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Crack (password software)

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Subject: Linux security software, John the Ripper, Crack, Dictionary attack, L0phtCrack
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Crack (password software)

Developer(s) Alec Muffett
Stable release 5.0a / June, 2000
Operating system Unix
Type password cracking

Crack is a Unix password cracking program designed to allow system administrators to locate users who may have weak passwords vulnerable to a dictionary attack. Crack was the first standalone password cracker for Unix systems[1][2][3][4] and (later) the first to introduce programmable dictionary generation.

Crack began in 1990 when Alec Muffett, a Unix system administrator at the University of Wales Aberystwyth was trying to improve Dan Farmer's 'pwc' cracker in COPS and found that by re-engineering its memory management he got a noticeable performance increase. This led to a total rewrite[5] which became "Crack v2.0" and further development to improve usability.


  • Public Releases 1
  • Legal issues arising from using Crack 2
  • Programmable dictionary generator 3
  • Network distributed password cracking 4
  • See also 5
  • References 6
  • External links 7

Public Releases

The first public release of Crack was version 2.7a, which was posted to the Usenet newsgroups alt.sources and on 15 July 1991. Crack v3.2a+fcrypt, posted to comp.sources.misc on 23 August 1991, introduced an optimised version of the Unix crypt() function but was still only really a faster version of what was already available in other packages.

The release of Crack v4.0a on 3 November 1991, however, introduced several new features that made it a formidable tool in the system administrators arsenal.

  • Programmable dictionary generator
  • Network distributed password cracking

Crack v5.0a[6] released in 2000 did not introduce any new features, but instead concentrated on improving the code and introducing more flexibility, such as the ability to integrate other crypt() variants such as those needed to attack the MD5 password hashes used on more modern Unix, Linux and Windows NT[7] systems. It also bundled Crack v6 - a minimalist password cracker and Crack v7 - a brute force password cracker.

Legal issues arising from using Crack

Randal L. Schwartz, a notable Perl programming expert, in 1995 was prosecuted for using Crack[8][9] on the password file of a system at Intel, a case the verdict of which was eventually expunged.[10]

Crack was also used by Kevin Mitnick when hacking into Sun Microsystems in 1993.[11]

Programmable dictionary generator

While traditional password cracking tools simply fed a pre-existing dictionary of words through the crypt() function Crack v4.0a introduced the ability to apply rules to this word list to generate modified versions of these word lists.

These could range from the simple (do not change) to the extremely complex - the documentation gives this as an example:

Reject the word unless it is less than 8 characters long, lowercase the word, reject it if it does not contain both the letter 'i' and the letter 'o', substitute all i's for 1's, substitute all o's for 0's, and append an = sign.

These rules could also process the GECOS field in the password file, allowing the program to use the stored names of the users in addition to the existing word lists. Crack's dictionary generation rule syntax was subsequently borrowed[12] and extended[13] by Solar Designer for John the Ripper.

The dictionary generation software for Crack was subsequently reused by Muffett[14] to create CrackLib, a proactive password checking library that is bundled with Debian[15] and Red Hat Enterprise Linux-derived[16] Linux distributions.

Network distributed password cracking

As password cracking is inherently embarrassingly parallel Crack v4.0a introduced the ability to use a network of heterogeneous workstations connected by a shared filesystem as parts of a distributed password cracking effort.

All that was required for this was to provide Crack with a configuration file containing the machine names, processing power rates and flags required to build Crack on those machines and call it with the -network option.

See also


  1. ^ David R. Mirza Ahmad; Ryan Russell (25 April 2002). Hack proofing your network. Syngress. pp. 181–.  
  2. ^  
  3. ^ Venema, Wietse (1996-07-01). "Murphy’s law and computer security". Proceedings of the Sixth USENIX UNIX Security Symposium. Retrieved 2012-02-17. 
  4. ^ Anonymous (2003). Maximum security. Sams Publishing. pp. 269–.  
  5. ^ Muffett, Alec. "Crypticide I: Thirteen Years of Crack". blog post. Retrieved 2012-02-17. 
  6. ^ Muffett, Alec. "Crack v5.0". Retrieved 2012-02-17. 
  7. ^ Sverre H. Huseby (15 March 2004). Innocent code: a security wake-up call for Web programmers. John Wiley & Sons. pp. 148–.  
  8. ^ Simson Garfinkel; Gene Spafford; Alan Schwartz (17 May 2011). Practical UNIX and Internet Security. O'Reilly Media, Inc. pp. 608–.  
  9. ^ Hakim, Anthony (2004-10-10), "Global Information Assurance Certification Paper Global Information Assurance Certification Paper", Intel v. Randal L. Schwartz (PDF), SANS Institute, p. 5, retrieved 2012-02-17 
  10. ^ "Randal Schwartz's Charges Expunged - Slashdot". Retrieved 2012-02-17. 
  11. ^  
  12. ^ Designer, Solar. "John the Ripper - credits".  
  13. ^ Designer, Solar. "John the Ripper - wordlist rules syntax".  
  14. ^ David N. Blank-Edelman (21 May 2009). Automating system administration with Perl. O'Reilly Media, Inc. pp. 461–.  
  15. ^ "Debian Package Search". Retrieved 2012-02-17. 
  16. ^ "CrackLib Enhancement Update". Retrieved 2012-02-17. 

External links

  • Password cracking - A quick guide to success
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