A Survey on Breaking Technique of Text-Based CAPTCHA

Table 1

Typical types of text-based CAPTCHA and their features.

Type	Example	Source	Features
Solid CAPTCHA		Discuz!	Character independent, texture background, some interference
		Slashdot	A large number of interference lines and noise points
		Gimpy	Multiple strings, overlap, distortion
		Google	Unfixed length, distortion, adhesion
		Microsoft	Double-string, unfixed length, uneven thickness, tilting, adhesion
Hollow CAPTCHA		QQ	Hollow, shadows, interference shapes
		Sina	Hollow, adhesion, interference lines
		Yandex	Hollow, virtual contours, distortion, adhesion, interference lines
Three-dimensional CAPTCHA		Scihub	Hollow, shadows, interference lines, noise points
		Teabag	Grids, protrusion, distortion, background and character blending
		Parc	Colorful, shadow, rotation, zoom
			Special characters
Animation CAPTCHA		Program generating	Multiple characters jumping
		Hcaptcha	Multilayer character images blinking transformation