Using SSL and SslStream for peer to peer authentication?

Question

I need to provide secure communication between various processes that are using TCP/IP sockets for communication. I want both authentication and encryption. Rather than re-invent the wheel I would really like to use SSL and the SslStream class and self-signed certificates. What I want to do is validate the remote process's certificate against a known copy in my local application. (There doesn't need to be a certificate authority because I intend for the certificates to be copied around manually).

To do this, I want the application to be able to automatically generate a new certifiate the first time it is run. In addition to makecert.exe, it looks like this link shows a way to automatically generate self-signed certificates, so that's a start.

I've looked at the AuthenticateAsServer and AuthenticateAsClient methods of SslStream. You can provide call-backs for verification, so it looks like it's possible. But now that I'm into the details of it, I really don't think it's possible to do this.

Am I going in the right direction? Is there a better alternative? Has anyone done anything like this before (basically peer-to-peer SSL rather than client-server)?

Answer 1

Step 1: Generating a self-signed certificate:

• I downloaded the Certificate.cs class posted by Doug Cook

• I used this code to generate a .pfx certificate file:

  byte[] c = Certificate.CreateSelfSignCertificatePfx(
          "CN=yourhostname.com", //host name
          DateTime.Parse("2000-01-01"), //not valid before
          DateTime.Parse("2010-01-01"), //not valid after
          "mypassword"); //password to encrypt key file
  
      using (BinaryWriter binWriter = new BinaryWriter(
          File.Open(@"testcert.pfx", FileMode.Create)))
      {
          binWriter.Write(c);
      }
  

Step 2: Loading the certificate

    X509Certificate cert = new X509Certificate2(
                            @"testcert.pfx", 
                            "mypassword");

Step 3: Putting it together

• I based it on this very simple SslStream example
• You will get a compile time error about the SslProtocolType enumeration. Just change that from SslProtocolType.Default to SslProtocols.Default
• There were 3 warnings about deprecated functions. I replaced them all with the suggested replacements.

• I replaced this line in the Server Program.cs file with the line from Step 2:

  X509Certificate cert = getServerCert();

• In the Client Program.cs file, make sure you set serverName = yourhostname.com (and that it matches the name in the certificate)

• In the Client Program.cs, the CertificateValidationCallback function fails because sslPolicyErrors contains a RemoteCertificateChainErrors. If you dig a little deeper, this is because the issuing authority that signed the certificate is not a trusted root.
• I don`t want to get into having the user import certificates into the root store, etc., so I made a special case for this, and I check that certificate.GetPublicKeyString() is equal to the public key that I have on file for that server. If it matches, I return True from that function. That seems to work.

Step 4: Client Authentication

Here's how my client authenticates (it's a little different than the server):

TcpClient client = new TcpClient();
client.Connect(hostName, port);

SslStream sslStream = new SslStream(client.GetStream(), false,
    new RemoteCertificateValidationCallback(CertificateValidationCallback),
    new LocalCertificateSelectionCallback(CertificateSelectionCallback));

bool authenticationPassed = true;
try
{
    string serverName = System.Environment.MachineName;

    X509Certificate cert = GetServerCert(SERVER_CERT_FILENAME, SERVER_CERT_PASSWORD);
    X509CertificateCollection certs = new X509CertificateCollection();
    certs.Add(cert);

    sslStream.AuthenticateAsClient(
        serverName,
        certs,
        SslProtocols.Default,
        false); // check cert revokation
}
catch (AuthenticationException)
{
    authenticationPassed = false;
}
if (authenticationPassed)
{
    //do stuff
}

The CertificateValidationCallback is the same as in the server case, but note how AuthenticateAsClient takes a collection of certificates, not just one certificate. So, you have to add a LocalCertificateSelectionCallback, like this (in this case, I only have one client cert so I just return the first one in the collection):

static X509Certificate CertificateSelectionCallback(object sender,
    string targetHost,
    X509CertificateCollection localCertificates,
    X509Certificate remoteCertificate,
    string[] acceptableIssuers)
{
    return localCertificates[0];
}