Graphical Passwords

Introduction

One of the key issues that every multi-user system has to manage is the identification of the users who are authorized to access its services. This problem has a long history in the field of computer security and its evolution has had a tremendous impact on user behaviour over the last few decades. Furthermore, it is not uncommon that the success of a service strongly depends on its ease of use, which includes the procedures that are needed to access it. From this point of view, crucial issues that every identification system has to explicitly validate are, on the one hand, its security and, on the other, the effort that users will need to use it, i.e., its usability.

The problem of identification has many different aspects, depending on factors such as the specific application scenario in which it has to be solved, the technological constraints (or freedom), the security level of the information to be protected, the users expectations as well as their willingness to trade effort with security.

Nowadays, it is possible to assume that all the potential users of a service are used to Password or PIN based identification systems. This assumption comes from the very simple fact that operations such as logging onto a personal computer or withdrawing money from an ATM are carried out by almost every human-being in developed countries on a daily basis.

These identification systems have the huge advantage of being (a) well known and (b) trusted by users. Such properties have clear benefits when deploying a new service in terms of user education and service acceptance. Nevertheless, password and PIN based authentication systems have well-known security issues such as guessing attacks, users problems in remembering different passwords and so forth. Moreover, user identification to distributed services poses the problem of securing the transmission of the user credentials through a potentially unprotected channel.

There have been many different improvements in password based authentication schemes over the years, including systems that prevent the selection of passwords “too easy to be guessed,” one-time password schemes as well as biometric identification devices. When considering the most important and well-consolidated solutions, the impression is that they all aim at solving certain security issues by introducing some form of overhead to the user operation. In other words, any security improvement is often paid in terms of ease of use.

Graphical Password schemes have emerged as a possible security enhancing and user-friendly alternative to the old-fashioned password based authentication schemes. The key idea behind this new approach is that the operational overhead as well as the increased size and complexity of passwords, introduced to improve security, could be mapped to some type of information that humans can easily handle: images.

Among several “human affordable” authentication protocols, Graphical Password schemes have been studied since the early nineties. However, there have been significant developments over the last decade, due to the increasing availability of human-computer interaction technologies. While earlier proposals were based on the idea of substituting letters and digits with images, more recent and sophisticated schemes have considered improving ergonomics by replacing classical interaction means such as the keyboard and screen with a plethora of new devices, including touch-screens, pointing devices at large or even devices capable of tracking user eyes movements as well as communicating through tactile stimulation.

This trend has had two significant consequences: (a) graphical identification systems are suitable for those pervasive computing devices (e.g., smart phones, PDAs) that no longer feature traditional human-computer interaction or have some computational constraint; (b) graphical identification systems can potentially outperform, in terms of user confidence and acceptance, the traditional identification systems in those applications where authentication and other security related tasks (e.g., stating authorization and access control rules) should be accomplished by poorly skilled users (e.g., elderly) or by people with impairments, someone who needs accessible interaction devices, for example, in the e-Health application scenario.

These considerations make graphical identification schemes a strategic research field that promises important innovations to ICT, especially to technologies for e-governance.

In this chapter, the world of Graphical Passwords will be reviewed. The review will not be a mere listing of existing schemes, it will extensively describe basic ideas that have been used by different systems, in order to try to evaluate how the different schemes deal with the two main concerns: security and usability.