Captchas

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
• Often not user-friendly to people with normal vision either; CAPTCHA system misbehavior is a common source of humor on the Internet.
• Spammers can simply pay humans in poorer nations to solve CAPTCHAs. In 2010, commercial CAPTCHA-solving services were seen charging $1 per thousand CAPTCHAs solved. At those rates, a fairly large spamming campaign can still be cost-effective.

References

• Wikipedia
• American Council of the Blind
• W3C Inaccessibility of CAPTCHA
• PARC history of Captchas