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In the past few years billions of user passwords have been exposed to the threat of offline cracking attempts. Such brute-force cracking attempts are increasingly dangerous as password cracking hardware continues to improve and as users continue to select low entropy passwords. Key-stretching techniques such as hash iteration and memory hard functions can help to mitigate the risk, but increased key-stretching effort necessarily increases authentication delay so this defense is fundamentally constrained by usability concerns. We introduce Just in Time Hashing (JIT), a client side key-stretching algorithm to protect user passwords against offline brute-force cracking attempts without increasing delay for the user. The basic idea is to exploit idle time while the user is typing in their password to perform extra key-stretching. As soon as the user types in the first character(s) of their password our algorithm immediately begins filling memory with hash values derived from the character(s) that the user has typed thus far. We conduct a user study to guide the development of JIT e.g. by determining how much extra key-stretching could be performed during idle cycles or how many consecutive deletions JIT may need to handle. Our security analysis demonstrates that JIT can substantially increase guessing costs over traditional key-stretching algorithms with equivalent (or less) authentication delay. Specifically an empirical evaluation using existing password datasets demonstrates that JIT increases guessing costs by nearly an order of magnitude in comparison to standard key-stretching techniques with comparable delay. We provide a proof-of-concept implementation of a Just in Time Hashing algorithm by modifying Argon2. 

We develop an economic model of an offline password cracker which allows us to make quantitative predictions about the fraction of accounts that a rational password attacker would crack in the event of an authentication server breach. We apply our economic model to analyze recent massive password breaches at Yahoo!, Dropbox, LastPass and AshleyMadison. All four organizations were using key-stretching to protect user passwords. In fact, LastPass' use of PBKDF2-SHA256 with $10^5$ hash iterations exceeds 2017 NIST minimum recommendation by an order of magnitude. Nevertheless, our analysis paints a bleak picture: the adopted key-stretching levels provide insufficient protection for user passwords. In particular, we present strong evidence that most user passwords follow a Zipf's law distribution, and characterize the behavior of a rational attacker when user passwords are selected from a Zipf's law distribution. We show that there is a finite threshold which depends on the Zipf's law parameters that characterizes the behavior of a rational attacker --- if the value of a cracked password (normalized by the cost of computing the password hash function) exceeds this threshold then the adversary's optimal strategy is {\em always} to continue attacking until each user password has been cracked. In all cases (Yahoo!, Dropbox, LastPass and AshleyMadison) we find that the value of a cracked password almost certainly exceeds this threshold meaning that a rational attacker would crack all passwords that are selected from the Zipf's law distribution (i.e., most user passwords). This prediction holds even if we incorporate an aggressive model of diminishing returns for the attacker (e.g., the total value of $500$ million cracked passwords is less than $100$ times the total value of $5$ million passwords). On a positive note our analysis demonstrates that memory hard functions (MHFs) such as SCRYPT or Argon2i can significantly reduce the damage of an offline attack. In particular, we find that because MHFs substantially increase guessing costs a rational attacker will give up well before he cracks most user passwords and this prediction holds even if the attacker does not encounter diminishing returns for additional cracked passwords. Based on our analysis we advocate that password hashing standards should be updated to require the use of memory hard functions for password hashing and disallow the use of non-memory hard functions such as BCRYPT or PBKDF2.