Awase-E: Image-based Authentication for
Mobile Phones using User’s Favorite Images
Tetsuji TAKADA1 and Hideki KOIKE2
1SONY Computer Science Laboratories
Muse Bldg. 3-14-13 Higashigotanda, Shinagawa-ku, Tokyo 141-0022, JAPANzetaka@computer.org
2
Graduate School of Information Systems, University of Electro-Communications
1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, JAPANkoike@acm.org

Abstract. There is a trade-off between security and usability in user
authentication for mobile phones. Since such devices have a poor input
interfaces, 4-digit number passwords are widely used at present. Therefore, a more secure and user friendly authentication is needed. This paper
proposes a novel authentication method called “Awase-E”. The system
uses image passwords. It, moreover, integrates image registration andnotification interfaces. Image registration enables users to use their favorite image instead of a text password. Notification gives users a trigger
to take action against a threat when it happens. Awase-E is implemented
so that it has a higher usability even when it is used through a mobilephone.

1Introduction

As mobile phones become widely used in various situations, a suitable method
for user authentication is strongly required because they are often used as a user
terminal for e-commerce and mobile banking. We currently, however, only havethe option of using text-based authentication, such as user ID and password.
This, however, is undesirable because of the trade-off problem between security
and usability. It is better to use a longer password to ensure security. This, however, brings about usability issues such as the difficulty of remembering, recallingand inputting passwords. In particular, the difficulty of inputting passwords is
critical in mobile phones because of the tedious input interface. In my mobile
phone, for example, string “zetaka” requires 11 times of key typing. A simplified password, therefore, has been used such as a 4-digit number. However, we
think that this type of authentication does not qualified enough to meet security
requirements for e-commerce and mobile banking.
This paper proposes a novel authentication method for a mobile phone called“Awase-E”. We assume that it will be used with a mobile phone with a digital
camera. It uses photographic images taken by the users instead of text-based
passwords. We also integrate two kinds of user interfaces into current authentication frameworks so that it not only improves usability but also enhances its
security.

2

Awase-E: Image-based Authentication with Image
Registration and Notification Interfaces

Awase-E is an authentication system using photographs instead of passwords. It,
moreover, integrates image registration and notification interfaces into current
authentication frameworks(Fig. 1).

Setting a password

Current
authentication

Authentication
ServerUser Terminals

Image registration
Setting a pass-image
Awase-E

Authentication

Notification
ServerMobile phone withdigital camera

Fig. 1. Transition between Current and Proposed Authentication Framework

The image registration interface enables users to add their favorite images tothe authentication system. As a result, this makes it possible for users to use theirfavorite image as a “pass-image”. Almost 20 million users currently have mobile
phones with digital cameras in Japan. Most of them send photos by E-mail with afew key clicks on the spot. The image registration interface is implemented using
this function. It is implemented separately from a pass-image setting in order toensure the security against impersonation attempts. This function simply enables
users to add a photo to the system and a registered photo does not automatically
become a pass-image. In other words, not all registered images become a passimage. A user must set at least one pass-image before authenticating oneself
using Awase-E.
The notification interface gives users a trigger to handle a threat practically.
It notifies users of the occurrence of all kinds of events related to the authentication process. For example, Awase-E sends an E-mail to the user who hasregistered a photo. The E-mail has a URL. The web page that is linked by that
URL contains the photo that a user has just registered. A user can thus confirm
the registered photo immediately through a web page. If a user receives such anE-mail even though the user had not registered the photo, it means that someone
has registered it masquerading as a legitimate user. A legitimate user, therefore,
quickly knows when an intrusive attempt has been made. From these scenarios,
we would strongly recommend using Awase-E with mobile phones to ensure auser’s prompt awareness of a security breach.

Awase-E keeps an event history of past usage for certain periods for thepurpose of auditing the user’s authentication usage. A user can investigate thehistory through a web page. It enables users to check the authentication usage
even if a user has lost their mobile phone.
Awase-E is implemented through both E-mail and Web. Prerequisite requirements for a user terminal is that it has access to the above two network service
types. This means that it is also possible to use Awase-E from computers.
The detail of the authentication process is shown in Fig. 2.
Verification stage
1st stage

2nd stage

3rd stage

4th stage

Selectlower rightimage

Select"No pass-image"Pass-Images
Correct answer
Selectupper center
image

Select"No pass-image"In this example, the authentication trial consists of four verification stages.
But, the number of verified pass-images are two.
Fig. 2. A Detailed Authentication Process in Awase-E(N=4, P=9)

One authentication trial consists of N times of verification stages. AwaseE, of course, authorizes a user as a legitimate user only if all verifications aresuccessful. In each verification stage, Awase-E shows P pieces of images on thescreen, a user must select a pass-image correctly from them. Only one passimage is included in each verification image set. The reason for this is to reduce
the possibility that a randomly selected attacker’s answer would be a correct
answer. We call an image that is not a pass-image as a “decoy image”. The
location of each image in the image set is randomly determined. This meansthat the location of both pass-image and decoy images can change each time. Itis also possible that there is no pass-image in an image set. In this case, the user
must answer “no pass-image”.
Awase-E is an easier method for users to complete the authentication process
than before, even when using a mobile phone. The numerical keys on a mobile
phone are uniquely correspond to each of the images on the screen at any given
stage. This enables users to choose any image in the screen with one click. Inusing Awase-E, it is possible to authenticate oneself by just N + 1 times of key
types. Moreover, Awase-E does not need to input any text in authenticating
oneself because it uses an E-mail address as a user ID.

3Considerations

There are some related works which make an attempt to improve the problemof password-based authentication and the experimental results clearly indicatethat image-based authentication has an advantage over password or PIN-based
authentication[1, 2] especially in regards to the human interface aspect. It iseasier for a user to memorize an image than a text. It is also easier to identify animage than recalling a text. However, previously proposed systems make use ofimages that are only provided by the system[3]. These system-assigned images
have little relation to the users, and it is possible that a user will forget it astime goes on, even though it is an image.
Awase-E, then, uses photographs that are taken by users for authentication.
Moreover, Awase-E enables users to add their favorite images to the system andto use them as pass-image in an easy way. This feature greatly reduces the loadof memorizing and recalling the secret information over using system provided
images. The reason is these images are closely related with the user’s experience.
This means that the user tends to remember the images subconsciously when theuser looks at it. Using photographs also make it easier to generate and select apass-image. We think that these advantages result in motivating users to change
their pass-image more frequently. This is a feature that has not been realized
in any current authentication methods. Another benefit of image registrationis that it increases the number of total images in the authentication database.
An image-based authentication, theoretically, has a vast information space forsecret information. It is, however, practically limited by the number of total
images that the system has in its database[1, 2]. In Awase-E, the database forimages continues to expand as time passes.
Awase-E also introduces the case that there is no pass-image in generatinga verification image set. This feature has two merits. One is that it reduces thenumber of pass-images that a user must remember. and therefore reduces thememory load on users. Of course, the case of a user selecting all“no pass-image”
is not allowed for security reasons. The other advantage is that it also enhances
the security level against “Intersection Attack”[1] which is a specialized attack
method to this kind of image-based authentication. The reason is that this kindof attack can only occur when a pass-image is included in the image set in all
verification stages.
The notification interface gives users the information of an occurrence of an
attack through E-mail and the web. In other words, it gives a trigger to usersto take a response against it. If a user is aware of an attempt that an attacker
impersonates yourself, a user should add a new photo and change their passimage to a new one. Current security assessments of existing authentications
are evaluated by statistical methods only. It is clear that Awase-E has the same
security level of N-digit number passwords. We think, however, the evaluation
method ignores the aspect of users in authentication. It is difficult to rectify
the well-known problem that “a user is the weakest link in the security chain”.
Awase-E provides a notification interface to address this issue. We expect that
this type of alerting mechanism has a positive effect on changing the user’s view

against computer security. The reason is that every user will probably encountera malicious attempt in the near future. We believe that notification will become
important in order to ensure the security level in any authentication system,
and mobile terminals will become an essential device that can receive thesenotifications immediately on the spot.
The number of verification stages in Awase-E is variable. Awase-E, therefore,provides a flexible authentication framework that can handle various situations.
For example, to emphasize security over usability, you could configure Awase-E
such that a user must verifies 5 sets and must select a pass-image in 4 of the sets.
On the contrary, if you put weight on usability over security, you can configure
the process such that the user verifies only 3 sets and must select a pass-image
in 1 of the sets.
From these considerations, we believe that Awase-E provides a better authentication framework that addresses both security and usability issues. In otherwords, Awase-E satisfies both security and usability issues at higher level thanexisting authentication methods.

4Conclusion

In this paper, we proposed a novel authentication method called “Awase-E” thatis used with mobile phones with digital cameras. We integrate image registrationand notification interfaces into image-based authentication. Image registration
enables users to use their favorite pictures as pass-images. And the notification interface gives users a trigger to take appropriate action against malicious
attempts.
Awase-E is easily operable even when used through a mobile phone. Using afavorite picture as a pass-image reduces the memory load on users regarding secure information and is less memory-intensive than simply using system-assigned
images. The notification function enables users to take appropriate action bythemselves. In other words, it provides users with a method of ensuring the security of their own right. This feature is important in order to keep the userfrom being the weakest link in the security chain. We think that Awase-E realizes a higher level of coexistence in both security and usability than previous
user authentication methods.

References
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9th Usenix Security Symposium, pp. 45–58, Aug (2000).
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3. A. Perrig and D. Song: Hash Visualization: a New Technique to improve RealWorld Security, In International Workshop on Cryptographic Techniques and ECommerce (CrypTEC), (1999).