Practice
In practice, the Xs are approximations to the S-boxes (substitution components) of block ciphers. Typically, X values are inputs to the S-box and Y values are the corresponding outputs. By simply looking at the S-boxes, the cryptanalyst can tell what the probability biases are. The trick is to find combinations of input and output values that have probabilities of zero or one. The closer the approximation is to zero or one, the more helpful the approximation is in linear cryptanalysis.
However, in practice, the binary variables are not independent, as is assumed in the derivation of the piling-up lemma. This consideration has to be kept in mind when applying the lemma; it is not an automatic cryptanalysis formula.
Read more about this topic: Piling-up Lemma
Famous quotes containing the word practice:
“They never consulted with books, and know and can tell much less than they have done. The things which they practice are said not yet to be known.”
—Henry David Thoreau (1817–1862)
“Those who make a practice of comparing human actions are never so perplexed as when they try to see them as a whole and in the same light; for they commonly contradict each other so strangely that it seems impossible that they have come from the same shop.”
—Michel de Montaigne (1533–1592)
“Communism to me is one-third practice and two-thirds explanation.”
—Will Rogers (1879–1935)