In cryptography, full entropy is a property of an output of a random number generator.
[2] The mathematical definition relies on a "distinguishing game": an adversary with an unlimited computing power is provided with two sets of random numbers, each containing W elements of length n. One set is ideal, it contains bit strings from the theoretically perfect random number generator, the other set is real and includes bit strings from the practical random number source after a randomness extractor.
The full entropy for particular values of W and positive parameter δ is achieved if an adversary cannot guess the real set with probability higher than
[3] The practical way to achieve the full entropy is to obtain from an entropy source bit strings longer than n bits, apply to them a high-quality randomness extractor that produces the n-bit result, and build the real set from these results.
depends on W and δ; the following table contains few representative values:[4] Not every randomness extractor will produce the desired results.
For example, the Von Neumann extractor, while providing an unbiased output, does not decorrelate groups of bits, so for serially correlated inputs (typical for many entropy sources) the output bits will not be independent.