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use of this technology when accessing the network or an application service. We propose an idM framework that offers cross-layer single sign-on (SSO) mechanisms for users who have some type of subscription with several service and identity pro-viders. It also protects privacy by preventing unauthorized parties from linking steps or actions with the same user identifiers, enforcing privacy rules at different provid-ers, and supporting the user’s autonomy in disclosing attributes. The framework provides the functionality to merge existing user subscriptions and generate a more general virtual identity—the union of one or more attributes from one or more identity providers. Users can create several virtual identities using a subset of attributes for each identity provider to which they subscribe. The concept of virtual identity is the key to creating federations of resources and an SSO mechanism across different idM systems. Users can access multiple services by referencing their virtual identity via a virtual identifier (VID). The main challenge for developers is how this additional level of indirection affects idM system elements and the require-ments on newly needed components. VIRTUAL IDENTITIES AND IDENTIFIERS Nowadays many users have numerous accounts with different service providers (SPs) on the Internet that play Published by the IEEE Computer Society 0018-9162/09/$25.00 © 2009 IEEE I dentity management is a growing Web trend with the goal of simplifying account management from a user’s perspective. In addition, idM seeks to seam-lessly maintain the user’s privacy while accessing different Web services. The Web is idM’s greatest strength, promoting rapid proliferation of the technology, but also its greatest weakness, in that idM mechanisms are not widely used in telecommunication networks or for more traditional services such as IP telephony or tele-vision. A single framework is crucial to the successful integrated deployment of idM on a global scale. Recent research activities, such as those of the EU’