CAPTCHA

CAPTCHA stands for “Completely Automated Public Turing Test to Tell Computers and Human Apart”. It is a program to differentiate humans from computers for security and privacy purpose. CAPTCHA perform a task which a human can perform easily but require great computer processing power for that task. CAPTCHA is used to prevent worms and spam, search engine bots, website registration, comment spam in blogs and many more.

Nowadays, a variety methods of CAPTCHA are available in market, such as require responder to type in the distorted word that shown to them, listen to a distorted sound file over clustered background noise and type in the word or require responder to differentiate which image are related to the word given. Some form of CAPTCHA also require user to solve a very tough mathematic question. Those variety forms of CAPTCHA can be categorized into 5 categories:

• Visual CAPTCHA
• Image CAPTCHA
• Audio CAPTCHA
• 3D CAPTCHA
• Question CAPTCHA

Visual CAPTCHA

Visual CAPTCHA are most wide applying on the internet today. Typically visual CAPTCHA will present a cluttered image of a string of characters to users and require users to type the characters. The string of characters can consist of either alphabet characters or alphanumeric characters, moreover the alphabet characters can be in upper case or lower case. The common techniques of this category CAPTCHA are text deformation, rotation, perturbation, colour variation, distortion, and words taken from dictionary words or arbitrary strings and many more.

• reCAPTCHA

• Baffle Text

• Gimpy

Image CAPTCHA

Typical this type of CAPTCHA will present some images or pictures as well as shapes to the users. To past the test, users required to choose a word most related in the drop-down list box. Moreover some image CAPTCHA requires users to identify object among several images or click on the object in the image.

• ASIRRA

• PIX

Audio CAPTCHA

Audio types CAPTCHA using one or several speakers to present an audio file to the users. Users must carefully listen to the audio and enter the answer regarding to the audio. Currently audio CAPTCHA available in two format, spoken letters or digits at randomly spaced intervals and playing sounds related to an image. To enhance the security of the systems such as prevent unauthorized person using automatic speech recognition (ASR) programs to solve the CAPTCHA, audio need to be distorted or background noise is inserted into the audio. This type of CAPTCHA is mainly focus to the blind or low visual users.

• Picture/Sound CAPTCHA

• KillBot

3D CAPTCHA

Typically designing a program to recognize 3D objects are essentially difficult. In addition, by simply change the angle of images will result multiple images. Furthermore by random translation, rotation, scaling or lighting effects, the 3D objects will significantly change. Some computer security expert claim that this type of CAPTCHA as a solution to the weakness of existing CAPTCHA.

• 3-D CAPTCHA

• tEABAG_3D

Question CAPTCHA

Question CAPTCHA will ask responders a simple question to prove that the response is given by humans. What colour is the sky?, what are three plus four? are the samples of the questions asking in this type of CAPTCHA. Moreover, some require responders to solve a complex mathematic problem such as derivatives or polynomial factorization.

• Ironclad

• MAPTCHA