Personally I think good identity system should have these properties: * Identity provider should not see which services users authenticate to. Afaik this is no

Personally I think good identity system should have these properties: * Identity provider should not see which services users authenticate to. Afaik this is now a major issue with current "Log in with X" systems * Services should not be able to discover which other services user has authenticated to, i.e. the identity presented to service A should not be linkable to identity presented to service B * Identities should be relatively stable. The identity presented to certain service at certain point in time should be the same identity presented to the same service in some other point of time. * The system probably needs to be able to allow users to reveal additional information about their identity. This is problematic requirement because in wider use it is ripe for abuse by service providers * Optionally it'd be nice if the system would have some facilities for offline usage, where neither user nor service needs to talk to identity provider. This is fairly hairy problem, with many more concerns in the details and other people probably having different requirements. From a quick search, Credenticas (now MS) U-Prove system comes closest, but I haven't delved really into it to see if it actually matches my thoughts.