Difference between revisions of "Captchas"

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* [http://en.wikipedia.org/wiki/Captcha Wikipedia]
* [http://en.wikipedia.org/wiki/Captcha Wikipedia]
* [http://www.acb.org/board-minutes/bm070802.html American Council of the Blind]
* [http://www.acb.org/board-minutes/bm070802.html American Council of the Blind]
* [http://www.w3.org/TR/turingtest/ www.w3.org] Inaccessibility of CAPTCHA
* [http://www.w3.org/TR/turingtest/ W3C Inaccessibility of CAPTCHA]
* [http://www2.parc.com/istl/projects/captcha/history.htm PARC history of Captchas]

Revision as of 09:12, 15 June 2009

Anti-spam technique: Captchas
Date of first use: ca. 2001
Effectiveness: Medium
Popularity: Medium
Difficulty of implementation: Medium
Where implemented: MTA or MUA
Harm: High

Captchas (from "Completely Automated Public Turing test to tell Computers and Humans Apart") is version of mail challenges, adapted from a method originally designed to protect web forms from being completed by robots.

When using to protect a mailbox, a confirmation message is sent to the sender when he writes to the protected mailbox for the first time, indicating a web page where he will be asked to enter a textual code hidden in an image. If the correct answer is given, the sender is added to a whitelist so subsequent messages will be accepted without any further action.

Although some defenders of this method claim it provides "100 % spam stopped without loss of messages", in practice this method has many drawbacks including:

  • All of the general problems with Challenges
  • Not user friendly with people with physical deficiencies - blind people may not be able to correctly decode captchas (see American Council of the Blind