Security Usability of Petname Systems  Md. Sadek Ferdous 1,2,3 , Audun Jøsang 2,3 Kuldeep Singh 1,2,5 and Ravishankar Borgaonkar 2,5,6 1 NTNU, 2 UNIK, 3 University of Oslo, 4 University of Tartu, 5 TKK, 6 KTH {sadek,josang,kuldeep,ravishankar}@unik.no Abstract. To have certainty about identities is crucial for secure com- munication in digital environments. The number of digital identities that people and organizations need to manage is rapidly increasing, and proper management of these identities is essential for maintaining secu- rity in online markets and communities. Traditional Identity Manage- ment Systems are designed to facilitate the management of identities from the perspective of the service provider, but provide little support on the user side. The difficulty of managing identities on the user side causes vulnerabilities that open up for serious attacks such as identity theft and Phishing. Petname Systems have been proposed to provide more user friendly and secure identity management on the user side. This paper provides an analysis of the Petname Model by describing its history and background, properties, application domains and usability issues with emphasis on Security Usability. By covering a broad set of aspects, this paper is intended to provide a comprehensive reference for the Petname System. 1 Introduction The purpose of digital communication protocols is to exchange information as efficiently and reliably as possible. Originally, these protocols were designed with- out authentication because the identities of communicating parties could be as- sumed, and did not have to be formally verified. Authentication was subsequently added for verifying the correctness of claimed and assumed identities. Authen- tication requires prior registration of identities, and is based on a set of security mechanisms combined with a credential or security token. As authentication became necessary for accessing many online services, more and more identities and credentials were issued, and their management became problematic, both for service providers and for users. Identity Management (IdM, in short) was introduced by the industry to facilitate server-side management of user identi- ties. Initially, client-side management of user identities was not considered to be an issue. However, many people currently feel overloaded with identities and passwords that security policies require them to memorize. The growing number of identities that users need to handle and the inability of users to comply with  Proceedings of the 14th Nordic Conference on Secure IT Systems (NordSec 2009), Oslo, October 2009