Nathaniel Johnston » ASP http://www.nathanieljohnston.com A blog of recreational math and coding Tue, 18 May 2010 12:49:57 +0000 http://wordpress.org/?v=2.8.4 en hourly 1 Math CAPTCHAs in ASP http://www.nathanieljohnston.com/index.php/2008/10/math-captchas-in-asp/ http://www.nathanieljohnston.com/index.php/2008/10/math-captchas-in-asp/#comments Fri, 10 Oct 2008 00:00:49 +0000 Nathaniel http://www.nathanieljohnston.com/?p=13 The idea of math-based CAPTCHAs has been around for a while, but it still hasn’t caught on much for some reason. Personally, I think the idea of a website asking you to enter the value of 7 + 8 is much less annoying than it asking you to squint and try to make out distorted letters and numbers. Thus, I have put together four ASP math-based CAPTCHA scripts that are freely-available for download and can be used or modified however you see fit. I have outlined the pros and cons of each of the scripts to help you decide which (if any) of them is right for you. Note that all four of these scripts make use of the AspJpeg component though the scripts could easily be modified to work with other components or produce text output.

Arithmetic CAPTCHA

This is the most basic of the math CAPTCHA scripts — it asks the user to provide the answer to a simple arithmetic question (either addition, subtraction, multiplication, or division). The script could easily be modified to ask more complicated questions such as “(12 / 4) + 7 = ?” or “33 = ?”.

Features and Benefits:

  • You can select the difficulty of the questions that the CAPTCHA asks.

Downsides:

  • If the difficulty is low, most answers will be in the range 1 – 25, which means that a SPAMbot has a decent chance of randomly guessing the correct answer.

Example:

 

Download:

 

Dictionary CAPTCHA

Quite a few websites now use CAPTCHAs that display words rather than random letters and numbers, and that is definitely a step in the right direction towards making CAPTCHAs less annoying. One step further though, is to use themed word lists that fit with the content of your website. This script does exactly that; it displays three words randomly selected from word lists that you create.

Features and Benefits:

  • Won’t alienate users with no math knowledge.
  • Comes with enough math terms to generate over 25 000 unique CAPTCHAs.
  • Word lists can easily be customized to fit any website.

Downsides:

  • Defeated reasonably easily by OCR software (this could be fixed by adding lines through the image and so on).

Example:

 

Download:

 

Sequence CAPTCHA

The Sequence CAPTCHA displays a sequence of numbers and asks the user to predict the next number in the sequence. The script generates one of three types of sequences: arithmetic, geometric, or Fibonacci-type. However, you could easily edit the code to generate some really nasty sequences if you dislike the visitors of your website that much.

Features and Benefits:

  • It’s reasonably immune to being broken by SPAMbots, unless someone spends a lot of time making a script specifically for your website.

Downsides:

  • Unless your visitors are very math-oriented, they will hate you.

Example:

 

Download:

 

Word Arithmetic CAPTCHA

This is a simple twist on the Arithmetic CAPTCHA that replaces the numbers by words and thus asks questions like “Ten × Four = ?”. The questions are slightly easier by default than the questions in the Arithmetic CAPTCHA script.

Features and Benefits:

  • Accepts both numeric and word input from the user.
  • Quite resilient against being broken by SPAMbots.

Downsides:

  • As with the Arithmetic CAPTCHA, a random answer in the range 1 – 25 has a decent chance of being correct.
  • Some questions like “Forty Three – Twenty Seven” might scare away certain users. Then again, those may be precisely the types of users that you want to scare away.

Example:

 

Download:

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