Elligator: elliptic-curve points indistinguishable from uniform random strings

  • Authors:
  • Daniel J. Bernstein;Mike Hamburg;Anna Krasnova;Tanja Lange

  • Affiliations:
  • University of Illinois at Chicago, Chicago, USA;Rambus, San Francisco, USA;Radboud University Nijmegen, Nijmegen, Netherlands;Technische Universiteit Eindhoven, Eindhoven, Netherlands

  • Venue:
  • Proceedings of the 2013 ACM SIGSAC conference on Computer & communications security
  • Year:
  • 2013

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Abstract

Censorship-circumvention tools are in an arms race against censors. The censors study all traffic passing into and out of their controlled sphere, and try to disable censorship-circumvention tools without completely shutting down the Internet. Tools aim to shape their traffic patterns to match unblocked programs, so that simple traffic profiling cannot identify the tools within a reasonable number of traces; the censors respond by deploying firewalls with increasingly sophisticated deep-packet inspection. Cryptography hides patterns in user data but does not evade censorship if the censor can recognize patterns in the cryptography itself. In particular, elliptic-curve cryptography often transmits points on known elliptic curves, and those points are easily distinguishable from uniform random strings of bits. This paper introduces high-security high-speed elliptic-curve systems in which elliptic-curve points are encoded so as to be indistinguishable from uniform random strings. At a lower level, this paper introduces a new bijection between strings and about half of all curve points; this bijection is applicable to every odd-characteristic elliptic curve with a point of order 2, except for curves of $j$-invariant 1728. This paper also presents guidelines to construct, and two examples of, secure curves suitable for these encodings.