International Association for Cryptologic Research

International Association
for Cryptologic Research


Paper: Password Hashing and Preprocessing

Pooya Farshim , University of York
Stefano Tessaro , University of Washington
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Conference: EUROCRYPT 2021
Abstract: How does the cryptanalytic effort needed to compromise t out of m instances of hashed passwords scale with the number of users when arbitrary preprocessing information on the hash function is available? We provide a formal treatment of this problem in the multi-instance setting with auxiliary information. A central contribution of our work is an (arguably simple) transcript-counting argument that allows us to resolve a fundamental question left open by Bellare, Ristenpart, and Tessaro (BRT; CRYPTO 2012) in multi-instance security. We leverage this proof technique to formally justify unrecoverability of hashed salted passwords in the presence of auxiliary information in the random-oracle model. To this end we utilize the recent pre-sampling techniques for dealing with auxiliary information developed by Coretti et al. (CRYPTO 2018). Our bounds closely match those commonly assumed in practice. Besides hashing of passwords through a monolithic random oracle, we consider the effect of iteration, a technique that is used in classical mechanisms, such as bcrypt and PBKDF2, to slow down the rate of guessing. Building on the work of BRT, we formulate a notion of KDF security, also in the presence of auxiliary information, and prove an appropriate composition theorem for it.
  title={Password Hashing and Preprocessing},
  author={Pooya Farshim and Stefano Tessaro},