The Role of Visual Features in Text-Based CAPTCHAs: An fNIRS Study for Usable Security

Table 2

Final security features of eighteen types of text-based CAPTCHA used in our experiment.

Type#	Line features	Waving features	Rotation features
1	No line	No wave	No rotation
2	Line thin
3	Line thick
4			Max rotation 20
5		Wave light
6		Wave high
7	Line thin		Max rotation 20
8	Line thick		Max rotation 20
9	Line thin	Wave light
10	Line thin	Wave high
11	Line thick	Wave light
12	Line thick	Wave high
13		Wave light	Max rotation 20
14		Wave high	Max rotation 20
15	Line thin	Wave light	Max rotation 20
16	Line thin	Wave high	Max rotation 20
17	Line thick	Wave light	Max rotation 20
18	Line thick	Wave high	Max rotation 20

Max rotation 20 = rotation (−20, −15, 15, 20).