C# 4 and .NET 4

e n c r y p t i o n ❘ 555
First, take a look at the major steps in the Main() method: Alice ’ s keys are created, and the string “Alice”
is signed and fi nally verifi ed if the signature is really from Alice by using the public key. The message
that is signed is converted to a byte array by using the Encoding class. To write the encrypted signature
to the console, the byte array that contains the signature is converted to a string with the method
Convert.ToBase64String() .
Never convert encrypted data to a string using the Encoding class. The Encoding
class verifi es and converts invalid values that are not allowed with Unicode, and thus
converting the string back to a byte array yields a different result.
using System;
using System.Security.Cryptography;
using System.Text;
namespace Wrox.ProCSharp.Security
{
class Program
{
internal static CngKey aliceKeySignature;
internal static byte[] alicePubKeyBlob;
static void Main()
{
CreateKeys();
byte[] aliceData = Encoding.UTF8.GetBytes("Alice");
byte[] aliceSignature = CreateSignature(aliceData, aliceKeySignature);
Console.WriteLine("Alice created signature: {0}",
Convert.ToBase64String(aliceSignature));
}
if (VerifySignature(aliceData, aliceSignature, alicePubKeyBlob))
{
Console.WriteLine("Alice signature verified successfully");
}
Code snippet SigningDemo/Program.cs
CreateKeys() is the method that creates a new key pair for Alice. This key pair is stored in a static fi eld,
so it can be accessed from the other methods. The Create() method of CngKey gets the algorithm as an
argument to defi ne a key pair for the algorithm. With the Export() method, the public key of the key pair
is exported. This public key can be given to Bob for the verifi cation of the signature. Alice keeps the private
key. Instead of creating a key pair with the CngKey class, you can open existing keys that are stored in
the key store. Usually Alice would have a certifi cate containing a key pair in her private store, and the store
could be accessed with CngKey.Open() .
static void CreateKeys()
{
aliceKeySignature = CngKey.Create(CngAlgorithm.ECDsaP256);
alicePubKeyBlob = aliceKeySignature.Export(CngKeyBlobFormat.GenericPublicBlob);
}
With the key pair, Alice can create the signature using the ECDsaCng class. The constructor of this class
receives the CngKey from Alice that contains both the public and private key. The private key is used,
signing the data with the SignData() method.
static byte[] CreateSignature(byte[] data, CngKey key)
{
var signingAlg = new ECDsaCng(key);
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