Table 1 Typical text-based CAPTCHAs

Gimpy [14]

Recognize three words from a list of seven from a dictionary at random

Multiple strings, overlap, distortion, rotation, noise, background

2D text-based

EZ-Gimpy [14]

Recognize a single English word from a distorted image

One string, distortion, gradients, non-linear deformation, noise, background

BaffleText [15]

Recognize a pronounced string of characters using difference masking

One string, distortion, difference masking

Megaupload CAPTCHA

Recognize four characters who are overlapping and have negative intersection areas

Fixed length, distortion, overlap, negative intersection

ReCAPTCHA V1 [16]

Recognize distorted text from old books that has been scanned

Two strings, distortion, noise, background, non-linear deformation

Teabag3D [17]

Recognize a character sequence that appears on a grid in 3D space

Grids, protrusion, distortion, rotation, background and character blending

3D text-based

Super CAPTCHA [18]

Recognize a string of 3D characters

Lines, protrusion, distortion, rotation, background, character blending

3DCAPTCHA [19]

Recognize a string of 3D characters

Texture, protrusion, distortion, background, character blending

DotCHA [20]

To identify each letter, drag and rotate the model, then type the answer

Twisted form, anti segmentation by small spheres, rotation, human interaction

HelloCAPTCHA [21]

Recognize a group of six characters in an animated GIF image

Multiple characters jumping, overlap, background, uppercase and lowercase mixing

Animated text-based

NuCaptcha [22]

Enter the final three red moving characters

Multiple characters jumping, rotation, coloring, adhesion, static and moving characters mixing, background

Dracon CAPTCHA [23]

Recognize five characters that fade and blur at different points throughout the animation frames

Fade and blur effect, changing character locations, background, noise, coloring, interference lines