The term Captcha is an acronym for "Completely Automated Public Turing-Test To Tell Computers and Humans Apart" and describes a fully automated test to distinguish computers from humans. Captcha has its roots in the Turing Test, invented by Alan Turing in 1950, used to determine whether a machine has an ability to think that is equivalent to that of a human being.

General information[edit]

The term captcha was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University. They tried to find a way to distinguish between man and machine on the Internet. The resulting Captcha system protects input forms from being filled out automatically by programmed robots (bots).

The use of captcha involves a test that is too difficult to be solved by a computer, but simple enough for a human to do without problems. Each time a user attempts to access the system, the captcha is generated automatically by a random generator. This ensures that there are no frequent repetitions.

The first variations of Captchas[edit]

Around the year 2005 Captchas could be divided into four basic classes. They are called Gimpy, Sounds, Bongo and PIX. Each of these classes has further variation possibilities, which differ however only minimally. The principle remains the same.

  • Gimpy: The Gimpy Captcha is the most commonly used form of the existing Captchas, in which the texts can consist of arbitrarily lined up letters as well as of normal words. The user is shown an image file that a machine cannot decipher. The principle is thus based on the human ability to read highly distorted and incorrect texts or words. The user is prompted to enter these into an authentication system via the keyboard. Originally, this captcha was developed for Yahoo to prevent bots from accessing their chat rooms. The developers also wanted to prevent access to scripts that automatically create email accounts to send spam emails.
  • Sounds: These captchas work similar to Gimpy. A random word or sequence of numbers is stored in a sound file, distorted and played back. The sound file is played to the user and they have to enter the content into an authentication system. The advantage is that this Captcha can also be solved by blind people
  • Bongo: With this type of captcha, the user has to solve a visual pattern recognition problem. Two blocks are generated that consist of several images and differ in one characteristic. The user must recognize this feature. They must then assign another, new image to one of the two blocks by means of a characteristically matching characteristic. For example, if two blocks differ in line width and the next figure displayed is thin, it must be assigned to the block where the line width is smaller.
  • Pix: This is a program that has a large database of inscribed images. Each of these images represents a concrete object, such as a tree, a chair or a house. The captcha program randomly selects an object and displays four images of the same type. The user has to enter the answer to the question, which object it is, into an authentication system. Possible variations are the number of images or a distortion.

New types of Captchas[edit]

In today's world, HTML can make captchas less frustrating and faster to solve for the user. The service "Are You a Human" provides Captchas without unreadable signs. The classic Captchas are replaced by mini-games, so-called PlayThrus. For example, the user has the task of placing certain items from a given selection in a box. The plug-in is available for common content management systems such as WordPress or Drupal.


Captchas offer easy to implement, simple protection mechanisms against unwanted access to a website. However, they do not guarantee 100 percent protection and also annoy users. Spam tools pose an ever-increasing threat.

Other ways of proving legitimate users, such as providing a mobile phone number to send a security code by text message, are also possible. This form, however, has the disadvantage that one has to intervene more deeply in the personal data of the user. Ultimately it is up to the individual to decide which approach to take.

Web Links[edit]