Chapter 2. What Is a Countermeasure?
In his book Secrets and Lies, Bruce Schneier talks about counter measures in three categories: protection, detection, and reaction.
In a military office, classified documents are stored in a safe. The safe provides protection against attack, but so does the system of alarms and guards. Assume the attacker is an outsider: someone who does not work in the office. If he is going to steal the documents inside the safe, he is not only going to have to break into the safe, he is also going to have to defeat the system of alarms and guards. The safe—both the lock and the walls—are protective countermeasures, and the guards are reactive counter measures.
If guards patrol the offices every 15 minutes, then the safe only ...