Algebraic captcha

Stumbled on the Internet for interesting technology from captcha - user is asked to solve a linear equation with one root and enter the solution as the test code.

When I was on your blog searching for the captcha, have tried many different, but with a curious approach does not come across. I decided to look at it closer, opened your page and was disappointed - the task for CAPTCHAs displayed text, but no picture.
So out of curiosity decided to break her

In general, this captcha is a linear algebraic equation (ax + b = 0). For solutions used free computer algebra system Maxima . (Oh, why did I not know about it at 1-3 degree? It even can work on your PDA!).

The line with zadchey for CAPTCHAs can be obtained from html-code of the page about this form:

.
The tags

can be eliminated at the stage of obtaining the correct line, but great harm them not, so for clarity left.

Next line is a bit processed to bring to a form suitable for solution in the Maxima, followed by the actual decision to continue - processing of the output for the isolation of a desired verification code (the output from Maxima sufficiently detailed, even if you use the option-quiet):

  #! / Usr / bin / perl
 use warnings;
 use strict;
 use encoding 'utf8';

 my $ input = '<b> (<i> x </ i> +9) ∙ 5 +4 = 64 </ b>';
 $ Input = ~ s |</?[ b, i]> | | ig;
 $ Input = ~ s / ∙ / * / ig;
 $ Input = ~ s /-/-/ ig;
 print $ input, "\ n";

 my $ out = qx / maxima - quiet - batch-string = solve "($ input );"/;
 my $ x = $ 1 if $ out = ~ m # \ [x = (-? \ s? \ d +) \] # gi;

 print $ x, "\ n";

 exit 0; 

That's how I was lucky in one evening to get acquainted with an interesting idea for the mechanism of captcha and learn about the wonderful computer algebra system Maxima.

I wish you a lot of new and pleasant discoveries!

More on similar topics:

• CAPTCHA Generation in perl using Authen:: Captcha
• Automatic detection of character encoding in terminal perl-script.
• Automatic verification of passport readiness

Filed under: Linux , Gadgets , Technocracy | Tags: captcha , linux , Maxima , Security , algebra , programming | No Comments