A method of comparing unmasked bits of an N-bit data key to an N-bit Rule includes dividing the key into C-bit chunks. Each of the chunks is used as an 5 address to extract from memories 12, 13, 14, 21, 22, 23, 24, 31, 32, 33, 34, 41, 42, 43, 44. The memory is preprepared, such that the data stored in the address corresponding to that chunk of the key is 1 or O according to whether a bitwise comparison of that chunk of the data key with the mask is equal to a bitwise comparison of that chunk of the mask and rule. This extracted bit therefore indicates whether the rule is obeyed for that chunk of the data key. The N/C extracted bits for each rule are compared, to determine if the rule is obeyed for the entire data key.