Chaffing at the bit: Thoughts on a note by Ronald Rivest

Abstract

Rivest developed the concept of chaffing, a technique based on blending authenticated plaintext information with non authenticated noise (chaff) to obscure the true message as a method of subverting controls on strong cryptography in an arena in which strong authentication is unregulated. Assuming a secret authentication key shared between the sender and the receiver, only the intended recipient can easily separate the message content from the noise (winnowing). We look at ways in which the winnowing and chaffing processes might be made more efficient. We begin by considering the winnowing process as error correction (another unregulated technology) and consider the effectiveness of shorter authentication codes. In the limit, we show that we can transmit only a single bit of authentication material in place of a single bit of data, but in this limit, our process is equivalent to XORing the message with a similar sized random bit stream.