Quantifying the security advantage of password expiration policies


Many security policies force users to change passwords within fixed intervals, with the apparent justification that this improves overall security. However, the implied security benefit has never been explicitly quantified. In this note, we quantify the security advantage of a password expiration policy, finding that the optimal benefit is relatively minor at best, and questionable in light of overall costs.

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    These probabilities are unknown and change across datasets; estimates are used, based on large datasets accumulated from prior compromises, or from heuristic tools.

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    More precisely, this is for \(\beta \) guesses per account. The optimal attack tries the most probable password on each account, then the next most probable, etc.

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    If this is counter-intuitive, note that an attack which guesses key candidates in a fixed sequence actually benefits from a key change if the original target key is more distant in the guessing sequence than the newly updated key. In our analogous problem herein, the implication is that a successful guessing attack cannot be prevented even if a user changes their password continuously, as quickly as system interfaces allow.


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We thank Joseph Bonneau and anonymous referees for insightful comments which have improved this paper. Both authors acknowledge funding from Canada’s NSERC for Canada Research Chair and Discovery Grant funding.

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Correspondence to P. C. van Oorschot.

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This is one of several papers published in Designs, Codes and Cryptography comprising the “Special Issue on Cryptography, Codes, Designs and Finite Fields: In Memory of Scott A. Vanstone”.

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Chiasson, S., van Oorschot, P.C. Quantifying the security advantage of password expiration policies. Des. Codes Cryptogr. 77, 401–408 (2015). https://doi.org/10.1007/s10623-015-0071-9

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  • Authentication
  • Password security in digital systems
  • Password aging
  • Password expiration
  • Guessing attacks

Mathematics Subject Classification

  • 94A62
  • 68W40
  • 68U35
  • 68N25
  • 94A60