Let us assume we have two legitimate users, A and B, in a computer systems who are known to type characters on a keyboard according to a Poisson process 1with rates A E {300,600} characters per minute. We are interested to see how reliable a spy who is listening to the sound of typing can infer the user's identity. in other words, let N(t) denote the number of characters typed on a keyboard; the conditional prof of N(t] given A = A be Poisson: Pain) = (A? e'\" for n = 0,1,2, 11. Let a spy measure random variable X denoting the time it took for the user to type up the rst character