Chaffinch:
Confidentiality in the Face of Legal Threats

Richard Clayton and George Danezis

University of Cambridge, Computer Laboratory, Gates Building, JJ Thompson Avenue, Cambridge CB3 0FD, United Kingdom

{ richard.clayton, george.danezis }@cl.cam.ac.uk

Abstract. We present the design and rationale of a practical system for passing confidential messages. The mechanism is an adaptation of Rivest's "chaffing and winnowing", which has the legal advantage of using authentication keys to provide privacy. We identify a weakness in Rivest's particular choice of his "package transform" as an "all-or-nothing" element within his scheme. We extend the basic system to allow the passing of several messages concurrently. Only some of these messages need be divulged under legal duress, the other messages will be plausibly deniable. We show how this system may have some resilience to the type of legal attack inherent in the UK's Regulation of Investigatory Powers (RIP) Act.

http://www.cl.cam.ac.uk/~rnc1/Chaffinch.html