Encryption and Authentication Algorithms

Rather than relying on secrecy to protect an encryption or authentication scheme (an approach known as " security through obscurity " ), TCP/IP security protocols always specify that cryptographic algorithms be well known and accessible. This is done for several reasons, not the least of which is that as an open protocol suite, TCP/IP protocol specifications must be published freely. The most important reason, however, is that secrecy is a poor safeguard over security.

Attempting to keep an encryption algorithm secret is almost impossible, particularly if it is being used by anyone other than the person who knows the secret. Attackers have many cryptanalysis tools at their disposal for breaking codes, and they need only have access to ciphertexts to break them. Having access to the software used to encrypt and/or decrypt data with the secret algorithm makes the task much easier: the attacker must only determine what the software does to the data to figure out how to reverse the operation.

The greatest advantage that published algorithms provide is the benefit of scrutiny by researchers and others seeking to find ways to further improve or break the algorithms. The more trained experts examine an algorithm, the less likely they are to overlook an " obvious " attack.

Security algorithms and protocols are hard to design because there are so many different ways to attack them-and designers can't always imagine them all. Although national security organizations as well as corporations may have their own top-secret codes, secrets are hard to keep. Spies and other criminals are well known for their skill at motivating (through bribery, extortion, or other means) people who know secrets to share them.

The prevailing wisdom in security holds that a good encryption or authentication algorithm should be secure even if an attacker knows what algorithm is being used. This is particularly important for Internet security, since an attacker with a sniffer will often be able to determine exactly what kind of algorithm is being used by listening as systems negotiate their connections.

In this section we'll cover five types of important cryptographic functions.

• Symmetric encryption
• Public key encryption
• Key exchange
• Secure hashes (message digests)
• Digital signature