Jeremiah Blocki, "Password Strength Signaling: A Counter-Intuitive Defense Against Password Cracking"

We introduce password strength information signaling as a novel,
yet counter-intuitive, defense mechanism against password cracking
attacks. Recent breaches have exposed billions of user passwords to
the dangerous threat of offline password cracking attacks. An
offline attacker can quickly check millions (or sometimes
billions/trillions) of password guesses by comparing their hash
value with the stolen hash from a breached authentication server.
The attacker is limited only by the resources he is willing to
invest. Our key idea is to have the authentication server store a
(noisy) signal about the strength of each user password for an
offline attacker to find. Surprisingly, we show that the noise
distribution for the signal can often be tuned so that a rational
(profit-maximizing) attacker will crack fewer passwords. The
signaling scheme exploits the fact that password cracking is not a
zero-sum game i.e., the attacker's profit is given by the value of
the cracked passwords minus the total guessing cost. Thus, a
well-defined signaling strategy will encourage the attacker to
reduce his guessing costs by cracking fewer passwords. We use an
evolutionary algorithm to compute the optimal signaling scheme for
the defender. As a proof-of-concept, we evaluate our mechanism on
several password datasets and show that it can reduce the total
number of cracked passwords by up to 12% (resp. 5%) of all users in
defending against offline (resp. online) attacks. Joint work with
Wenjie Bai and Ben Harsha