Friday, June 27, 2008

The basics of secure data exchange under TCP

Doing data exchange in plain text is very convenient and easy to implement but what can you do to prevent eavesdropping, tampering, and message forgery of the data you send back and forth? Here's where secure communication comes into play. At present the most common secure communication method is using Transport Layer Security (TLS) or Secure Sockets Layer (SSL). In web context you can see secure data exchange in action when browsing web-sites with HTTPS prefix

In .NET framework secure communications can be done with SslStream class. It can use both TLS and SSL protocols.

TLS and SSL for authentication process use public key infrastructure or PKI. It requires certificates.

Here's nice explanation how to create certificate using makecert utility

After reading and doing what was said in the above mentioned blogpost we should end up with 2 installed certificates. They're depicted on the picture below.

The certificate we'll use will be "vadmyst-enc".

SslStream gives you the look and feel of a common .NET stream.

So, what are the basic steps to start secure communication with SslStream?
Very often the communication happens between server (e.g web server) and client (e.g. browser).
Here are the steps for the server side:

  • Start listening on specific address and port

  • When connection is accepted wrap obtained NetworkStream with SslStream

  • Call SslStream::AuthenticateAsServer

  • Start doing I/O (in our case that's basic echo server

In code it looks like this:
TcpListener listener = new TcpListener(ipEndpoint);
TcpClient tcpClient = listener.AcceptTcpClient();

SslStream secureStream = new SslStream(tcpClient.GetStream(), false);

//use anonymous delegate for simplicity
ThreadPool.QueueUserWorkItem(new WaitCallback(delegate(object unused)
//simple echo server
byte[] tempBuffer = new byte[1024];
int read = 0;
while ((read = secureStream.Read(tempBuffer, 0, tempBuffer.Length)) > 0)
secureStream.Write(tempBuffer, 0, read);
}), null);
serverCertificate is obtained from certificate storage on the local machine:
X509Store store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
X509Certificate serverCertificate = null;
for (int i = 0; i < store.Certificates.Count; i++)
serverCertificate = store.Certificates[i];
if (serverCertificate.Subject.Contains("vadmyst-enc"))
In this post I'll will not cover usage of client certificates to perform client authentication for the simplicity's sake. Client will only authenticate server.
The steps required by the client:
  • Open TCP connection to the remote server

  • Wrap obtained NetworkStream with SslStream instance

  • Call SslStream::AuthenticateAsClient

  • Begin do the I/O

Source code below demonstrates basic TCP client that transfers data in a secure way.
TcpClient client = new TcpClient();
SslStream sslStream = new SslStream(client.GetStream(), false);

byte[] plaintext = new byte[5*1024 + 35];
byte[] validation = new byte[plaintext.Length];

RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();


int read = 0;
int totalRead = 0;
while( (read = sslStream.Read(validation, totalRead,
validation.Length - totalRead)) > 0)
totalRead += read;
if (totalRead == plaintext.Length)
break; //we've received all sent data
//check received data
for(int i=0; i < plaintext.Length; i++)
if ( validation[i] != plaintext[i] )
throw new InvalidDataException("Data is not the same");

SslStream appeared in .NET framework on version 2.0. As you can see doing secure communications with it is very easy. However, there are number of situations that require additional coding: client authentication using client certificates, using other algorithms when doing secure I/O. I will cover these advanced topics on the next posts. Stay tuned!

Tuesday, June 17, 2008

Hashing in .NET (cryptography related battle tactics)

Those who think I'm going to talk about stuff related to hashish or hash brown are totally not right. (By the way I do like hash brown as well as this great Japanese liquor ;) )

I will be talking about hashing that is related to cryptography and security. Hashing can be described as a process of getting small digital "fingerprint" from any kind of data. Those interested in general information can get it here.

In .NET, security and cryptography related stuff is located in System.Security.Cryptography namespace. Our hero of the day will be SHA1 algorithm. .NET class SHA1Managed implements it. According to .NET cryptography model this class implements abstract class SHA1. The same, by the way, is valid for other hash algorithms e.g. MD5. They both inherit from HashAlgorithm class. It is very likely if new hashing algorithm is added to the .NET framework it will inherit from HashAlgorithm.

There are three ways how to calculate hash value for some data.

  1. Use ComputeHash method

  2. Use TransformBlock/TransformFinalBlock directly

  3. Use CryptoStream

I'll show how to use above mentioned approaches. Let's assume we have some application data

RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
byte[] data = new byte[5 * 4096 + 320];
//fill data array with arbitrary data
//initialize HashAlgorithm instance
SHA1Managed sha = new SHA1Managed();

The first way:

byte[] hash1 = sha.ComputeHash(data);

It very straightforward and simple: pass data and get hash output. But this method is not suitable when hash has to be calculated for several byte arrays or when data size is very large (calculate hash value for the binary file).
This leads us to the second way:

int offset = 0;
int blockSize = 64;
//reset algorithm internal state
while (data.Length - offset >= blockSize)
offset += sha.TransformBlock(data, offset, blockSize,
data, offset);
sha.TransformFinalBlock(data, offset, data.Length - offset);
//get calculated hash value
byte[] hash2 = sha.Hash;

This way is much more flexible because: we can reuse HashAlgorithm instance (using Initialize method) and calculate hash value for large data objects.
However, to do that we still have to write additional code to read chunks from file and then pass them to TransformBlock method.

Finally, the third way:

//reuse SHA1Managed instance
MemoryStream memoryStream = new MemoryStream(data, false);
CryptoStream cryptoStream = new CryptoStream(memoryStream, sha,
//temporary array used by the CryptoStream to store
//data chunk for which hash calculation was performed
byte[] temp = new byte[16];
while (cryptoStream.Read(temp, 0, temp.Length) > 0) { }
hash3 = sha.Hash;

Isn't it beautiful? CryptoStream can use any Stream object to read from. Thus calculating hash value for a large file isn't a problem - just pass FileStream to CryptoStream constructor!
Under the hood CryptoStream uses TransformBlock/TransformFinalBlock, so the third way is derivative from the second one.
CryptoStream links data streams to cryptographic transformations: it can be chained together with any objects that implement Stream, so the streamed output from one object can be fed into the input of another object.

The first approach is good when you're calculating hash values from time to time.
The second and third are best when large part of your application's operation is connected with hash calculations (like using cryptography in network I/O).