Chaffing and winnowing

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

Chaffing and winnowing is a cryptography technique, though as it works by hiding which part of a transmission is the message, it is classified as steganography. A message is broken into blocks, and each one is transmitted with a message authentication code, 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.