Consider a system that generates 10 million audit records per day.Also assume that there are on average 10 attacks per day on this system and that each such attack is reflected in 20 records.If the intrusion-detection system has a true-alarm rate of 0.6 and a false-alarm rate of 0.0005,what percentage of alarms generated by the system correspond to real intrusions?
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The probability of occurrence of intrusive records is 10*20/ 10⁶= 0 .0002.Using Bayes' theorem,the probability that an alarm corresponds to a real intrusion is simply 0 ,0002 *0 .6/(0.0002 *0 .6 +0.9998 * 0 0005) = 0.193.