C# 5.0 Programmer's Reference

602 ❘ CHAPTER 27 Cryptography
This chapter describes only a few of the most useful cryptographic methods provided by the .NET
Framework. Those include generating cryptographically secure random numbers, symmetric encryption,
and asymmetric encryption.
This chapter explains only how to use .NET Framework tools to perform those tasks, but it
should be enough to get you started with cryptography. Unfortunately, there isn’t room here to
explain all the methods for performing even those limited tasks or to explain how those methods
work. See a book about cryptography (such as Bruce Schneier’s book) for details about how some
of these algorithms work.
Leisure Reading
The history of cryptography contains truly remarkable stories involving spies and
counterspies, ingenious contraptions, breaking unbreakable codes, and using clever
artifice to convince the enemy that its code is secure even after it’s broken. If you
have the time, it makes fascinating reading.
Cryptography was originally used to send messages from one person to another securely. Modern
cryptography has expanded to include other sorts of secure communication. The following section
describes some of the operations that modern cryptographic programs perform. The sections after
that explain some of the cryptographic tools you can use in your C# programs.
Cryptography Namespaces
The System.Security.Cryptography namespace includes cryptographic classes.
To make using these classes easier, you may want to include the following using
directive in your code.
using System.Security.Cryptography;
Cryptographic Operations
The following list describes some of the most common cryptographic operations.
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Symmetric key encryption (private key encryption)—This technique uses a secret key
to encrypt and decrypt messages. You use the key to encrypt a message. You then send the
result to someone else who knows the key. That person uses the key to decrypt the message.
Asymmetric key encryption (public key encryption)—This technique uses two keys, a public
key and a secret key, to encrypt and decrypt messages. Everyone knows the public key that is
used to encrypt messages. Only those who know the secret key (typically a single person) can
decrypt the messages. (This is sort of like a mail slot in a door. Anyone can slip a message
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