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Pro PHP Security: From Application Security Principles to the Implementation of XSS Defenses, Second Edition by Thomas Myer, Michael Southwell, Chris Snyder

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Kinds of Captchas

There are many different kinds of captchas. We discuss the advantages and disadvantages of the most common ones in this section.

Text Image Captchas

Text image captchas are the most common version, as they are relatively easy to deploy. The user sees a possibly distorted image, which may be either an ordinary word or a miscellaneous collection of typographic symbols (letters, numbers, and even punctuation), and then enters into a form whatever she has seen. The submitted entry is compared to the known answer to the challenge, and if it matches, the user is permitted to continue.

Figure 9–2 shows a simple text image captcha, where no attempt has been made to distort the word contained in the image beyond setting it in an unusual ...

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