Chaffing and winnowing is a cryptographic technique to achieve confidentiality without using encryption when sending data over an insecure channel.
The name is derived from agriculture: after grain has been harvested and threshed, it remains mixed together with inedible fibrous chaff.
However, a third party can make their communication confidential by simultaneously sending specially crafted messages through the same channel.
Many MACs use a secret key Alice shares with Bob, but it is sufficient that the receiver has a method to authenticate the packets.
Rivest notes an interesting property of chaffing-and-winnowing is that third parties (such as an ISP) can opportunistically add it to communications without needing permission or coordination with the sender/recipient.
But an eavesdropper between Charles and Bob would have to tell which packets are bogus and which are real (i.e. to winnow, or "separate the wheat from the chaff").
To make the transmission more efficient, Alice can process her message with an all-or-nothing transform and then send it out in much larger chunks.
[citation needed] Ron Rivest suggests that laws related to cryptography, including export controls, would not apply to chaffing and winnowing because it does not employ any encryption at all.