Chaffing and winnowing

Chaffing and winnowing is separation of chaff from grain, done by a threshing machine; the phrase is also used metaphorically.

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Chaffing and winnowing is a technique in cryptography which works by hiding which part of a transmission is the message, and so can be viewed as a form of steganography. A message is broken into blocks, and each one is transmitted with a message authentication code (MAC), interspersed at random with other similar blocks, called chaff. The recipient checks the MAC of each block; those that do not match are chaff, and are winnowed out.

See also: null cipher