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PWNtcha - captcha decoder

PWNtcha stands for "Pretend We’re Not a Turing Computer but a Human Antagonist", as well as PWN capTCHAs. This project’s goal is to demonstrate the inefficiency of many captcha implementations.

For an overview on why visual captchas are a bad idea, see Matt May’s excellent presentation, Escape from CAPTCHA, as well as the W3C’s Inaccessibility of Visually-Oriented Anti-Robot Tests working draft.

History

I created PWNtcha in 2004 as a personal research project, but only published my results, not the program itself. Given the number of captcha-breaking software available for sale now, I changed my mind and decided to publish the PWNtcha source code. It can be downloaded from Subversion:

 svn co svn://svn.zoy.org/caca/pwntcha/trunk pwntcha

Note that PWNtcha is now lagging 3 years behind captcha technology and is therefore no longer a very interesting piece of software.

Defeated captchas

PWNtcha is able to detect and decode the following captchas:

Origin	Samples	PWNtcha efficiency	Comments
Authimage		100%	Vendor site: http://www.gudlyf.com/index.php?p=376 Weaknesses: constant font, aligned glyphs, constant glyph position, constant rotation, no deformation, non-textured background, constant colours, no perturbation.
Clubic		100%	Weaknesses: constant font, no rotation, no deformation, aligned glyph, constant background, weak colour variation, weak perturbation.
linuxfr.org		100%	Weaknesses: constant font, aligned glyphs, no rotation, no deformation, non-textured background, weak colour variation, weak perturbation.
LiveJournal?		99%	Weaknesses: constant font, constant character position.
lmt.lv		98%	Weaknesses: constant font, almost aligned glyphs, no rotation, no deformation, constant background, no colour variation, weak perturbation.
Ourcolony		100%	Weaknesses: constant font, no rotation, no deformation, no colour variation, no perturbation.
Paypal		88%	Weaknesses: constant font, almost aligned glyphs, no rotation, no deformation, constant background, no colour variation, no additional perturbation.
phpBB		97%	Vendor site: http://www.phpbb.com/ Weaknesses: constant font, no rotation, no deformation, constant colours, weak perturbation.
Scode and derivatives		100%	Vendor site: http://james.seng.cc/archives/000145.html Weaknesses: at most 3 different fonts, no rotation, no deformation, weak colour variation, useless perturbation (separate colour key).
Slashdot		89%	Weaknesses: constant font, no deformation, constant colours, weak perturbation.
vBulletin		100%	Vendor site: http://www.vbulletin.com/ Weaknesses: constant font, fixed glyph position, no rotation, no deformation, almost constant colours, weak perturbation.
Xanga		49%	Weaknesses: fixed horizontal glyph position, no rotation, no deformation, constant colours, insufficient perturbation.

Other captchas and hard captchas

These captchas can currently not be defeated by PWNtcha. Note however that this is not an acknowledgement of efficiency; for instance, EZ-Gimpy can be easily defeated by other projects. However, the Passport/Yahoo? and CFXCaptcha captchas are probably going to last for a long time.

Origin	Samples	Comments
Drupal
Trencaspammers
Xanga (2)
20six		Extremely weak captcha, easily removed perturbation.
Authimage (3)		A very good captcha, but not always human-solvable.
CFXCaptcha		A very good captcha.
Clearscreen		Weak perturbation, but interesting use of non-alphanumeric characters.
Cwazymail		An excellent idea, but a critically buggy implementation.
EZ-Gimpy (eg. Yahoo! Briefcase)		Already defeated by another project.
Hoke		A very weak captcha.
.Mac		A weak captcha that is not always human-solvable.
ICQ		A pretty good captcha that uses a wide variety of backgrounds and fonts.
IMDb		A very good captcha with a very well thought implementation, but a small dictionary.
MS MVPS		Already defeated by another project.
MVN Forum		A pretty good captcha that uses a wide variety of backgrounds and fonts.
Passport		A very good captcha, but not always human-solvable.
Pichan		An apparently weak captcha.
Screenname		A pretty good captcha that uses a wide variety of backgrounds, fonts and deformations.
Yahoo!		A very good captcha, but not always human-solvable.