La procedura per costruire le approssimazioni è differente da cifrario a cifrario. ###>For example, the following equation, from a hypothetical cipher, states the XOR sum of the first and third plaintext bits (as in a block cipher's block) and the first ciphertext bit is equal to the second bit of the key:formula_1In an ideal cipher, any linear equation relating plaintext, ciphertext and key bits would hold with probability 1/2.