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Cost-Asymmetric Memory Hard Password Hashing

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Security and Cryptography for Networks (SCN 2022)

Part of the book series: Lecture Notes in Computer Science ((LNCS,volume 13409))

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Abstract

In the past decade billions of user passwords have been exposed to the dangerous threat of offline password cracking attacks. An offline attacker who has stolen the cryptographic hash of a user’s password can check as many password guesses as s/he likes limited only by the resources that s/he is willing to invest to crack the password. Pepper and key-stretching are two techniques that have been proposed to deter an offline attacker by increasing guessing costs. Pepper ensures that the cost of rejecting an incorrect password guess is higher than the (expected) cost of verifying a correct password guess. This is useful because most of the offline attacker’s guesses will be incorrect. Unfortunately, as we observe the traditional peppering defense seems to be incompatible with modern memory hard key-stretching algorithms such as Argon2 or Scrypt. We introduce an alternative to pepper which we call Cost-Asymmetric Memory Hard Password Authentication which benefits from the same cost-asymmetry as the classical peppering defense i.e., the cost of rejecting an incorrect password guess is larger than the expected cost to authenticate a correct password guess. When configured properly we prove that our mechanism can only reduce the percentage of user passwords that are cracked by a rational offline attacker whose goal is to maximize (expected) profit i.e., the total value of cracked passwords minus the total guessing costs. We evaluate the effectiveness of our mechanism on empirical password datasets against a rational offline attacker. Our empirical analysis shows that our mechanism can significantly reduce the percentage of user passwords that are cracked by a rational attacker by up to \(10\%\).

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Notes

  1. 1.

    The salt value protects against pre-computation attacks such as rainbow tables and ensures that the attacker must crack each individual password separately. For example, even if Alice and Bob select the same password \(pw_A=pw_B\) their password hashes will almost certainly be different i.e., \(h_A = H(pw_A, salt_A) \ne H(pw_B, salt_B) = h_B\) due to the different choice of values and collision resistance of the cryptographic hash function H.

  2. 2.

    We use the concept and notation of subset and superset for ordered sequences the way they were defined for regular set. If all elements of sequence A are also elements of sequence B regardless the order, we say \(A \subseteq B\).

  3. 3.

    The password datasets we analyze and experiment with are publicly available and widely used in literature research. We did not crack any new passwords. Thus, our usage of the datasets would not cause further harm to users.

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Acknowledgments

The research was supported in part by the National Science Foundation under awards CNS #2047272 and by IARPA under the HECTOR program. Mohammad Hassan Ameri was also supported in part by a Summer Research Grant from Purdue University.

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Authors and Affiliations

  1. Purdue University, West Lafayette, IN, 47907, USA

    Wenjie Bai, Jeremiah Blocki & Mohammad Hassan Ameri

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  1. Wenjie Bai
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  2. Jeremiah Blocki
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  3. Mohammad Hassan Ameri
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Correspondence to Jeremiah Blocki .

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Editors and Affiliations

  1. Università degli Studi di Salerno, Fisciano, Italy

    Clemente Galdi

  2. University of California at Irvine, Irvine, CA, USA

    Stanislaw Jarecki

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Bai, W., Blocki, J., Ameri, M.H. (2022). Cost-Asymmetric Memory Hard Password Hashing. In: Galdi, C., Jarecki, S. (eds) Security and Cryptography for Networks. SCN 2022. Lecture Notes in Computer Science, vol 13409. Springer, Cham. https://doi.org/10.1007/978-3-031-14791-3_2

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  • DOI: https://doi.org/10.1007/978-3-031-14791-3_2

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