Spelling-Error Tolerant, Order-Independent Pass-Phrases via the Damerau-Levenshtein String-Edit Distance Metric

Bard, G.V.

    It is well understood that passwords must be very long and complex to have sufficient entropy for security purposes. Unfortunately, these passwords tend to be hard to memorize, and so alternatives are sought. Smart Cards, Biometrics, and Reverse Turing Tests (human-only solvable puzzles) are options, but another option is to use pass-phrases. This paper explores methods for making passphrases suitable for use with password-based authentication and key-exchange (PAKE) protocols, and in particular, with schemes resilient to server-file compromise. In particular, the -method of Gentry, MacKenzie and Ramzan, is combined with the Bellovin-Merritt protocol to provide mutual authentication (in the random oracle model (Canetti, Goldreich & Halevi 2004, Bellare, Boldyreva & Palacio 2004, Maurer, Renner & Holenstein 2004)). Furthermore, since common password-related problems are typographical errors, and the CAPSLOCK key, we show how a dictionary can be used with the Damerau- Levenshtein string-edit distance metric to construct a case-insensitive pass-phrase system that can tolerate zero, one, or two spelling-errors per word, with no loss in security. Furthermore, we show that the system can be made to accept pass-phrases that have been arbitrarily reordered, with a security cost that can be calculated. While a pass-phrase space of 2128 is not achieved by this scheme, sizes in the range of 252 to 2112 result from various selections of parameter sizes. An attacker who has acquired the server-file must exhaust over this space, while an attacker without the serverfile cannot succeed with non-negligible probability.
Cite as: Bard, G.V. (2007). Spelling-Error Tolerant, Order-Independent Pass-Phrases via the Damerau-Levenshtein String-Edit Distance Metric. In Proc. Fifth Australasian Information Security Workshop (Privacy Enhancing Technologies) (AISW 2007), Ballarat, Australia. CRPIT, 68. Brankovic, L. and Steketee, C., Eds. ACS. 117-124.
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