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CAPTCHAS and img name /displayed value association

Unfortunately I am having trouble summing up this question in one phrase/line, thus I am forced to initially hint as to what my understanding is before asking the question in fear of me asking a question based on wrong facts or assumptions. As I understand “and please correct me if am wrong”, as I understand captchas work like so:

  1. Have numerous images, and associate each image name/source to its displayed characters value.
  2. Display image, then have user input what they see.
  3. Match user's input against the character value which is associated with that image's name/source.

Assuming my understanding is correct: Given an unlimited amount of time,

  • Can't one associate image names/sources with the displayed characters increasing the chance of cracking the 开发者_JAVA百科captcha as they gather more associations?

  • In that case, wouldn't the security strength of captchas be parallel the size of the image database?

    NOTICE:

    As i suspected my question was based on wrong understanding.


Short answer! These are dynamic images and they are not stored anywhere. You wont even find them in the source code..

Wikipedia has good explanation about this. Alternatively check out the related questions in SO.

Edit: Goto this page where you can see an example of a captcha. Use firebug to see the HTML code for this image and you will see something like this.

<img height="57" width="300" src="http://www.google.com/recaptcha/api/image?c=03AHJ_VutaG4ahxWuQv0e6edYypp_FM8QuFIZkG75AnAm8iu3WRmwQ41jqcvojmKmbSKXxkf_s9fk61-axEp77_omKZZEYliE35BND_hXNh3Jac6ZUAeD08wOMZPj4W2s-A39vAI84eim5q-z9kFnmoSmon1jG2LmmFw" style="display: block;">

Did you notice the source? It does not point to an image file.

You can copy this url and generate the image (just open it in a browser). So you can develop an application which can download this image and then scan for color change in pixels and try to match for alphabets and numbers but if you notice almost all the alphabets and numbers are connected and closer so it is difficult to seperate different alphabets.

Even if you manage to seperate most of the alphabtes are not perfect. example :

CAPTCHAS and img name /displayed value association


(source: watblog.com)

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