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arXiv 2006.05023cs.CR
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离线密码破解的经济学

On the Economics of Offline Password Cracking

Jeremiah Blocki, Ben Harsha, Samson Zhou

摘要:

我们开发了一个离线密码破解器的经济模型,它允许我们对合理的密码攻击者在身份验证服务器违规的情况下会破解的帐户的比例进行定量预测。我们应用我们的经济模型来分析雅虎最近发生的大规模密码泄露事件!,Dropbox、LastPass和AshleyMadison。这四个组织都在使用密钥扩展来保护用户密码。事实上,LastPass使用PBKDF 2-SHA 256与$10^5$哈希迭代的性能超过了2017年NIST最低推荐值一个数量级。然而,我们的分析描绘了一幅黯淡的画面:所采用的密钥拉伸级别对用户密码提供的保护不足。特别是,我们提出了强有力的证据表明,大多数用户密码遵循Zipf定律分布,并描述了一个理性的攻击者的行为时,用户密码从Zipf定律分布。我们表明,有一个有限的阈值,这取决于Zipf的法律参数,其特征在于一个合理的攻击者的行为-如果破解的密码的值(规范化的计算密码哈希函数的成本)超过这个阈值,然后对手的最佳策略是总是继续攻击,直到每个用户的密码已被破解。在所有情况下(雅虎,Dropbox,LastPass和AshleyMadison),我们发现破解的密码的值几乎肯定会超过这个阈值,这意味着理性的攻击者会破解从Zipf定律分布中选择的所有密码(即,大多数用户密码)。即使我们采用攻击者收益递减的侵略性模型(例如,$500$百万破解密码的总值小于$100$乘以$5$百万密码的总值)。见论文全文摘要。

英文摘要:

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 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). See paper for full abstract.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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