C# 4 and .NET 4

encryption ❘ 557
}
alicePubKeyBlob = aliceKey.Export(CngKeyBlobFormat.EccPublicBlob);
bobPubKeyBlob = bobKey.Export(CngKeyBlobFormat.EccPublicBlob);
In the method AliceSendsData(), the string that contains text characters is converted to a byte array
by using the Encoding class. An ECDiffieHellmanCng object is created and initialized with the key pair
from Alice. Alice creates a symmetric key by using her key pair and the public key from Bob calling the
method DeriveKeyMaterial(). The returned symmetric key is used with the symmetric algorithm AES to
encrypt the data. AesCryptoServiceProvider requires the key and an initialization vector (IV). The IV
is generated dynamically from the method GenerateIV(). The symmetric key is exchanged with the help
of the EC Diffie Hellman algorithm, but the IV must also be exchanged. From the security standpoint, it is
okay to transfer the IV unencrypted across the network — just the key exchange must be secured. The IV
is stored first as content in the memory stream, followed by the encrypted data where the CryptoStream
class uses the encryptor created by the AesCryptoServiceProvider class. Before the encrypted data is
accessed from the memory stream, the crypto stream must be closed. Otherwise, end bits would be missing
from the encrypted data.
private static byte[] AliceSendsData(string message)
{
Console.WriteLine("Alice sends message: {0}", message);
byte[] rawData = Encoding.UTF8.GetBytes(message);
byte[] encryptedData = null;
}
using (var aliceAlgorithm = new ECDiffieHellmanCng(aliceKey))
using (CngKey bobPubKey = CngKey.Import(bobPubKeyBlob,
CngKeyBlobFormat.EccPublicBlob))
{
byte[] symmKey = aliceAlgorithm.DeriveKeyMaterial(bobPubKey);
Console.WriteLine("Alice creates this symmetric key with " +
"Bobs public key information: {0}",
Convert.ToBase64String(symmKey));
var aes = new AesCryptoServiceProvider();
aes.Key = symmKey;
aes.GenerateIV();
using (ICryptoTransform encryptor = aes.CreateEncryptor())
using (MemoryStream ms = new MemoryStream())
{
// create CryptoStream and encrypt data to send
var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write);
// write initialization vector not encrypted
ms.Write(aes.IV, 0, aes.IV.Length);
cs.Write(rawData, 0, rawData.Length);
cs.Close();
encryptedData = ms.ToArray();
}
aes.Clear();
}
Console.WriteLine("Alice: message is encrypted: {0}",
Convert.ToBase64String(encryptedData));;
Console.WriteLine();
return encryptedData;
Bob receives encrypted data in the argument of the method BobReceivesData(). First, the unencrypted
initialization vector must be read. The BlockSize property of the class AesCryptoServiceProvider
returns the number of bits for a block. The number of bytes can be calculated by doing a divide by 8, and
the fastest way to do this is by doing a bit shift of 3 bits. Shifting by 1 bit is a division by 2, 2 bits by 4,
and 3 bits by 8. With the for loop, the first bytes of the raw bytes that contain the IV unencrypted are
written to the array iv. Next, an ECDiffieHellmanCng object is instantiated with the key pair from Bob.
Using the public key from Alice, the symmetric key is returned from the method DeriveKeyMaterial().
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