30-03-2012, 02:44 PM
CAPTCHA
captcha.ppt (Size: 569 KB / Downloads: 105)
Intro
CAPTCHA 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
Background
Why CAPTCHA was needed?
Sabotage of online polls
Spam emails
Abusing free online accounts
Tampering with rankings on recommendation systems (like EBay, Amazon)
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
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
Applications
Protect online polls
Prevent Web registration abuse, protect passwords from brute-force attack
Prevent comment spam and spam emails
E-Ticketing, prevent scalping
Summary
CAPTCHAs are an effective way to counter bots and reduce spam
They serve dual purpose– help advance AI knowledge
Applications are varied– from stopping bots to character recognition & pattern matching
Some issues with current implementations represent challenges for future improvements