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How entropy is acquired is up to the implementer of any protocol. The literature on measurement of entropy and on methods of harvesting entropy in computer systems is extensive and well known to anyone skilled in the cryptographic art. The best entropy source is probably a properly verified hardware random-bit generator that has circuitry attached to monitor all bits produced and to verify the entropy of the bits, raising an error condition if the hardware starts to malfunction. Such a hardware source of entropy can be used to drive a conditioning function (sometimes called "a whitening function") and might be used to drive a pseudo-random number generator (PRNG). If a PRNG is used, it should be compliant with recognized standards, such as FIPS 140-2 Annex C, as specified in [FIPS140].