The cryptanalyst computes y = Dx' (c0), where D is the decipherment function corresponding to E, for each possible key x', and then checks the table...

The cryptanalyst computes y = Dx' (c0), where D is the decipherment function corresponding to E, for each possible key x', and then checks the table to see if y is in it. If so, (x, x') is a candidate for the key pair. How should the table be organized to allow the cryptographer to find a match for y in time O(1)? How many time units will pass before a match must occur?