Question

A hacker has a list of 10,000 commonly used login names, and a list of 100,000 commonly used passwords. On this computer, there are 100 accounts that have both a login name on the first list and a password on the second list. For simplicity, assume that no other accounts have either a login name or password on these lists.

The hacker has written code that can attempt 1000 logins per second on this computer. This program repeatedly chooses a random login name from the first list and a random password from the second list. Compute the expected number of seconds it will take to break into some account on the machine, and identify that number from the list below.

Hint: Since names and passwords are chosen randomly, repetitions are possible. It may indeed be easier to forget what names/passwords have been tried than to keep a list of what has been tried and avoid duplicate attempts. As a result, there is an easy formula for the expected number of tries. If the probability of success on any given try is p, then the expected number of tries until the first success is exactly 1/p.

• A. 100.1
• B. 1.1
• C. 10,000
• D. 1000.1

Answer 1

The expected number of seconds it will take to break into some account on the machine is computed here as:

= 1 / (1000p) where p is the probability of getting the combination correct , and we used 1000 in the denominator with p as there are 1000 logins are done per second.

But as there are 100 accounts on the list, therefore the total number of seconds expected to get any one of those 100 accounts is computed here as:

Therefore c) 10,000 is the expected number of seconds here.