Monday, December 8, 2008

SslStream Class

SslStream Class

Provides a stream used for client-server communication that uses the Secure Socket Layer (SSL) security protocol to authenticate the server and optionally the client.

Namespace: System.Net.Security
Assembly: System (in System.dll)
Visual Basic (Declaration)
Public Class SslStream _
Inherits AuthenticatedStream
Visual Basic (Usage)
Dim instance As SslStream
public class SslStream : AuthenticatedStream
Visual C++
public ref class SslStream : public AuthenticatedStream
public class SslStream extends AuthenticatedStream

SSL protocols help to provide confidentiality and integrity checking for messages transmitted using an SslStream. An SSL connection, such as that provided by SslStream, should be used when communicating sensitive information between a client and a server. Using an SslStream helps to prevent anyone from reading and tampering with information while it is in transit on the network.

An SslStream instance transmits data using a stream that you supply when creating the SslStream. When you supply this underlying stream, you have the option to specify whether closing the SslStream also closes the underlying stream. Typically, the SslStream class is used with the TcpClient and TcpListener classes. The GetStream method provides a NetworkStream suitable for use with the SslStream class.

After creating an SslStream, the server and optionally, the client must be authenticated. The server must provide an X509 certificate that establishes proof of its identity and can request that the client also do so. Authentication must be performed before transmitting information using an SslStream. Clients initiate authentication using the synchronous AuthenticateAsClient methods, which block until the authentication completes, or the asynchronous BeginAuthenticateAsClient methods, which do not block waiting for the authentication to complete. Servers initiate authentication using the synchronous AuthenticateAsServer or asynchronous BeginAuthenticateAsServer methods. Both client and server must initiate the authentication.

The authentication is handled by the Security Support Provider (SSPI) channel provider. The client is given an opportunity to control validation of the server's certificate by specifying a RemoteCertificateValidationCallback delegate when creating an SslStream. The server can also control validation by supplying a RemoteCertificateValidationCallback delegate. The method referenced by the delegate includes the remote party's certificate and any errors SSPI encountered while validating the certificate. Note that if the server specifies a delegate, the delegate's method is invoked regardless of whether the server requested client authentication. If the server did not request client authentication, the server's delegate method receives a null certificate and an empty array of certificate errors.

If the server requires client authentication, the client must specify one or more certificates for authentication. If the client has more than one certificate, the client can provide a LocalCertificateSelectionCallback delegate to select the correct certificate for the server. The client's certificates must be located in the current user's "My" certificate store. Client authentication via certificates is not supported for the Ssl2 (SSL version 2) protocol.

If the authentication fails, you receive a AuthenticationException, and the SslStream is no longer useable. You should close this object and remove all references to it so that it can be collected by the garbage collector.

When the authentication process, also known as the SSL handshake, succeeds, the identity of the server (and optionally, the client) is established and the SslStream can be used by the client and server to exchange messages. Before sending or receiving information, the client and server should check the security services and levels provided by the SslStream to determine whether the protocol, algorithms, and strengths selected meet their requirements for integrity and confidentiality. If the current settings are not sufficient, the stream should be closed. You can check the security services provided by the SslStream using the IsEncrypted and IsSigned properties. The following table shows the elements that report the cryptographic settings used for authentication, encryption and data signing.


The security protocol used to authenticate the server and, optionally, the client.
The SslProtocol property and the associated SslProtocols enumeration.

The key exchange algorithm.
The KeyExchangeAlgorithm property and the associated ExchangeAlgorithmType enumeration.

The message integrity algorithm.
The HashAlgorithm property and the associated HashAlgorithmType enumeration.

The message confidentiality algorithm.
The CipherAlgorithm property and the associated CipherAlgorithmType enumeration.

The strengths of the selected algorithms.
The KeyExchangeStrength, HashStrength, and CipherStrength properties.

After a successful authentication, you can send data using the synchronous Write or asynchronous BeginWrite methods. You can receive data using the synchronous Read or asynchronous BeginRead methods.

If you specified to the SslStream that the underlying stream should be left open, you are responsible for closing that stream when you are done using it.

If the application that creates the SSLStream object runs with the credentials of a Normal user, the application will not be able to access certificates installed in the local machine store unless permission has been explicitly given to the user to do so.

The following code example demonstrates creating an TcpListener that uses the SslStream class to communicate with clients.

C# Copy Code
using System;
using System.Collections;
using System.Net;
using System.Net.Sockets;
using System.Net.Security;
using System.Security.Authentication;
using System.Text;
using System.Security.Cryptography.X509Certificates;
using System.IO;

namespace Examples.System.Net
public sealed class SslTcpServer
static X509Certificate serverCertificate = null;
// The certificate parameter specifies the name of the file
// containing the machine certificate.
public static void RunServer(string certificate)
serverCertificate = X509Certificate.CreateFromCertFile(certificate);
// Create a TCP/IP (IPv4) socket and listen for incoming connections.
TcpListener listener = new TcpListener(IPAddress.Any, 8080);
while (true)
Console.WriteLine("Waiting for a client to connect...");
// Application blocks while waiting for an incoming connection.
// Type CNTL-C to terminate the server.
TcpClient client = listener.AcceptTcpClient();
static void ProcessClient (TcpClient client)
// A client has connected. Create the
// SslStream using the client's network stream.
SslStream sslStream = new SslStream(
client.GetStream(), false);
// Authenticate the server but don't require the client to authenticate.
false, SslProtocols.Tls, true);
// Display the properties and settings for the authenticated stream.

// Set timeouts for the read and write to 5 seconds.
sslStream.ReadTimeout = 5000;
sslStream.WriteTimeout = 5000;
// Read a message from the client.
Console.WriteLine("Waiting for client message...");
string messageData = ReadMessage(sslStream);
Console.WriteLine("Received: {0}", messageData);

// Write a message to the client.
byte[] message = Encoding.UTF8.GetBytes("Hello from the server.");
Console.WriteLine("Sending hello message.");
catch (AuthenticationException e)
Console.WriteLine("Exception: {0}", e.Message);
if (e.InnerException != null)
Console.WriteLine("Inner exception: {0}", e.InnerException.Message);
Console.WriteLine ("Authentication failed - closing the connection.");
// The client stream will be closed with the sslStream
// because we specified this behavior when creating
// the sslStream.
static string ReadMessage(SslStream sslStream)
// Read the message sent by the client.
// The client signals the end of the message using the
// "" marker.
byte [] buffer = new byte[2048];
StringBuilder messageData = new StringBuilder();
int bytes = -1;
// Read the client's test message.
bytes = sslStream.Read(buffer, 0, buffer.Length);

// Use Decoder class to convert from bytes to UTF8
// in case a character spans two buffers.
Decoder decoder = Encoding.UTF8.GetDecoder();
char[] chars = new char[decoder.GetCharCount(buffer,0,bytes)];
decoder.GetChars(buffer, 0, bytes, chars,0);
messageData.Append (chars);
// Check for EOF or an empty message.
if (messageData.ToString().IndexOf("") != -1)
} while (bytes !=0);

return messageData.ToString();
static void DisplaySecurityLevel(SslStream stream)
Console.WriteLine("Cipher: {0} strength {1}", stream.CipherAlgorithm, stream.CipherStrength);
Console.WriteLine("Hash: {0} strength {1}", stream.HashAlgorithm, stream.HashStrength);
Console.WriteLine("Key exchange: {0} strength {1}", stream.KeyExchangeAlgorithm, stream.KeyExchangeStrength);
Console.WriteLine("Protocol: {0}", stream.SslProtocol);
static void DisplaySecurityServices(SslStream stream)
Console.WriteLine("Is authenticated: {0} as server? {1}", stream.IsAuthenticated, stream.IsServer);
Console.WriteLine("IsSigned: {0}", stream.IsSigned);
Console.WriteLine("Is Encrypted: {0}", stream.IsEncrypted);
static void DisplayStreamProperties(SslStream stream)
Console.WriteLine("Can read: {0}, write {1}", stream.CanRead, stream.CanWrite);
Console.WriteLine("Can timeout: {0}", stream.CanTimeout);
static void DisplayCertificateInformation(SslStream stream)
Console.WriteLine("Certificate revocation list checked: {0}", stream.CheckCertRevocationStatus);

X509Certificate localCertificate = stream.LocalCertificate;
if (stream.LocalCertificate != null)
Console.WriteLine("Local cert was issued to {0} and is valid from {1} until {2}.",
} else
Console.WriteLine("Local certificate is null.");
// Display the properties of the client's certificate.
X509Certificate remoteCertificate = stream.RemoteCertificate;
if (stream.RemoteCertificate != null)
Console.WriteLine("Remote cert was issued to {0} and is valid from {1} until {2}.",
} else
Console.WriteLine("Remote certificate is null.");
private static void DisplayUsage()
Console.WriteLine("To start the server specify:");
Console.WriteLine("serverSync certificateFile.cer");
public static int Main(string[] args)
string certificate = null;
if (args == null ||args.Length < 1 )
certificate = args[0];
SslTcpServer.RunServer (certificate);
return 0;

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