Following are specialized CAPTCHAs belonging to the genre 'Captchas With A Purpose'.
Apart from preventing online abuse they help in serving some 'practical purpose' as well which benefits human community.

1)Sentence Based Captcha


(A)None of us is as smart as all of us - Word 'of' and 'all' are swapped and garbage alphabet is filled in the white space.
Intelligence Quote.

(B)Anna Hazare breaks fast after 288 hours - Words 'Hazare' and 'after' are swapped and garbage is filled - A News Headline

(C)Bad officials are elected by good citizens who do not vote - Words 'elected' and  'citizens' are swapped. - Humorous Quote                                                    

Algorithm -
A sentence is selected. Two random words are selected from the sentence and are swapped. A Random alphabet is filled in each whitespace present in the sentence.Further distortions are added to the sentence image to make it harder for computers.Challenge is to guess the sentence and write the sentence in correct format by inserting whitespaces between words.

Purpose -
Sentence can come from variety of sources of different genre,it can comes from news website,jokes website,educational/general knowledge websites.Thus,this CAPTCHA can serve as a source of education,entertainment as well as enlightenment!
How cool! it is to catch up with latest news just while doing routine activities such as checking regular email,posting at blogs.

Read more here  

See Captcha 'live' in action at -

Try Sentence Based Captcha !

The gadget spec URL could not be found


2)Emotion Based Captcha


(a)Happiness                                                                    (b)Shock                                                                   (c)beautiful

Algorithm -
A statement or a graphic is displayed to the user depicting a particular human emotion. User has to choose an emotion describing his feeling as an answer .We also add  'NULL' images to the database which does not depict any emotion corresponding to which user has to type 'NULL' as an answer.


One measure for comparing emotions can be the Russell circumplex model of emotion illustrated in (1) . The two principal axes are excitement and pleasure. Russell states that these axes are orthogonal. For example, the emotion distress implies high excitement and displeasure, and so distress falls in the upper left quadrant of the model. However, low excitement and displeasure correspond to depression in the lower left quadrant. Emotions that are close to each other on the model are perceptually similar to humans, and vice versa. Opposite, or most dissimilar, emotions are diagonal to each other on the model. So, we can categorize emotions on the basis of the quandrant in which they lie and in this way group similar and dissimilar emotions. This model can be deployed in Emotion based CAPTCHA for identifying correct and incorrect answers when tags depicting emotion attached with the image are not sufficient, thus will improve the accuracy of CAPTCHA.

Purpose -
Database for this CAPTCHA can be generated from an online game with a purpose.A simple game can be "Identical Emotions‟ - player and partner are given same image and they have to type emotion corresponding to that image, they keep on getting images till their emotions are same, when their emotions differ, game is over, more their emotions are identical, more points they get. Also, image for which their emotions differ is displayed to them and they also see each others responses for it.
It will help to categorize images belonging to different genre such as 'sorrowful', funny', 'shocking', 'beautiful' etc.Also captcha can be indirectly used for conducting online polls on a very large scale. 

See Captcha 'live' in action at -

Try Emotion based Captcha!

The gadget spec URL could not be found

Try Emotion based Advertisement Captcha

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Multiple Advertisements Emotion Captcha!

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Identical Emotions - Game With a Purpose 

'Game with a Purpose' was innovative idea developed at CMU. These games help computers learn to think more like humans.One plays the game computers get smarter.Some of these games include ESP game which helps to label images on the web, Verbosity which help to collect common sense facts, Matchin which helps to generate best images etc.These Games are available on

This game is an extension of 'ESP' Game whose main purpose was to label images on the web. Identical emotions,instead of assigning description to the image,helps to assign emotions to the images,thus provides tags describing emotions depicted by the images.This helps to classify images according to different genre such as 'sorrowful', funny', 'shocking', 'beautiful' etc.  

Description of the system -

The game is played by two partners and is meant to be played online by a large number of pairs at once. Partners are randomly assigned from among all the people playing the game.
Players are not told whom their partners are, nor are they allowed to communicate with their partners. The only thing partners have in common is an image they can both see.Challenge imposed by Identical Emotion to players is that they have to type 'emotion' the image is depicting and player and his partner have to type same emotion to win points.Once both players have typed the same string, they move on to the next image and we say that their emotions are 'identical' .

                                                                       (1)                                                                                                  (2)  

In the figure (1) when player and partner type the same emotion depicted by image for example love,they move on to the next image.

Partners strive to agree on as many images as they can in 3 minutes. Every time two partners have 'Identical Emotions' on an image,they get a certain number of points.More their emotions are Identical,more points they get.Players can also choose to pass or opt out on difficult images. If a player clicks the pass button, a message is generated on their partner’s screen; a pair cannot pass on an image until both have hit the pass button. As the player and his partner have to type same string describing emotion for the image to move on,they have to agree on the same emotion depicted by the image,thus string which they type can act as a good tag for the image, describing emotion conveyed by the image. 

Off -Limit Words - If an image has been labeled many times before,system generated off limit words and players cannot use this words for labeling.These words act as Taboo words.These words make the game harder as the players now, will have to think of another possible emotion which image is conveying.Taboo words are obtained from the game itself. The first time an image is used in the game, it will have no taboo words.When it has been labeled by players in previous games, it will start having taboo words.


For example - Figure 3 can have off limit word 'love' associated with it and players will have to type another emotion depicted.In this example it can be 'happy'.
These taboo words guarantee that image will get different tags describing emotions associated with it.

When the game is over,player and partner can review the labels they submitted for the image as shown in (4)
Selecting the images-
For game to be successful images presented to the user should be evocative. Images are retrieved from flickr database. Images are searched for different keywords such as "face", "nature", "flowers”,"expression","celebration","emotion","movie","baby","children","books", "god" etc. Keywords are selected intelligently such as search will return images depicting different emotions. Moreover, for the game to be able to create a good database and to be able to serve Emotion based Captcha, images should pass through the system multiple times. This mean same image should be labeled by different players. More "times" a particular label depicting emotion has been assigned by different players to the image, more accurate the tag is for the image. Thus, system also selects the images which have already been presented to previous players and are present in the database. This, also helps in obtaining multiple tags for the images which is also essential for accuracy. In some cases, if Images are not evocative, Images could also be labeled "NULL" or simply passed for smooth functioning of the game.

When is an image done ?

One image passes through the system multiple times.When it has acquired many labels which become off limit words ,image becomes too difficult for players such that they begin to pass it each time.As this point of time,image is removed from the database.

Results -

Below collages illustrate different images on flickr with their emotional tags obtained from 'Identical Emotions'.



                                                                             Tag - Happy                                                                                      Tag - Love

3)Time Based Captcha

At t=1sec                                                                            At t=2sec                                                                    At t=3sec

Solution - ayt

Algorithm -
Splash random alphabets on random positions on a screen at different time instants.User will have to type alphabets in a sequence they appeared. Moreover, these Captchas expire if they don't receive a response within particular time interval. Please note that letters are distorted to make it harder for computers.Current implementation makes use of animated gif. This captcha can be made harder by asking users to enter the repeated sequence as well and playing repeated sequence random no. of times. In this way even if, program is able to extract frames from animated gif and recognize individual alphabets, it may not know, how many times sequence of alphabets is to be entered.

It can be a suitable means to measure certain reaction times of human beings. A good data about human beings which is otherwise difficult to obtain and is very useful.

See Captcha 'live' in action at -

Try Time Based Advertisement Captcha

The gadget spec URL could not be found

Animated Advertisement Captcha

Security of CAPTCHAs -

Most of the websites today use a single CAPTCHA to secure the login interface. As there are plenty of CAPTCHAs available today, different CAPTCHA problems can be used to secure a single login interface. Set of different CAPTCHA problems is maintained and a random CAPTCHA is selected from these CAPTCHAs before a login prompt is granted.Random CAPTCHA script, (A random CAPTCHA problem) is selected each time before giving login prompt to the user. If a login is unsuccessful, a random CAPTCHA problem is selected and presented to the user. As CAPTCHAs of different genre are presented randomly, this makes it very difficult to attack these CAPTCHAs. Even if a program is written to attack one CAPTCHA problem, devising an online password guessing attack becomes infeasible as after one unsuccessful try, a different random CAPTCHA problem will be presented to the user and attacking program should be capable to solve that as well. This problem, introduces a new problem of identifying CAPTCHAs, before attacking a CAPTCHA, attacking program will have to identify which CAPTCHA, it is as well.

How it aids in enabling CAPTCHAs serve some practical purpose?

CAPTCHAs such as sentence based CAPTCHA presented earlier might be bit annoying to people if presented to people everytime, but as in the above technique, a random CAPTCHA is presented each time, people might encounter it sometimes and might be willing to solve it to gain some useful knowledge. Moreover, as random CAPTCHAs are presented to the user, it is not possible to attack them as a graphic can be a sentence, image with different challenges such as figuring out sentence, typing emotion corresponding to image, typing letters, so each CAPTCHA has a different solution which can be figured out by humans more easily but is difficult for computer as it does not know what to give as a answer.

Increasing the Usability of CAPTCHAs using Partial Credit Algorithm-

Partial credit scheme is introduced in paper describing Asirra by Microsoft Research. It can also be used to make CAPTCHAs bit easy for people which are otherwise difficult. For eg- If in a Sentence based CAPTCHA, sentence being presented to the user is bit difficult and he is able to figure out say 5 out 7 words , he is given partial credit and a second CAPTCHA is presented to the user, this scheme can be combined with the above the system. Second CAPTCHA presented to the user may or may not be Sentence Based CAPTCHA, for example it might be Emotion based CAPTCHA and if person is able to get that partially correct as well, he is believed to have solved CAPTCHA challenge completely.

At present, most websites deploy reCAPTCHA which helps to digitize books, this CAPTCHA can be used in conjunction with other "CAPTCHAs with a purpose" to broaden the application domain of CAPTCHAs and make CAPTCHA solving more interesting and beneficial to the human community.

For more details -
Read my paper available here in the below folder.

Contribute to Solve Complex Problems while Playing a game!

See section on gameswithapurpose 

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  1047k v. 2 May 16, 2012, 7:15 AM Suhas Aggarwal