C# 4 and .NET 4

encryption ❘ 553
encrypted using a public key can be decrypted with the corresponding private key. This also works the other
way around: something encrypted using a private key can be decrypted by using the corresponding public
key, but not the private key.
Public and private keys are always created as a pair. The public key can be made available to everybody,
and it can even be put on a web site, but the private key must be safely locked away. Following are some
examples where these public and private keys are used to explain encryption.
If Alice sends a message to Bob (see Figure 21-2), and Alice wants to make sure that no one else but Bob can
read the message, she uses Bob’s public key. The message is encrypted using Bob’s public key. Bob opens the
message and can decrypt it using his secretly stored private key. This key exchange guarantees that no one
but Bob can read Alice’s message.
Alice
Bob
figure 21-2
Eve
There is one problem left: Bob can’t be sure that the mail comes from Alice. Eve can use Bob’s public key
to encrypt messages sent to Bob and pretend to be Alice. We can extend this principle using public/private
keys. Let’s start again with Alice sending a message to Bob. Before Alice encrypts the message using Bob’s
public key, she adds her signature and encrypts the signature using her own private key. Then she encrypts
the mail using Bob’s public key. Therefore, it is guaranteed that no one else but Bob can read the mail.
When Bob decrypts the message, he detects an encrypted signature. The signature can be decrypted using
Alice’s public key. For Bob, it is not a problem to access Alice’s public key because the key is public. After
decrypting the signature, Bob can be sure that it was Alice who sent the message.
The encryption and decryption algorithms using symmetric keys are a lot faster than those using
asymmetric keys. The problem with symmetric keys is that the keys must be exchanged in a safe manner.
With network communication, one way to do this is by using asymmetric keys first for the key exchange and
then symmetric keys for encryption of the data that is sent across the wire.
With the .NET Framework, you find classes for encryption in the namespace System.Security
.Cryptography. Several symmetric and asymmetric algorithms are implemented. You can find algorithm
classes for many different purposes. Some of the classes added in .NET 3.5 have a Cng prefix or suffix.
Cng is short for Cryptography Next Generation, which can be used since Windows Vista and Windows
Server 2008. This API makes it possible to write a program independently of the algorithm by using a
provider-based model. If you are targeting Windows Server 2003 as well, you need to pay attention to what
encryption classes to use.
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