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:
“It is by the goodness of God that in our country we have those three unspeakably precious things: freedom of speech, freedom of conscience, and the prudence never to practice either of them.”
—Mark Twain [Samuel Langhorne Clemens] (1835–1910)
“She, too, would now swim down the river of matrimony with a beautiful name, and a handle to it, as the owner of a fine family property. Women’s rights was an excellent doctrine to preach, but for practice could not stand the strain of such temptation.”
—Anthony Trollope (1815–1882)
“If I had my life over again I should form the habit of nightly composing myself to thoughts of death. I would practise, as it were, the remembrance of death. There is no other practice which so intensifies life. Death, when it approaches, ought not to take one by surprise. It should be part of the full expectancy of life. Without an ever- present sense of death life is insipid. You might as well live on the whites of eggs.”
—Muriel Spark (b. 1918)