International Journal of Research In Science & Engineering
Volume: 1 Special Issue: 1
e-ISSN: 2394-8299
p-ISSN: 2394-8280
REVIEW PAPER ON CAPCHA (Completely Automated Public Turing test
to tell Computers and Humans Apart)
Kavita R. Menake 1 , Prof. A D Raut2
1
Student, Information Technology, JDIET, Yavatmal, [email protected]
2
HOD, Information Technology, JDIET, Yavatmal, India,[email protected]
ABSTRACT
Now a days, several daily activities are carry out through internet like communication, education,
Entertainment, E-commerce and tasks. For that user have to register with respective websites. During
registration, some introducer write malicious program for making automatic false enrolment that affect the
system resources. A CAPTCHA is a (an acronym for “Completely Automated Public Turing Test to tells
Computer and Human Apart”) type of challenge-response test to insure that response not generated by computer.
This form of CAPTCHA requires that the user type the letters of a distorted image, sometimes with the addition of
an conceal sequence of letters or digits that display on the screen. Because the test is conducted by a computer, in
distinct to the standard Turing test that is administered by a human. CAPTCHA is based on identifying the
distorted text, the colour of image, object or the background that human can easily distinguish but computer
cannot. CAPTCHAs are used by Yahoo, Hotmail, PayPal any many other Web sites to prevent automated
registrations, and it work due to no computer program concurrently read distorted text as well as humans can.
Keywords: Automated Turing Tests, CAPTCHA.
----------------------------------------------------------------------------------------------------------------------------1.Introduction
CAPTCHA was invented in 2000 at CMU by Luis Von Ahn, Manuel Blum, Nicholas J. Hooper and John
Langford. In our daily life online activities play an important role such as communication, commerce, banking, and
voting. But unfortunately, these online services are often misused by undesirable automated programs, or
“bots,”(computer program) that abuse services by posing as human beings to (for example) repeatedly vote in a
scale, add spam to online message , or open thousands of email accounts for various wicked purposes. One
approach to prevent such misuse has been the introduction of online security systems called CAPTCHAs. A
Captchas is nothing but the simple challenge-response tests that are generated by computers, and that are designed to
be easily solvable by humans but that are beyond the capabilities of current computer programs. If a correct solution
of this test is received, it is assumed that a human user (and not a bot) is requesting an Internet service [1].
There are three main categories of Captchas have been introduced. Text-based Captchas generate distorted
images of text which are very hard to be recognized by state-of-theart optical character recognition (OCR) programs
but which is easily recognizable by most humans. In Sound-based Captchas, that requires the user to solve a speech
recognition task, while others require the user to read out a given sentence to authenticate that he/she is a human.
Finally, image-based Captchas require the user to solve an image recognition task, in which user have to entering a
label to describe an image. Other work has combined multiple of these categories into multi-modal Captchas [2],
which can increase security when giving users a choice of the type of Captcha they wish to solve.
IJRISE| www.ijrise.org|[email protected] [255-258]
International Journal of Research In Science & Engineering
Volume: 1 Special Issue: 1
e-ISSN: 2394-8299
p-ISSN: 2394-8280
2. Types of CAPTCHA
CAPTCHAs are classified based on what is distorted and presented as a task to the user. They are:
2.1Text CAPTCHAs
These are simple to implement. In text CAPTCHAs, it involves text distortions and then asked to identify
the text hidden.
2.1.1 Gimpy
Gimpy is a very reliable text CAPTCHA built by CMU in collaboration with Yahoo for their Messenger
service. Gimpy works by ten words randomly choosing from a dictionary, distorted it and overlapped manner and
then displaying them. After that it asks the users to enter a subset of the words in the image. The human user is
capable of identifying the words in correct manner, whereas a computer program cannot [7].
2.1.2 Ez-Gimpy
This is a simplified version of the Gimpy CAPTCHA, accept by Yahoo for their signup page. Ez – Gimpy
work by randomly picks a single word from a dictionary and applies distortion to th e text and then user is asked to
identify the text correctly [7].
2.1.3 MSN CAPTCHA
For providing services under MSN umbrella Microsoft uses a different CAPTCHA. These are popularly
called MSN Passport CAPTCHAs. MSN CAPTCHA works by use eight characters (in upper case) and digits.
Background colour is grey Foreground colour is dark blue and Warping is used to distort the characters, to produce a
ripple effect, due to this computer recognition very difficult [7].
Fig.2.1.1:-Gimpy
Fig.2.1.2:-Ez – Gimpy
Fig.2.1.3:-MSN captcha
2.2 Graphic CAPTCHAs
Graphic CAPTCHAs are challenges because it involves pictures or objects that have some sort of similarity
that the users have to think virtually. This is one type of are visual puzzles like Mensa tests. Computer generates the
puzzles and grades the answers, but is itself is not able to solve it.
2.2.1 Bongo
Bongo is developed by M.M. Bongard who is a pattern recognition expert. In this user have to solve visual
pattern recognition problem. Bongo CAPTCHA work by two series of blocks are displayed, in which one series of
block differ from another block of series. The user has to find the distinct characteristics between that two series [3].
2.2.2 Pix
PIX includes large database of labeled images. In PIX, user has to find the common feature among the set
of images. Before displaying those to the users this set of images are distorted randomly. For example - pick the
common features among the following 3 pictures =” AEROPLANE”.[4]
IJRISE| www.ijrise.org|[email protected] [255-258]
International Journal of Research In Science & Engineering
Volume: 1 Special Issue: 1
Fig.2.2.1:Bongo
e-ISSN: 2394-8299
p-ISSN: 2394-8280
Fig.2.2.2:- pix[5]
2.3 Audio CAPTCHAs
In Audio Captch, the program picks a word or a sequence of numbers at random after that renders the word
or the numbers into a sound clip and distorts the sound clip; it then presents the distorted sound clip to the user and
task to the users to enter its contents .
Fig.2.3:- Audio CAPTCHAs
3. CAPTCHA Logic
The CAPTCHA image (or question) is generated in different way. The classic approach is to generate some
random text, apply some random effects to it and convert it into an image.
1. The CAPTCHA image (or question) is generated. There are different ways to do this. The one approach
is to generate some random text, apply some random effects to it and convert it into an image.
2. Step 2 is not really sequential. During step 1, the original text (pre-altered) is persisted somewhere, as
this is the correct answer for this captcha query. There are different ways to access the answer, as a serverside session variable, file, database entry, or cookie.
3. The generated CAPTCHA is presented to the user, who is assist to answer.
4. The answer supplied by the user by comparing it with the persisted (correct) answer by using the backend script. If the value is empty or incorrect, then go to step 1: a new CAPTCHA is generated. Users should
not get a second chance at answering the same CAPTCHA.
5. If the answer given by the user is correct, the form post is correct and user can continuous with this
processing.
4. Breaking the CAPTCHA
Breaking the CAPTCHA is the challenge lies in real hard task of teaching a computer how to process
information in a way similar to how humans think. artificial intelligence (AI) Algorithms will have to be designed in
way to make the computer think like humans when it have to recognizing the patterns in CAPTCHA images.
However, there is no universal algorithm that could pass through and break any CAPTCHA system. Thus,
each CAPTCHA algorithm must have to be tackled individually. It might not work hundred percent, but it can work
often enough to be worthwhile to the spammers [8].
IJRISE| www.ijrise.org|[email protected] [255-258]
International Journal of Research In Science & Engineering
Volume: 1 Special Issue: 1
e-ISSN: 2394-8299
p-ISSN: 2394-8280
5. CONCLUSION
CAPTCHAs are one of the mechanisms that protect web applications against abuse of automated form
submissions on many free online services that are we use in our daily life. This paper gives a detail description about
captcha and their various types of captcha with a literature survey and it also provides a description about working
of various Captcha. Further it provides the description about it logic.
REFERENCES
[1] L. Ahn, M. Blum, N. Hopper, and J. Langford, “CAPTCHA Using hard AI problems for Security, Springer
Verlag , 2003.
[2] A. Almazyad, Y. Ahmad, and S. Kouchay, “Multi-modal captcha: A user verification Scheme”, IEEE, 2011.
[3] Lienhart, R., Hartmann, A. “Classifying images on the web automatically”, J. Electron. Imaging Vol 11, 2002
[4] Mikhail M. Bongard, “Pattern Recognition. Rochelle Park, N.J.: Hayden Book Co.”, Spartan Books, 1970.
[5] www.slideshare.net/kunalkiit/seminar-report-on -c aptcha
[6] Lienhart, R., Hartmann, A. “Classifying images on the web automatically”, J. Electron. Imaging Vol 11,2002.
[7] Brighten Godfrey, “Text-based CAPTCHAs”,Unpublished Manuscript. 2001.
[8] http://www.captcha.net
IJRISE| www.ijrise.org|[email protected] [255-258]