C# 5.0 Programmer's Reference

Exercises ❘ 619
Length Limitations
RSA is relatively slow compared to symmetric key algorithms, so the .NET
Framework limits the size of the messages it encrypts with RSA. If you exceed the
allowed length, the provider throws a CryptographicException with the rather
unhelpful message “Bad Length.”
Symmetric algorithms break messages into blocks and then encrypt them separately,
so they can handle messages of any length.
Together the two kinds of algorithms are effective. For example, you could use the
following steps to send a long message to a receiver even if you have not exchanged
private keys for symmetric encryption.
1. Generate a private key for symmetric encryption.
2. Use asymmetric encryption to send the private key to the receiver.
3. Send the symmetrically encrypted message to the receiver.
Summary
Cryptography is a fascinating topic. Modern cryptographic tools use sophisticated mathematics to
let you perform such operations as encrypting and decrypting files and strings, hashing files to see if
they have been modified, digitally signing files to provide assurance that you actually wrote the file,
and generating cryptographically secure random numbers.
There isn’t room to cover all these topics here, so this chapter focused on three of the most useful
tasks: generating cryptographically secure random numbers, symmetric encryption, and asymmetric
encryption. This should provide enough introduction for you to continue your study of the .NET
Framework’s cryptographic methods on your own.
For more information about cryptographic tools in the .NET Framework, see “Cryptographic Services”
at msdn.microsoft.com/library/92f9ye3s.aspx.
Exercises
1. Write a program that uses the Random class to encrypt and decrypt messages.
Unfortunately, if you use the XOR operator ^ to combine a letter with a random byte, the
result may not be a letter, so it’s hard to visualize the result. To make it easier to visualize
the results, assume the message contains only uppercase letters A through Z. (For bonus points,
use string methods and LINQ to convert the plaintext.) To encrypt a letter, shift it by a random
number of letters between 0 and 25. For example, A + 1 = B, A + 2 = C, and so forth.
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