To set up a system in practice, we need to implement the six principles covered in the previous section using mechanisms that tend to be similar from one system to the next. It doesn't really matter whether you are implementing a system to send secret letters by pigeon or a security method for a wireless LAN. Some common processes and terms should be understood. This section briefly describes some of the main terms used in security. Sometimes words in common use have a specific meaning in security. For example, the word "encryption" tends to be used in common speech to refer to an entire security protocol, whereas in security it refers to a single specific process.
Threat model: We need a means to measure whether a security system meets its goals. One way to understand the security goals in a given situation is to make a list of all the types of attack that are known. This "list" is used to create the threat model, which is the basis for designing and evaluating security. Having created the list, we then identify all those threats against which we plan to defend. From a practical standpoint, some of the threats on the list may be too low risk and too expensive to defend against. As an example, the threat model for protecting wired Ethernet LANs does not (usually) include the threat of being monitored via the tiny radiations coming from the wires. By contrast, unwanted monitoring of radio emissions is central to the threat model of wireless LANs.
Security protocol: Many people use the word "encryption" in a general way to talk about security. You often hear people talking about "sending data over an encrypted link," and so on. This is dangerous because encryption is only one part of the process, albeit a very important part. Real security is provided by a set of processes and procedures that are carefully linked together. This set of procedures and processes is called the security protocol. It is important to realize that even if the most advanced encryption techniques are used, you have no security if they are used together in the wrong way.
Keys and passwords: These terms are often used interchangeably, although there is a slight difference in meaning. Both refer to a piece of information that is intended to be secret to two or more parties. Conventionally, the term password is used to refer to keys that are chosen by humans. The term key is more often used to describe information generated by a machine that is usually not human-readable. You will often see references to the length of the key. For example, the original IEEE 802.11 had "40-bit" keys, whereas most Wi-Fi WEP systems have "128-bit keys." In general, longer keys are more difficult to crack than shorter keys, but not always?it depends on the key entropy (described next).
Key entropy: What is important about passwords and keys is the number of different possible values a key can take. Theoretically, a 40-bit key has 240 or 1,000 billion possible values. However, if we restrict the values that are allowed, the effectiveness of the key goes down. For example, suppose the user enters a 40-bit key as five uppercase letter symbols (assume each letter uses 8 bits, hence 40 bits total). An example of such a password is the string "LASER." Because each symbol is limited to only 26 letters, you can have only 265 (or about 12 million) different passwords. By limiting the type of password, you have reduced the number of possible passwords by a factor of 100,000. The number of possible key values determines the strength of the key and is known as the key entropy. In our earlier example, the restriction to using uppercase letters has reduced the key entropy (and hence its effectiveness) from 40 bits to 23 bits, even though the key remains 40 bits long. If we restricted ourselves to known words and names, it would be reduced even more.
Authentication: The heart of security is the ability to distinguish the "good guys" from the "bad guys." If you can't be sure whom you are talking to, you can't protect yourself against attack. The term authentication is used at two different levels in security protocols, and this sometimes leads to confusion. The first level is user authentication and the second level is message authentication. The objective of user authentication is to prove that the other party with whom you want to communicate is who she says she is. Note that although we talk about "user" here, it could be that the other party is a computer or even a software process running on a server rather than a person. Message authentication has a different objective: to prove that a received message has not been tampered with, delayed, altered, or copied. A message is said to be authentic if it passes these tests. Typically, user authentication must be performed to identify the other party, and message authentication is done to ensure that subsequent communications come from that other party and are unaltered.
Authorization: The process of user authentication is difficult to perform correctly. Therefore, it is discussed extensively in this book and in others. You often see statements such as,"When the mobile device is authenticated, the access point allows it to communicate with the network." This is not quite true. We saw a cartoon by Gary Larson recently in which a ghoulish specter was peeking through a partly open front door held by a security chain. The old lady inside was saying, "Ah, but how do I know you really are the angel of death?" The message is simple: the fact that you know who someone is (authenticate) doesn't mean you always want to give him access. The decision to "let him in" is called authorization and comes after authentication.
Encryption: The process of combining a piece of data and a key to produce random-looking numbers is called encryption. It is useful only if a known key can be used to transform the random-looking numbers back to the original data. Note that we have said nothing about LANs, packets, wires, or even time. Encryption is just a computational algorithm of which there are many variants. Encryption algorithms are used to create security protocols.