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:
“I am out of practice at living.
You are as brave as a motorcycle.”
—Anne Sexton (1928–1974)
“It has always been my practice to cast a long paragraph in a single mould, to try it by my ear, to deposit it in my memory, but to suspend the action of the pen till I had given the last polish to my work.”
—Edward Gibbon (1737–1794)
“My paternal grandmother would not light a fire on the Sabbath and piled all Sunday’s washing-up in a bucket, to be dealt with on Monday morning, because the Sabbath was a day of rest—a practice that made my paternal grandfather, the village atheist, as mad as fire. Nevertheless, he willed five quid to the minister, just to be on the safe side.”
—Angela Carter (1940–1992)