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Enhancing Security Against Hard AI Problems in User Authentication Using Captcha as Graphical Passwords

Murugavalli S1, Jainulabudeen SAK2*, Senthil Kumar G2, Anuradha D2

1HOD/Professor, Department of CSE, Panimalar Engineering College, Chennai, India

2Assistant Professor, Department of CSE, Panimalar Engineering College, Chennai, India

*Corresponding Author:
Jainulabudeen SAK
HOD/Professor, Department of CSE, Panimalar Engineering College, Chennai, India
E-mail: [email protected]

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Information and computer security are supported by the passwords, as passwords play a vital role in authentication process. The traditional authentication method uses text- based passwords, which is also called as alphanumeric passwords, is not reliable in data security, and to overcome these drawbacks, graphical password scheme is introduced as an alternative to text-based passwords. But graphical password scheme is vulnerable to shoulder surfing attacks, spyware attacks. To overcome this vulnerability of graphical passwords, an emerging technique CAPTCHA, as a challenge response test is generated to distinguish humans from bots in authentication. To ensure security on hard AI problems, CAPTCHA as graphical Password (CaRP) scheme is introduced as an alternative method to textual CAPTCHA’s. As CaRP scheme has scope of refinements in cyber security a two-way authentication method is proposed in one of the CaRP techniques of Recognition-based scheme.


Textual CAPTCHAs, Authentication, Shoulder surfing attacks, Cyber security, CaRP


Information Security is an important factor in security systems nowadays. The security of the systems is provided by the user authentication. Authentication is the process of verifying the identity of a particular person to ensure security in any security systems. The most famous method is password authentication. A user gaining access into any security system should be validated by an authentication followed in that system. This secure authentication primitive is now almost used in all online transactions (such as accessing email accounts, entering a secure vault and so on). When that sensitive information is accessed under unauthorized user, the entire security of the system will collapse and become unreliable. Hence for this secure authentication purpose conventionally we made use of the textual passwords which is also called as alphanumeric passwords. These alphanumerical password [1] can be personal names of family members, phone number, pet name etc., and vulnerable to various attacks like password guessing, dictionary attacks, spyware attacks etc., and hence found that the textual passwords are inefficient to resistant some security and usability problems. This short come in alphanumerical passwords led to the development of graphical password schemes [2]. The graphical password scheme uses images as password to remember easily than text.

The images and password space used in the graphical password technique is large enough, and thus can offer resistance to all possible attacks of text-based password. In such way graphical passwords are tricky to guess and uncomplicated to remember. But also there are some drawbacks of graphical passwords [3], such as password registration and log-in process require much more storage space than text based passwords, exposed to shoulder surfing attacks. So after graphical password an emerging security technique CAPTCHA (Completely Automated Public Turing-test to tell Computers and Humans Apart) was developed as a challenge- response test to identify the interruptions of bots during user authentication. This technique is found difficult for bots and easier for human discernment and followed in some foremost web sites of Microsoft, Google, Yahoo etc., have their own CATCHAs for addressing malicious programs.

The most widely used CAPTCHAs are the textual CAPTCHA, displayed as a distorted textual image along with noise for visually impaired ones. These textual CAPTCHAs were found to be inefficient towards online dictionary attacks and several other attacks, CAPTCHA as graphical password (CaRP) [4] technique is introduced in this paper to ensure security on hard Artificial Intelligence (AI) problems; a two-way authentication technique is used in one of the CaRP techniques of Recognitionbased method [5]. The rest of this paper will deal with the related work in the field of CAPTCHAs and security [3], a more detailed description of our proposed work [6], the methods and algorithms to be used [7], conclusion and future developments [8].

Related work-an overview of textual Captcha

CAPTCHA [9] is a technique first used by computer scientists at Carnegie Mellon University in 2000 to resist against some malicious programs. A CAPTCHA ("Completely Automated Public Turing test to tell Computers and Humans Apart") is a challenge-response turing test used for computing, to determine whether the user is human or bot. Textual CAPTCHAs [10] as shown in Figure 1 are designed as distorted text images and sometimes with noise addition which are identified by humans easily. The authentication of the user is validated only after the distorted image on the screen is entered.

Text-based CAPTCHAs are the most widely used for security reasons in web application to tend for authentication process like registration, query processing, login validation etc., as shown in Figure 1 but there are some common weaknesses in handling textual CAPTCHAs. The Number of characters and digits used in this technique follow a same font patterns, and hence they are identified easily through OCR or Optical Character Recognition Technique (Figure 1).


Figure 1: Textual CAPTCHA [2].

When the noise is added to the text based CAPTCHA it creates a problem in recognition [11] during login, as the characters in that image have different shapes as depicted in Table 1. This problem is more prevalent in Text based CAPTCHA. In addition to the above mentioned disadvantages, users suffer from blurred vision and wave motion in distinguishing those characters in textual CAPTCHAs (Table 1). Text Captchas Remarks
1. image Here the alphabets “c” and “l” maybe misinterpreted as “d”
2. image The first two alphabets “T” and “I” maybe misinterpreted as “T” and “l”

Table 1: Confusing Text- based CAPTCHAs.

Graphical Password

The concept of graphical passwords was first described by Greg Blonder [Blonder. G, Graphical Passwords, Patent 5559961 at 1996 in United States] [12]. The idea of this is to allow a user to click on the set of images displayed on the screen rather than text- based passwords [13]. To gain access in the system, the user has to click on the same images sequentially again which they have chosen already in the registration. As this is easier for human memory rather than text-based it provides a way of userfriendly passwords (Table 2).

There are numerous graphical password schemes [6,7] which can be classified into three categories according to the recording and entering passwords are recognition, recall, and cued recall. Some of the graphical passwords schemes are listed below depicted in Table 2 along with their merits and demerits. Text Captchas Remarks
1. image Here the alphabets “c” and “l” maybe misinterpreted as “d”
2. image The first two alphabets “T” and “I” maybe misinterpreted as “T” and “l”

Table 1: Confusing Text- based CAPTCHAs.

Method Used Merits Demerits
Novel Authentication Scheme Better Protection against Denial of Service Attacks Increases the costs of Online Dictionary Attack
Cued Recall Technique Complex Real World images can be used Only Individual Click Points are considered
Draw-A- Secret (DAS) and STORY It overcomes a drawback of Recall- Based systems Hotspot was still a serious Security Problem
CORTCHA Technique is Scalable It modifies images to generate challenge and images appear unpleasantly
Blowfish Algorithm, Window Clustering Algorithm and Dictionary Generation Algorithm Preventing Dictionary Attacks [25] and
E-Mail Spam [26]
Supports Mobile User
Verification Only

Table 2: Graphical Password Scheme.

Appending graphical passwords with Textual captchas

CaRP : CAPTCHA as Graphical Password Algorithm

In CaRP, for every login attempt a new image as shown in Figure 2 is generated for the same user. CaRP, which uses the alphabet is from visual objects (e.g., alphanumerical characters, similar figures) to generate a CaRP image, is an major challenge in Captcha **Source: Internet (Textual CAPTCHA) A major difference between CaRP images and Captcha images [14] is that all the visual objects in the string should appear in a CaRP image allows a user to input any password but not inevitably in a Captcha representation. Many Captcha schemes can be converted to CaRP schemes by following the above. CaRP Schemes is classified into two categories are Recognition-Based (Act of recognizing or condition of being recognized) [5,11] and Recognition-Recall [15]. Among these we discuss about Recognition-based CaRP [16] technique here (Figures 2 and 3).

Recognition-Based CaRP

In this scheme, a password is a progression of optical stuff in the speckled alphabets. The traditional recognition-based graphical passwords, detection-based CaRP seems to have access to an infinite number of different visual objects on the screen. The 3 techniques under this scheme are discussed below.


A recognition-based CaRP scheme is similar as text Captcha, with whose alphabets consists of characters are without any visually confusing characters. The ClickText [17] image has mostly 33 characters based on the servers. These characters are randomly arranged on 2D space for user access (Figure 4).

Here the password preferred for authentication is a sequence of characters e.g. =“# 9CBYCU”. It is same as text password with difference in their spatial arrangements. The ClickText image generated by the Captcha engine is authenticated by the server according to user-clicked points on the ClickText image as shown in Figure 4 by authentication user.


Figure 4: A ClickText Image [1].

The idea behind the ClickText image [18,19] is different from normal text Captcha. In text Captcha user has to type the characters from left to right sequentially and in ClickText user has to click the characters in password. In the above example user has to click the characters in the order as „#‟, „9‟, „C‟, „B‟, „Y‟, „C‟, and „U‟. If this orders for given password example is followed by user, then user is an authorized user.


It is also a recognition-based CaRP scheme, based on Captcha Zoo images [20] Here an alphabet consists of similar animal figures e.g. dog, horse, pig, donkey etc., for every animal 3D model [21] is used for image representation. In accordance with the Captcha generation process, Images for authentication purpose is generated eg. ClickAnimal images as shown in Figure 5 are generated for authentication process. The 2D models are generated from the 3D animal image with minor variations in views, colours, textures, and lightning effects and if entail distortions are also included. These resulting 2D animals are placed on the cluttered background for authentication. In the 2D model of ClickAnimal image [22], a possibility of overlapping of animals may occur, but without any change in their core parts.

This will lead a difficult identification for bots and easier for humans during login as shown in Figure 2. Here the password is a sequence of animal names such as password = “Dog, Cat, Turtle, Fox” etc. The ClickAnimal has a less significant alphabet and so the password legroom obligatory is also less as compared to ClickText as number of analogous animals is less than the number of available characters.


Figure 2: Appending textual CAPTCHA with graphical password.

AnimalGrid: A Two-way Authentication Technique

To resist the human guessing attacks like shoulder surfing [23], spyware attacks [24] etc.,the password space required for CaRP scheme should be sufficiently larger than other schemes. So here in AnimalGrid CaRP scheme the password space is increased by combining click animal with the grids depending on the size of the selected animal. AnimalGrid as shown in Figure 5 is a amalgamation of ClickAnimal and CAS (Click-ASecret) (Figure 5).

In CAS, a user clicks the grid cells of the corresponding animal in a password. In this AnimalGrid, ClickAnimal image is displayed first for two-way authentication technique [25]. After an animal is selected from a given images, an n*n grid equalizing their size will appear for user identification. As shown in Figure 3, when the red turkey in the left image was selected a 6*6 grid equalizing an animal is generated.


Figure 3: Image with textual CAPTCHAs **


Figure 5: A ClickAnimal Image [1].

In this scheme password is a sequence of selected animals interleaving with grid cells [26]. Here password must begin with animal name. Eg. pwd=“Cat, Fox, Grid (3), Dog, Grid (2), Grid (1)”.Where Grid (3) means the grid-cell indexed as 3 and grid cells after an animal name means the grid is determined by the bounding rectangle of that animal as shown in Figure 6. Here the correct animal should be clicked for the correct follow up grid. If wrong animal is clicked, the follow up grid is also made wrong and entire registration will collapse. Figure 6 gives more concentration on enhancing security, here the user will get more secured options for their substantiation the illustration is given as aCaRP as key in it can be used in new user registration and login attempts. Captcha covers the gap between the User and System in finding certain AI problems. It can be visualized via text or Image recognition.


Figure 6: 6 x 6 Grid Cells determined by colour bounding rectangle.

In Cued Click Points (CCP) [10], for every login attempts subsequent click point grids are retrieved to determine the tolerance of the original point. With CCP, we use f (username, currentImage, current Tolerance Square) this function to determine next image that distinctively maps each forbearness four-sided figure to a next-image (Figure 7).


Figure 7: ClickAnimal Image.

As its limitation in duplicate image during multiple tolerance points a two-way authentication, which helps in identifying an error immediately after the click point is used as an enhanced security in Recognition-based CaRP scheme.


Login Attempts for user attempts leads to 25% failure that Tests when a bigger interval cared-for have a lot of failed makes an attempt. Some participants contributed considerably more failing makes an attempt than others. At the tip of tests, in a total of 50 participants 100% participants remembered their passwords, 97.5% remembered their passwords of each ClickText and Click Animal, and 83% remembered their Normal AlphaNumerical passwords. One of the users forgot the AnimalGrid parole at the one hour test, and another one forgot the ClickText parole at the one-week check depicted in Figure 8. For Text, three participants forgot their passwords at the one-week check, and a more forgot at the three week test. ClickAnimal scored the most effective in memo ability whereas Text scored the worst. This could be part attributable to the very fact that hotspots were allowed for PassPoints passwords, which Text passwords had a far larger alphabet than each ClickText and AnimalGrid. Graphical passwords schemes are compared based on the ease of usage. CaRP has excellent budding refinements it combines both the Captcha and Graphical Password Scheme (Figure 8).


Figure 8: Comparison of Graphical Scheme.

Conclusion and Future Enhancements

In this paper, the various password techniques such as textual password, graphical password, Captcha password and CaRP has been studied. The best alternative for textual password is a graphical password. The graphical password can reduce the burden of human memory as humans tend to remember graphics and images better than text. As graphical passwords are vulnerable to shoulder surfing and spyware attacks [27], the best alternative to graphical scheme Captcha technique [28] is used. Captcha can be recognized by humans and not by bots, and with its limitation in providing robust security a CaRP technique, which is the combination of captcha and graphical password is developed. It is relying on hard AI problems [29].

The Recognition-Based CaRP includes ClickText, ClickAnimal and Animal Grid techniques. In all these techniques every time a new image is generated and so all the techniques are resistant to shoulder surfing attack and secure than graphical password techniques. Also for attackers [30] to hack CaRP [31] more incentives are required as compare to Captcha as CaRP does not rely on any specific scheme. At present all the CaRP techniques are more secure as compare to other password techniques [32]. But also CaRP has a scope for refinements. So to increase a security the difficulty level of images can be increased at every login attempt and this level is based on the machine used to login and on the login history of the user. Another scope of improvement here is some CaRP techniques can be made three-way for authentication. E.g. If AnimalGrid and ClickText are combined then it will become three-way authentication technique.


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