Computer Laboratory

Technical reports

Cryptography and evidence

Michael Roe

May 2010, 75 pages

This technical report is based on a dissertation submitted April 1997 by the author for the degree of Doctor of Philosophy to the University of Cambridge, Clare College.


The invention of public-key cryptography led to the notion that cryptographically protected messages could be used as evidence to convince an impartial adjudicator that a disputed event had in fact occurred. Information stored in a computer is easily modified, and so records can be falsified or retrospectively modified. Cryptographic protection prevents modification, and it is hoped that this will make cryptographically protected data acceptable as evidence. This usage of cryptography to render an event undeniable has become known as non-repudiation. This dissertation is an enquiry into the fundamental limitations of this application of cryptography, and the disadvantages of the techniques which are currently in use. In the course of this investigation I consider the converse problem, of ensuring that an instance of communication between computer systems leaves behind no unequivocal evidence of its having taken place. Features of communications protocols that were seen as defects from the standpoint of non-repudiation can be seen as benefits from the standpoint of this converse problem, which I call “plausible deniability”.

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BibTeX record

  author =	 {Roe, Michael},
  title = 	 {{Cryptography and evidence}},
  year = 	 2010,
  month = 	 may,
  url = 	 {},
  institution =  {University of Cambridge, Computer Laboratory},
  number = 	 {UCAM-CL-TR-780}