBirth = DateOfBirth.ToString("yyyy-MM-dd"),
                     Email = Email,
                     Name = Name,
+                    CertificateRequest = _privateKeyHandler.CreateCertificateRequest(),
                     RegisterAsMerchant = false
                 };
 
@@ -136,6 +146,9 @@ namespace QRBee.ViewModels
                     settings.ClientId = response.ClientId;
                     await _settings.SaveSettings(settings);
 
+                    // Attach certificate to privateKey (replace self-sighed with server issued certificate)
+                    _privateKeyHandler.AttachCertificate(_securityService.Deserialize(response.Certificate));
+
                     var page = Application.Current.MainPage.Navigation.NavigationStack.LastOrDefault();
                     await page.DisplayAlert("Success", "You have been registered successfully", "Ok");
                 }

QRBeeApi/Program.cs

@@ -4,6 +4,7 @@ using MongoDB.Driver;
 using QRBee.Api;
 using QRBee.Api.Services;
 using QRBee.Api.Services.Database;
+using QRBee.Core.Security;
 
 var builder = WebApplication.CreateBuilder(args);
 
@@ -24,10 +25,12 @@ builder.Services.AddEndpointsApiExplorer();
 builder.Services.AddSwaggerGen();
 
 builder.Services
-    .AddSingleton<IQRBeeAPI,QRBeeAPI>()
+    .AddSingleton<IQRBeeAPI,QRBeeAPIService>()
     .AddSingleton<IStorage, Storage>()
     .Configure<DatabaseSettings>(builder.Configuration.GetSection("QRBeeDatabase"))
     .AddSingleton<IMongoClient>( cfg => new MongoClient(cfg.GetRequiredService<IOptions<DatabaseSettings>>().Value.ToMongoDbSettings()))
+    .AddSingleton<IPrivateKeyHandler, ServerPrivateKeyHandler>()
+    .AddSingleton<ISecurityService, SecurityService>()
     ;
 
 var app = builder.Build();

QRBeeApi/Services/IQRBeeAPI.cs

@@ -5,7 +5,7 @@ using QRBee.Core.Data;
 namespace QRBee.Api.Services
 {
     /// <summary>
-    /// QRBeeAPI interface
+    /// QRBeeAPIService interface
     /// </summary>
     public interface IQRBeeAPI
     {

QRBeeApi/Services/QRBeeAPIService.cs

@@ -1,23 +1,43 @@
 using System.Globalization;
+using System.Text;
 using System.Text.RegularExpressions;
 using QRBee.Api.Services.Database;
 using QRBee.Core;
 using QRBee.Core.Data;
+using QRBee.Core.Security;
 
 namespace QRBee.Api.Services
 {
     /// <summary>
     /// Implementation of <see href="IQRBeeAPI"/>
     /// </summary>
-    public class QRBeeAPI: IQRBeeAPI
+    public class QRBeeAPIService: IQRBeeAPI
     {
         private readonly IStorage _storage;
+        private readonly ISecurityService _securityService;
+        private readonly IPrivateKeyHandler _privateKeyHandler;
+        private static readonly object _lock = new ();
+
         private const int MaxNameLength = 512;
         private const int MaxEmailLength = 512;
 
-        public QRBeeAPI(IStorage storage)
+        public QRBeeAPIService(IStorage storage, ISecurityService securityService, IPrivateKeyHandler privateKeyHandler)
         {
             _storage = storage;
+            _securityService = securityService;
+            _privateKeyHandler = privateKeyHandler;
+            Init(_privateKeyHandler);
+        }
+
+        private static void Init(IPrivateKeyHandler privateKeyHandler)
+        {
+            lock (_lock)
+            {
+                if (!privateKeyHandler.Exists())
+                {
+                    privateKeyHandler.GeneratePrivateKey();
+                }
+            }
         }
 
         public async Task<RegistrationResponse> Register(RegistrationRequest request)
@@ -28,8 +48,14 @@ namespace QRBee.Api.Services
             var info = Convert(request);
 
             var clientId = await _storage.PutUserInfo(info);
+            var clientCertificate =  _securityService.CreateCertificate(clientId,System.Convert.FromBase64String(request.CertificateRequest.RsaPublicKey));
+            //TODO save certificate to certificate mongoDB collection
 
-            return new RegistrationResponse{ClientId = clientId};
+            return new RegistrationResponse
+            {
+                ClientId = clientId,
+                Certificate = _securityService.Serialize(clientCertificate)
+            };
         }
 
         public Task Update(string clientId, RegistrationRequest request)
@@ -45,7 +71,7 @@ namespace QRBee.Api.Services
             return _storage.PutTransactionInfo(info);
         }
 
-        private static void Validate(RegistrationRequest request)
+        private void Validate(RegistrationRequest request)
         {
             if (request == null)
             {
@@ -55,6 +81,7 @@ namespace QRBee.Api.Services
             var name = request.Name;
             var email = request.Email;
             var dateOfBirth = request.DateOfBirth;
+            var certificateRequest = request.CertificateRequest;
 
             if (string.IsNullOrEmpty(name) || name.All(char.IsLetter)==false || name.Length>=MaxNameLength)
             {
@@ -76,6 +103,15 @@ namespace QRBee.Api.Services
                 throw new ApplicationException($"DateOfBirth \"{dateOfBirth}\" isn't valid");
             }
 
+            //Check digital signature
+            var verified = _securityService.Verify(
+                Encoding.UTF8.GetBytes(certificateRequest.AsDataForSignature()),
+                Encoding.UTF8.GetBytes(certificateRequest.Signature),
+                _privateKeyHandler.LoadPrivateKey());
+            if (!verified)
+            {
+                throw new ApplicationException($"Digital signature is not valid.");
+            }
         }
 
         private static UserInfo Convert(RegistrationRequest request)

QRBeeApi/Services/SecurityService.cs

@@ -10,6 +10,10 @@ namespace QRBee.Api.Services
     internal class SecurityService : SecurityServiceBase
     {
 
+        private const int CertificateValidityPeriodDays = 365;
+        private const string CertHeader = "-----BEGIN CERTIFICATE-----";
+        private const string CertFooter = "-----END CERTIFICATE-----";
+
         public SecurityService(IPrivateKeyHandler privateKeyHandler)
         : base(privateKeyHandler)
         {
@@ -18,7 +22,20 @@ namespace QRBee.Api.Services
         /// <inheritdoc/>
         public override X509Certificate2 CreateCertificate(string subjectName, byte[] rsaPublicKey)
         {
-            throw new ApplicationException("Client never issues certificates");
+            using var rsa = new RSACryptoServiceProvider();
+            rsa.ImportRSAPublicKey(rsaPublicKey, out _);
+
+            var distinguishedName = new X500DistinguishedName($"CN={subjectName}");
+            var req = CreateClientCertRequest(distinguishedName, rsa);
+
+            var pk = PrivateKeyHandler.LoadPrivateKey();
+            var clientCert = req.Create(pk,
+                DateTimeOffset.UtcNow - TimeSpan.FromDays(1),
+                DateTimeOffset.UtcNow + TimeSpan.FromDays(CertificateValidityPeriodDays),
+                Guid.NewGuid()
+                    .ToByteArray());
+
+            return clientCert;
         }
 
         /// <summary>
@@ -27,7 +44,7 @@ namespace QRBee.Api.Services
         /// <param name="distinguishedName"></param>
         /// <param name="rsa"></param>
         /// <returns></returns>
-        private static CertificateRequest CreateRequest(X500DistinguishedName distinguishedName, RSA rsa)
+        private static CertificateRequest CreateClientCertRequest(X500DistinguishedName distinguishedName, RSA rsa)
         {
             var request = new CertificateRequest(
                 distinguishedName,
@@ -41,19 +58,11 @@ namespace QRBee.Api.Services
                     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)
         {

QRBeeApi/Services/ServerPrivateKeyHandler.cs

@@ -1,20 +1,180 @@
-using QRBee.Core.Security;
+using System.Security.Cryptography;
+using System.Security.Cryptography.X509Certificates;
+using System.Text;
+using QRBee.Core.Security;
 
 namespace QRBee.Api.Services
 {
     /// <summary>
     /// Private key handler for API server
     /// </summary>
-    public class ServerPrivateKeyHandler : PrivateKeyHandlerBase
+    public class ServerPrivateKeyHandler : IPrivateKeyHandler
     {
-        private const string CACommonName = "QRBee-CA";
+        private X509Certificate2? _certificate;
+        private readonly object   _syncObject = new object();
 
-        private const string VeryBadAndInsecureCertificatePassword = "U…Š)+œ¶€=ø‘ c¬Í↨ð´áY/ÿ☼æX";
+        private const string FileName             = "private_key.p12";
+        protected string CommonName { get; set; }
+        private const int RSABits                 = 2048;
+        private const int CertificateValidityDays = 3650;
       
-        public ServerPrivateKeyHandler()
+        protected string CertificatePassword { get; set; }
+
+        private string PrivateKeyFileName => $"{System.Environment.SpecialFolder.LocalApplicationData}/{FileName}";
+
+        /// <inheritdoc/>
+        public bool Exists()
+            => File.Exists(PrivateKeyFileName);
+
+        /// <inheritdoc/>
+        public ReadableCertificateRequest GeneratePrivateKey(string? subjectName)
         {
-            CertificatePassword = VeryBadAndInsecureCertificatePassword;
+            // locking used to make sure that only one thread generating a private key
-            CommonName = CACommonName;
+            lock (_syncObject)
+            {
+                var pk = CreateSelfSignedServerCertificate(subjectName ?? CommonName);
+                var pkcs12data = pk.Export(X509ContentType.Pfx, CertificatePassword);
+                File.WriteAllBytes(PrivateKeyFileName, pkcs12data);
+
+                _certificate?.Dispose();
+                _certificate = new X509Certificate2(pkcs12data, CertificatePassword);
+            }
+
+            return CreateCertificateRequest();
+        }
+
+        /// <inheritdoc/>
+        public ReadableCertificateRequest CreateCertificateRequest()
+        {
+            //TODO in fact server should create certificate request in standard format if we ever want to get externally sighed certificate.
+            var pk = LoadPrivateKey();
+            var rsa = pk.GetRSAPublicKey();
+
+            var request = new ReadableCertificateRequest
+            {
+                RsaPublicKey = Convert.ToBase64String(rsa.ExportRSAPublicKey()),
+                SubjectName = pk.SubjectName.Name
+            };
+            var data = Encoding.UTF8.GetBytes(request.AsDataForSignature());
+
+            //We can't use SecurityService here because it uses this class. This creates cyclic dependency.
+            var signature = rsa?.SignData(data, 0, data.Length, HashAlgorithmName.SHA256, RSASignaturePadding.Pkcs1) ?? throw new CryptographicException("No private key found");
+            request.Signature = Convert.ToBase64String(signature);
+            return request;
+        }
+
+        //private static string AsCsr(CertificateRequest request)
+        //{
+        //    // https://stackoverflow.com/questions/65943968/how-to-convert-a-csr-text-file-into-net-core-standard-certificaterequest-for-s
+
+        //    var encoded = request.CreateSigningRequest();
+        //    var payload = Convert.ToBase64String(encoded, Base64FormattingOptions.InsertLineBreaks);
+        //    using var stream = new MemoryStream();
+        //    using (var writer = new StreamWriter(stream, System.Text.Encoding.UTF8, 512, true))
+        //    {
+        //        writer.WriteLine("-----BEGIN CERTIFICATE REQUEST-----");
+        //        writer.WriteLine(payload);
+        //        writer.WriteLine("-----END CERTIFICATE REQUEST-----");
+        //        writer.Flush();
+        //    }
+
+        //    stream.Position = 0;
+        //    using (var reader = new StreamReader(stream))
+        //    {
+        //        return reader.ReadToEnd();
+        //    }
+        //}
+
+        /// <summary>
+        /// Generate EXPORTABLE certificate
+        /// </summary>
+        /// <param name="subjectName"></param>
+        /// <returns></returns>
+        private X509Certificate2 CreateSelfSignedServerCertificate(string subjectName)
+        {
+            // https://stackoverflow.com/questions/42786986/how-to-create-a-valid-self-signed-x509certificate2-programmatically-not-loadin
+
+            var distinguishedName = new X500DistinguishedName($"CN={subjectName}");
+
+            using RSA rsa = RSA.Create(RSABits);
+            var request = CreateClientCertificateRequest(distinguishedName, rsa);
+
+            var certificate = request.CreateSelfSigned(
+                new DateTimeOffset(DateTime.UtcNow.AddDays(-1)),
+                new DateTimeOffset(DateTime.UtcNow.AddDays(CertificateValidityDays))
+            );
+
+            return certificate;
+        }
+
+        /// <summary>
+        /// Generate CA certificate request (i.e. with KeyCertSign usage extension)
+        /// </summary>
+        /// <param name="distinguishedName"></param>
+        /// <param name="rsa"></param>
+        /// <returns></returns>
+        private static CertificateRequest CreateClientCertificateRequest(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,
+            false));
+
+            return request;
+        }
+
+        /// <inheritdoc/>
+        public X509Certificate2 LoadPrivateKey()
+        {
+            if (_certificate != null)
+                return _certificate;
+
+            // double locking 
+            lock ( _syncObject )
+            {
+                if (_certificate != null)
+                    return _certificate;
+
+                if (!Exists())
+                    throw new CryptographicException("PrivateKey does not exist");
+
+                _certificate = new X509Certificate2(PrivateKeyFileName, CertificatePassword);
+                return _certificate;
+            }
+        }
+
+        /// <inheritdoc/>
+        public void AttachCertificate(X509Certificate2 cert)
+        {
+            // heavily modified version of:
+            // https://stackoverflow.com/questions/18462064/associate-a-private-key-with-the-x509certificate2-class-in-net
+
+            // we can't use LoadPrivateKey here as it creating non-exportable key
+            var pk = new X509Certificate2(PrivateKeyFileName, CertificatePassword, X509KeyStorageFlags.Exportable);
+            using var rsa = pk.GetRSAPrivateKey();
+            if (rsa == null)
+                throw new CryptographicException("Can't get PrivateKey");
+
+            var newPk = cert.CopyWithPrivateKey(rsa);
+
+            var pkcs12data = newPk.Export(X509ContentType.Pfx, CertificatePassword);
+            File.WriteAllBytes(PrivateKeyFileName, pkcs12data);
+
+            lock ( _syncObject )
+            {
+                _certificate?.Dispose();
+                _certificate = null;
+                // it will be loaded on the next access
+            }
         }
 
     }