Breaking the CAPTCHA

Posted by Kurt on May 12th, 2008

The CAPTCHA suffers several weaknesses: fixed font face, fixed font size, no distortions, trivial background noise, and it’s easy to segment. In this experiment, a three step algorithm has been developed to break the PayPal CAPTCHA. The image is preprocessed to remove noise using thresholding and a simple cleaning technique, and then segmented using vertical projections and candidate split positions. Four classification methods have been implemented: pixel counting, vertical projections, horizontal projections and template correlations. The system was trained on a sample of twenty PayPal CAPTCHAs to create thirty-six training templates (one for each character: 0-9 and A-Z). A separate sample of 100 PayPal CAPTCHAs were used for testing. The following success rates have been achieved using the different classifiers: 8% pixel counting, vertical projections 97%, horizontal projections 100%, template correlations 100%. Three of the trained classifiers out perform the 88% success rate of Pwntcha.



  1. Original:
  2. Grey Scale:
  3. Thresholding:
  4. Further Cleaning:


  1. Segmented:
  2. Padded:


  • Pixel Counting: 8% Break Rate
  • Vertical Projections: 97% Break Rate
  • Horizontal Projections: 100% Break Rate
  • Template Correlations: 100% Break Rate


The final paper including MATLAB source code, sample runs, and results can be downloaded here or from the RIT Digital Media Library.


A copy of the slides used for a presentation of this experiment can be downloaded here.


The 20 training and 100 testing PayPal CAPTCHA images are available to download here.

Source Code

Complete MATLAB code (281 lines, well commented) for preprocessing, segmenting, and classifying the images is available here.

YouTube Video

Note that this video wasn’t created by me. Skip forward to approximately the 1 minute mark.

GCCIS Welcomes 1st Graders from Canandaigua

Posted by Kurt on May 5th, 2008

RIT hosted the Golisano College Kids of 2023 for an activity inspired by CS Unplugged. We had a ton of fun with the 26 first graders from Canandaigua Primary School and even taught them how to convert to and from binary!  There’s a short blurb about it in the GCCIS Women in Computing 2007/2008 Year in Review.  The class was celebrating their internationally award-winning video that promotes women in technology.  You can watch the video below:

Modified version of Webby Blue
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