We present a novel approach to improving the security of passwords In our approach, the legitimate user's typing patterns (e.g, durations of keystrokes, and latencms between keystrokes) are combined with the user's password to generate a hardened password that is convincingly more secure than conventmnal passwords against both online and offime attackers. In additmn, our scheme automatically adapts to gradual changes in a user's typing patterns while maintaining the same hardened password across multiple logms, for use m file encryption or other applications requiring a longterm secret key Using empirical data and a prototype implementatmn of our scheme, we give evidence that our approach Is viable m practice, m terms of ease of use, improved security, and performance Textual passwords have been the primary means of authenticating users to computers since the introduction of access controls m computer systems Passwords remain the dominant user authentication technology today, despite the fact that they have been shown to be a fairly weak mechamsm for authenticating users Studies have shown that users tend to choose passwords that can be broken by an exhaustive search of a relatively small subset of all possible passwords. In one case study of 14,000 Unix passwords, almost 25% of the passwords were found by searching for words from a carefully formed "dictionary" of only 3 × 10 ° words [10] (see also [21, 4, 27, 29] ) This high success rate is not unusual despite the fact that there are roughly 2 x 10 TM 8-character passwords consisting of digits and upper and lower case letters alone In this paper, we propose a technique for improving the security of password-based apphcatmns by incorporating binmetric information into the password Specifically, our technique generates a hardened password based on both the password characters and the user's typing patterns when typing the password. This hardened password can be tested for logm purposes or used as a cryptographm key for file encryptlon, wrtual private network access, etc. An attacker who obtmns all stored system information for password ver~caUon (the analog of the/etc/passwd file m a typical Unix environment) is faced with a convincingly more difficult task Permmsmn to make dlgffal or hard copies of all or part of this work for personal or classroom use is granted without fee prov)ded that cop)es are not made or d)strlbuted for profit or commercial advent -age and that copies bear thts notme and the full citatmn on the first page To copy otherwise, to republish, to post on servers or to red)strlbute to hsts, requires prior specific permlss)on and/or a fee CCS '99 11/99 Singapore © 1999 ACM 1-58113-148-8199/0010 $5 O0 to exhaustwely search for the hardened password than in a traditional password scheme Moreover, an attacker who learns the user's textual password (e g., by observing it bemg typed) must type it like the legitimate user to log into an account protected by our scheme There are several challenges to realizing this goal. The first is to identify features of a user's typing patterns (e.g, latencies between keystrokes, or duration of keystrokes) that the user reliably repeats (approximately) when typing her password The second is to use these features when the user types her password to generate the correct hardened password At the same time, however, the attacker who captures system reformation used to generate or verify hardened passwords should be unable to determine which features are relevant to generating a user's hardened password, since reveahng this reformation could reveal reformation about the characters related to that password feature. For example, suppose the attacker learns that the latency between the first and second keystrokes Is a feature that is rehably repeated by the user and thus is used to generate her hardened password Then this may reveal information about the first and second characters of the text password, since due to keyboard dynamms, some digraphs are more amenable to reliable latency repetitions than others. Our approach effectively hides reformation about which of a user's features are relevant to generating her hardened password, even from an attacker that captures all system information. At the same time, It employs novel techniques to impose an additional (multiplicative) work factor on the attacker who attempts to exhaustwely search the password space. Using empirical data, we evaluate both this work factor and the reliabihty with whmh legitimate users can generate their hardened passwords Our empirical studies demonstrate various choices of parameters that yield both increased security and sufficient ease of use Our scheme Is very attractive for use in practice. Unhke other b]ometnc authentmation procedures (e.g., fingerprint recognition, retina or ins scans), our approach is unmtrusive and works with off-the-shelf keyboards. Our scheme initially is as secure as a "normal" password scheme and then adapts to the user's typing patterns over time, gradually hardening the password with biometrm information Moreover, while fully able to adapt to gradual changes m user typing patterns, our scheme can be used to generate the same hardened password indefimtely, despite changes in the user's typing patterns. Therefore, the hardened password can be used, e.g, to encrypt files, without needing to decrypt and re-encrypt files with a new hardened password on each logm. The main limitation of our scheme is that a user whose typing patterns change substantially between consecutive instances of typing her password may be unable to generate 73
Keystroke biometric is among the first to be used to generate biometric keys REF .
Password hardening based on keystroke dynamics
CCS '99
Computer Science