04-02-2013, 03:07 PM
CAPTCHA
CAPTCHA.pptx (Size: 815.58 KB / Downloads: 30)
History
First developed by Alta Vista in 1997
The term coined in 2000 at CMU by Luis Von , Manuel Blum And Nicholas Hopper of Carnegie Mellon university.
He decided to add a test to the submission page for protection against bots.
He reversed the Turing test.
What is a Turing test?
Proposed by Alan Turing
It is a test of a machine’s ability to exhibit intelligence equivalent to a human.
Human judge asks questions to two participants, one is a machine , one is human. He doesn’t know which is which
If judge can’t tell which is the machine, the machine passes the test
CAPTCHA employs a reverse Turing test,
judge = CAPTCHA program,
participant = user
if user passes CAPTCHA, he is human
if user fails, it is a machine
Background
Why CAPTCHA was needed?
Sabotage of online polls (happened in 1999 in www.slashodot.org ,CMU flooded the polls by automated generated program)
Abusing free online accounts by multiple registration( In 2000,Yahoo’s popular messenger chat service was hit by bots )
Constructing CAPTCHAs
Things to keep in mind:
Don’t store CAPTCHA solution in Web page’s metadata
A CAPTCHA is no good if it doesn't distort
Need a large database of different CAPTCHA questions
Avoid repetition of questions
Breaking CAPTCHAs
Greg Mori and Jitendra Malik have broken text CAPTCHAs, e.g., Ez-Gimpy
To break this CAPTCHA
Preprocessing: Removal of
background clutter and noise
Segmentation: Locate possible
letters in the image
Classification : Identifying the
character in each region
Summary
CAPTCHAs are an effective way to counter bots and reduce spam
Applications are varied– from stopping bots to character recognition & pattern matching.
Recaptcha is adding a lot to digitization of books . So next time if you are solving a captcha feel good as you are solving something which computers still can’t and preserving a part of history.