Validating and examining server certificate 

When connecting securely, you should ensure you are really connected to the desired server. TLS/SSL provides it with a security certificate of the server. On the client side you should just verify that the server certificate of the current connection really comes from the server you want to connect to. There are following ways, how to do it using the Rebex HTTPS:

• By default, the server certificate is validated against Windows certificate infrastructure.
• If you cannot rely on the certificate can be validated against your Windows certificate infrastructure, you can write your own certificate validation.
• The certificate validation can be simply skipped by setting the SslAcceptAllCertificates property:
  • C#
  • VB.NET

  // create client instance (Rebex.Net.WebClient or Rebex.Net.HttpRequestCreator)
  // ...
  
  // skip certificate validation (don't do this in production environment!)
  client.Settings.SslAcceptAllCertificates = true;
  

  ' create client instance (Rebex.Net.WebClient or Rebex.Net.HttpRequestCreator)
  ' ...
  
  ' skip certificate validation (don't do this in production environment!)
  client.Settings.SslAcceptAllCertificates = True
  

Built-in certificate verifier 

When calling the Connect method and no custom validation is configured, the server certificate is validated using Windows CryptoAPI.

See what to do when a server certificate is not trusted.

Legacy Windows CE platforms don't natively support certificates signed using algorithms based on SHA-2 hashes. As a workaround for this major OS limitation, we introduced a built-in certificate validator in the 2016 R3 release.

Custom certificate validation 

If you cannot relay on the certificate can be trusted by the Windows certificate infrastructure, you can implement your own validation:

Register an event handler before calling the Connect method

// create client instance (Rebex.Net.WebClient or Rebex.Net.HttpRequestCreator)
// ...

// register custom validation event handler
client.ValidatingCertificate += client_ValidatingCertificate;

' create client instance (Rebex.Net.WebClient or Rebex.Net.HttpRequestCreator)
' ...

' register custom validation event handler
AddHandler client.ValidatingCertificate, AddressOf client_ValidatingCertificate

Implement the custom validation within the event handler

public void client_ValidatingCertificate(object sender, SslCertificateValidationEventArgs e)
{
    // first try to use the default validation (against the Windows certificate store)
    ValidationResult res = e.CertificateChain.Validate(e.ServerName, 0);
    if (res.Valid)
    {
        e.Accept();
        return;
    }

    // get server certificate of the current connection
    Certificate cert = e.CertificateChain[0];

    // check the certificate is already known
    if (trustedThumbprints.Contains(cert.Thumbprint))
    {
        e.Accept();
        return;
    }

    // the certificate is not know yet - show it to user
    Console.WriteLine("Do you trust to the following certificate?");
    Console.WriteLine(" Common name: {0}", cert.GetCommonName());
    Console.WriteLine(" Thumbprint:  {0}", cert.Thumbprint);
    Console.WriteLine(" Expires on:  {0:d}", cert.GetExpirationDate());

    // ask user for the answer
    // ...

    // accept or reject the certificate
    if (userAnswer)
    {
        e.Accept();

        // OPTIONALY store the current certificate as trusted
        trustedThumbprints.Add(cert.Thumbprint);
    }
    else
    {
        e.Reject();
    }
}

Public Sub client_ValidatingCertificate(sender As Object, e As SslCertificateValidationEventArgs)
    ' first try to use the default validation (against the Windows certificate store)
    Dim res As ValidationResult = e.CertificateChain.Validate(e.ServerName, 0)
    If res.Valid Then
        e.Accept()
        Return
    End If

    ' get server certificate of the current connection
    Dim cert As Certificate = e.CertificateChain(0)

    ' check the certificate is already known
    If trustedThumbprints.Contains(cert.Thumbprint) Then
        e.Accept()
        Return
    End If

    ' the certificate is not know yet - show it to user
    Console.WriteLine("Do you trust to the following certificate?")
    Console.WriteLine(" Common name: {0}", cert.GetCommonName())
    Console.WriteLine(" Thumbprint:  {0}", cert.Thumbprint)
    Console.WriteLine(" Expires on:  {0:d}", cert.GetExpirationDate())

    ' ask user for the answer
    ' ...

    ' accept or reject the certificate
    If userAnswer Then
        e.Accept()

        ' OPTIONALY store the current certificate as trusted
        trustedThumbprints.Add(cert.Thumbprint)
    Else
        e.Reject()
    End If
End Sub

Client certificate authentication 

Client certificates are an optional way to authenticate the client to the server. This is only possible when connecting/authenticating to a TLS/SSL-capable HTTP server.

A certificate with an associated private key is needed for client authentication. Set Settings.SslClientCertificateRequestHandler property to an implementation of certificate request handler that is called when the HTTP server asks for client certificate.

a) Use the built-in StoreSearch handler, that searches the user's certificate store for a first suitable certificate:

// set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = CertificateRequestHandler.StoreSearch;

' set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = CertificateRequestHandler.StoreSearch

b) Use the built-in PFX-based certificate request handler:

// load a certificate chain from a .P12 or .PFX file
CertificateChain certificate = CertificateChain.LoadPfx("mycert.p12", "password");

// set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = CertificateRequestHandler.CreateRequestHandler(certificate);

' load a certificate chain from a .P12 or .PFX file
Dim certificate As CertificateChain = CertificateChain.LoadPfx("mycert.p12", "password")

' set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = CertificateRequestHandler.CreateRequestHandler(certificate)

c) Write a custom handler, for example to load the certificate from a .pfx/.p12 file:

private class MyCertRequestHandler : ICertificateRequestHandler
{
    // This method is called during TLS/SSL negotiation
    // when the server requests client certificate authentication
    public CertificateChain Request(TlsSocket socket, DistinguishedName[] issuers)
    {
        // provide a certificate loaded from a .pfx/.p12 file
        return CertificateChain.LoadPfx(clientCertPath, clientCertPassword);
    }
}

Private Class MyCertRequestHandler
    Implements ICertificateRequestHandler
    ' This method is called during TLS/SSL negotiation
    ' when the server requests client certificate authentication
    Public Function Request(socket As TlsSocket, issuers As DistinguishedName()) _
                        As CertificateChain _
                        Implements ICertificateRequestHandler.Request
        ' provide a certificate loaded from a .pfx/.p12 file
        Return CertificateChain.LoadPfx(clientCertPath, clientCertPassword)
    End Function
End Class

Don't forget to register the handler:

// set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = new MyCertRequestHandler();

' set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = New MyCertRequestHandler()

Overriding SSL server name 

Server certificate's common name should specify the server hostname - this ensures that the server presents a certificate that actually belongs to it. If the names are mismatched, you can still work around it.

Security settings and algorithms 

When connecting with Rebex HTTPS, you can choose from a number of security algorithms and standards.

TLS/SSL cipher suites

The Settings.SslAllowedSuites property (on the client object) makes it possible to specify a combination of following algorithms:

TLS/SSL extensions

Rebex HTTPS supports Server Name Identification (SNI) extension and Renegotiation Indication extension.

TLS/SSL elliptic curves

The Settings.SslAllowedCurves property (on client object) makes it possible to specify a set of enabled elliptic curves used by ECDHE_RSA ciphers above. Supported elliptic curves:

* These curves require a plugin on non-Windows and legacy Windows platforms.

** These curves require a plugin on non-Windows platforms and on Windows earlier than Windows 10 / Windows Server 2016.

*** These curves require a plugin.

TLS/SSL protocol versions

Use Settings.SslAllowedVersions property (on client object) to enable or disable specific versions of the TLS/SSL protocol. Supported versions:

• TLS 1.2
• TLS 1.1
• TLS 1.0
• SSL 3.0

Certificate validation options

Use Settings.SslServerCertificateValidationOptions property to specify various options for the build-in certificate verifier. For example:

• IgnoreCnNotMatch (Ignore common name mismatch)
• SkipRevocationCheck (Skip revocation check)
• IgnoreCnNotMatch (Ignore invalid common name)
• IgnoreWrongUsage (Ignore wrong usage)
• IgnoreTimeNotNested (Ignore improperly nested validity periods)
• AllowUnknownCa (Allow unknown certification authority)