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Chaffing and Winnowing

Chaffing and Winnowing: Confidentiality without Encryption Ronald L. Rivest MIT Lab for Computer Science March 18, 1998 (rev. April 24, 1998) http://theory.lcs.mit.edu/~rivest/chaffing.txt A major goal of security techniques is ``confidentiality''---ensuring that adversaries gain no intelligence from a transmitted message. There are two major techniques for achieving confidentiality: -- Steganography: the art of hiding a secret message within a larger one in such a way that the adversary can not discern the presence or contents of the hidden message. For example, a message might be hidden within a picture by changing the low-order pixel bits to be the message bits. (See Wayner (1996) for more information on steganography.) -- Encryption: transforming the message to a ciphertext such that an adversary who overhears the ciphertext can not determine the message sent. The legitimate receiver possesses a secret decryption key that allows him to reverse the encryption transformation and retrieve the message. The sender may have used the same key to encrypt the message (with symmetric encryption schemes) or used a different, but related key (with public-key schemes). DES and RSA are familiar examples of encryption schemes. This paper introduces a new technique, which we call ``chaffing and winnowing''---to winnow is to ``separate out or eliminate (the poor or useless parts),'' (Webster's Dictionary), and is often used when referring to the process of separating grain from chaff. Novel techniques for confidentiality are interesting in part because of the current debate about cryptographic policy as to whether law enforcement should be given when authorized surreptitious access to the plaintext of encrypted messages. The usual technique proposed for such access is ``key recovery,'' where law enforcement has a ``back door'' that enables them to recover the decryption key. Winnowing does not employ encryption, and so does not have a ``decryption key.'' Thus, t

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