CAPTCHA is a program that is a challenge –response test to separate humans from computer programs.

Why CAPTCHA was needed???

To prevent the following :-

Multiple votes in online pollsWorms and Spam e-mailsSearch engine BOTsPreventing dictionary attacks(password

cracking)Tampering with rankings on

recommendation systems (e.g. e-bay, amazon etc.)

Online Poll In 1999-

MIT- 21156 voted

CMU – 21036 voted

AND……

Every Other College with less than 1000……….!!!!!!

First developed by Alta Vista in 1997.

The term coined in 2000 by Luis von Ahn , Manuel Blum and Nicholas J. Hopper of Carnegie Mellon University and John Langford of IBM.

Primitive CAPTCHAs seem to have been developed in 1997 by Andrei Broder, Martin Abadi, Krishna Bharat, and Mark Lillibridge to prevent bots from adding URLs to their search engine.

Luis von Ahn Manuel Blum

Picks a random string of letters:

o m a g

Renders it to a distorted image:-

HOW TO GENERATE

..... and generates a test

TYPE THE CHARACTERS THAT APPEAR IN THE IMAGE

1. Generic CAPTCHAs distort letters and Numbers

2. Distorted characters are presented to user

3. User has to recognize the distorted Letters

4. If the guessed letters are correct, the user is inferred to be a human and allowed access

5. Humans can read the distorted and noisy text.

6. Current Optical Character Recogniser(OCR)s can not read them.

PROCESS TO RECOGNISE CAPTCHA LOGIC

Proposed by Alan Turing

To test a machine’s level of intelligence.

Human judge asks questions to two participants, one is a

machine and other is a human, the judge doesn’t know

which is machine or which is human.

If judge can’t tell which is the machine, the machine

o passes the test.

oCAPTCHA employs a Reverse Turing Test,

judge = CAPTCHA program,

participant = user

if user passes CAPTCHA, he is human

if user fails, it is a machine

o Text Based CAPTCHAs

Gimpy, Ez-gimpy Gimpy-r, Google CAPTCHA Simard’s HIP (MSN)

o Graphics Based CAPTCHAs

Bongo Pix

o Audio or Sound Based CAPTCHAs

Typically relay on sophisticated distortion of text images rendering them unrecognizable to the state of the art of the pattern recognition programs but recognizable by humans.

Very effective, needs a large question bank.

Cognitively challenged users find it hard .

Gimpy:Originally designed by Yahoo and CMU.Based on human ability to read heavily distorted and

corrupted text. Picks up 10 random words from dictionary and distorts.User has to recognize at least 3 words.If user is correct, he is admitted.

EZ-Gimpy:A modified version of Gimpy.Used in Yahoo Messenger Service.It contains only one random character string.The word is random and not picked from the

dictionary, so not prone to dictionary attack.Its not a good implementation of CAPTCHA

and already broken By OCRs.

aesdp

MSN Passport service CAPTCHAs:

Its provided for Microsoft MSN services.Uses 8 characters. Warping is used to distort.Its very strongly implemented and hasn’t been broken.It is segmentation –resistant.

Requires user to perform image recognition test.

Ex:

WHAT ARE THESE PICTURES OF ?

BONGO:

After M.M.Bongard, pattern recognition expert.User has to solve a pattern recognition problem.Has to tell the distinct characteristics between two sets of figure.

PIX: Uses a large database of labelled images. It shows a set of images, user has to recognize the

common feature among those.

Ex:- Pick the common characteristic among the following four pictures

“Airplane”

Require user to solve a speech recognition test. Helps visually disabled users.Consist of downloadable audio clipUser listens and enters the spoken word

Advantages:

•Distinguishes between a human and a machine•Makes online polls more genuine.•Reduces spam and viruses•Makes online shopping safer.•Diminishes abuse of free email account services.

Disadvantages:

•Sometimes very difficult to read.•Are not compatible with users with disabilities.•Time-consuming to decipher.•Technical difficulties with certain internet browsers.•May greatly enhance Artificial Intelligence.

Usability issues: W3C mandates Web to be accessible to all

people.

Some CAPTCHAs are inaccessible to visually impaired, cognitively challenged people

Compatibility issues: JavaScript may need to be activated in browsers.

Some may need Adobe Flash plugin installed

• Alta vista first used a crude CAPTCHA in their Sites.

• Resulted in 95% spam reduction.

• Yahoo partnered CMU to counter these threats in

Messenger chat service.

• AI knowledge is advanced if CAPTCHAs are broken.

INTERESTING FACTS

CAPTCHAs are an effective way to counter bots and reduce spam.

They serve to 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.

Can I answer your questions about CAPTCHA?

YOU BETCHA!!!!