Computers allow us to encrypt rich messages in real time, but the underlying principle is the same as in the simple baseball example. For encryption to be effective, the sender and the recipient must be the only parties who know how to encrypt and decrypt the messages. Microsoft Windows and the .NET Framework provide robust algorithms for doing encryption, and we’ll use these routines in this chapter. Unless you’re an encryption expert, you shouldn’t try to write your own encryption algorithm, for exactly the same reason that only aviation engineers should build their own airplanes.

It’s a common misconception that encryption algorithms and hash functions must be secret to be secure. The encryption algorithms and hash functions used in this book are commonly understood , and the associated source code is distributed freely on the Internet. They are, however, still secure because they are designed to be irreversible (in the case of hash functions) or they require the user to supply a secret key (in the case of encryption algorithms). As long as only the authorized parties know the secret key, the encrypted message is safe from intruders. Encryption helps to ensure three things:

• Confidentiality Only the intended recipient will be able to decrypt the message you send.

• Authentication Encrypted messages you receive have originated from a trusted source.

• Integrity When you send or receive a message, it won’t be tampered with in transit.

Some cryptography mechanisms are one way; that is, they produce cipher text that can’t be decrypted. A good example of a one-way cryptography is a hash . A hash is a very large number (the hashes in this chapter are 160 bits in size ) mathematically generated from a plain-text message. Because the hash contains no information about the original message, the original message can’t be derived from the hash. “What use is cipher text that can’t be decrypted?” you might ask. As you’ll see soon, a hash is useful for verifying that someone knows a secret without actually storing the secret.

In the examples in this chapter, you’ll learn how to create and use a hash for verifying passwords. You’ll also learn how to use private key encryption for storing and retrieving information in a database. We’ll also begin building a library of easy-to-use encryption functions that you can reuse in your Visual Basic programs.