Therefore let PE(A) denote the probability that the adversary A wins this game against E, and define the advantage of A as 2(PE(A) - 1/2). It follows that if A guesses randomly, its advantage will be 0; on the other hand, if A always wins, then its advantage is 1. The block cipher E is a pseudo-random permutation (PRP) if no adversary has an advantage significantly greater than 0, given specified restrictions on qand the adversary's running time. If in Step 2 above adversaries have the option of learning f−1(X) instead of f(X)(but still have only small advantages) then E is a strong PRP (SPRP). An adversary is non-adaptive if it chooses all qvalues for X before the game begins (that is, it does not use any information gleaned from previous queries to choose each X as it goes).