04-04-2011, 10:21 AM
Presented by
Aparna
[attachment=11637]
Introduction
CAPTCHAà Completely Automated Public Turing test to tell Computers and Humans Apart
Invented at CMU by Luis von Ahn, Manuel Blum, et. al
A program that is a challenge – response test to separate humans from computer programs
Generic CAPTCHAs distort letters and numbers
Distorted characters are presented to user
User has to recognize the distorted letters
If the guessed letters are correct, the user is inferred to be a human and allowed access
Else, user is a bot and denied access
Humans can read the distorted and noisy text
Current OCRs cannot read them
Background
Why CAPTCHA was needed?
Sabotage of online polls
Spam emails
Abusing free online accounts
Tampering with rankings on recommendation systems (like EBay, Amazon)
Altavista first used a crude CAPTCHA in their sites
Resulted in 95% spam reduction
Yahoo partnered CMU to counter these threats in Messenger chat service.
Luis von Ahn and Manuel Blum of CMU trademarked CAPTCHA in 2000
What is a Turing test?
Proposed by Alan Turing
To test a machine’s level of intelligence
Human judge asks questions to two participants, one is a machine, 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
Types of CAPTCHAs
Text based:
Simple, normal language questions:
What is sum of three and thirty-five?
If today is Saturday, what is day after tomorrow?
Which of mango, table, water is a fruit?
Very effective, needs a large question bank
Cognitively challenged users find it hard
Aparna
[attachment=11637]
Introduction
CAPTCHAà Completely Automated Public Turing test to tell Computers and Humans Apart
Invented at CMU by Luis von Ahn, Manuel Blum, et. al
A program that is a challenge – response test to separate humans from computer programs
Generic CAPTCHAs distort letters and numbers
Distorted characters are presented to user
User has to recognize the distorted letters
If the guessed letters are correct, the user is inferred to be a human and allowed access
Else, user is a bot and denied access
Humans can read the distorted and noisy text
Current OCRs cannot read them
Background
Why CAPTCHA was needed?
Sabotage of online polls
Spam emails
Abusing free online accounts
Tampering with rankings on recommendation systems (like EBay, Amazon)
Altavista first used a crude CAPTCHA in their sites
Resulted in 95% spam reduction
Yahoo partnered CMU to counter these threats in Messenger chat service.
Luis von Ahn and Manuel Blum of CMU trademarked CAPTCHA in 2000
What is a Turing test?
Proposed by Alan Turing
To test a machine’s level of intelligence
Human judge asks questions to two participants, one is a machine, 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
Types of CAPTCHAs
Text based:
Simple, normal language questions:
What is sum of three and thirty-five?
If today is Saturday, what is day after tomorrow?
Which of mango, table, water is a fruit?
Very effective, needs a large question bank
Cognitively challenged users find it hard
