A user-portable computing device configured as a smart card enables a user to carry identification information and to generate security tokens for use in authenticating the user to a service provider. The device includes memory for storing user identities as information cards that are exported to a host computer, presented to a user in visual form, and then selected for use in the authentication process. A security token service installed on the device issues a security token in response to a token request sent from the host computer that references the selected user identity. The security token service uses user attribute information stored on the user device to compose the claim assertions needed to issue the security token. The token is returned to the host computer and used to facilitate the authentication process.