Intrusion detection systems. A computer intrusion

Chapter 3, Problem 69E

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QUESTION:

Problem 69E

Intrusion detection systems. A computer intrusion detection system (IDS) is designed to provide an alarm whenever an intrusion (e.g., unauthorized access) is being attempted into a computer system. A probabilistic evaluation of a system with two independently operating intrusion detection systems (a double IDS) was published in the Journal of Research of the National Institute of Standards and Technology (November/December 2003). Consider a double IDS with system A and system B. If there is an intruder, system A sounds an alarm with probability .9, and system B sounds an alarm with probability .95. If there is no intruder, the probability that system A sounds an alarm (i.e., a false alarm) is .2, and the probability that system B sounds an alarm is .1. Assume that under a given condition (intruder or not), systems A and B operate independently.

a. Using symbols, express the four probabilities given in the example.

b. If there is an intruder, what is the probability that both systems sound an alarm?

c. If there is no intruder, what is the probability that both systems sound an alarm?

d. Given an intruder, what is the probability that at least one of the systems sounds an alarm?

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QUESTION:

Problem 69E

Intrusion detection systems. A computer intrusion detection system (IDS) is designed to provide an alarm whenever an intrusion (e.g., unauthorized access) is being attempted into a computer system. A probabilistic evaluation of a system with two independently operating intrusion detection systems (a double IDS) was published in the Journal of Research of the National Institute of Standards and Technology (November/December 2003). Consider a double IDS with system A and system B. If there is an intruder, system A sounds an alarm with probability .9, and system B sounds an alarm with probability .95. If there is no intruder, the probability that system A sounds an alarm (i.e., a false alarm) is .2, and the probability that system B sounds an alarm is .1. Assume that under a given condition (intruder or not), systems A and B operate independently.

a. Using symbols, express the four probabilities given in the example.

b. If there is an intruder, what is the probability that both systems sound an alarm?

c. If there is no intruder, what is the probability that both systems sound an alarm?

d. Given an intruder, what is the probability that at least one of the systems sounds an alarm?

ANSWER:

Solution:

Step 1 of 5:

Let probability of A sounds an alarm is 0.9 and probability of B sounds an alarm is 0.95

If there is no intruder, the probability that system A sounds an alarm is 0.2 and the probability that system B sounds an alarm is 0.1. A and B operate independently.

A : {Alarm A sounds alarm}

B: {Alarm B sounds alarm}

I: {Intruder}


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