Number of values
A persistent, non-reassigned, privacy-preserving identifier for a user shared between an identity provider and service provider. An identity provider uses the appropriate value of this attribute when communicating with a particular service provider or group of service providers, and does not reveal that value to any other service provider except in limited circumstances.

Persistence: Does not require a specific lifetime, but the association should be maintained longer than a single user interaction and long enough to be useful as a key for a particular service that is consuming it.

Privacy: This attribute is designed to preserve the user's privacy and inhibit the ability of multiple unrelated services from correlating user activity by comparing values. It is therefore required to be opaque.

Uniqueness: A value of this attribute is intended only for consumption by a specific audience of applications (often a single one). Values of this attribute therefore must be unique within the namespace of the identity provider and the namespace of the service provider(s) for whom the value is created. The value is "qualified" by these two namespaces and need not be unique outside them. Logically, the attribute value is made up of the triple of an identifier, the identity provider, and the service provider(s).

Reassignment: A distinguishing feature of this attribute is that it prohibits reassignment. Since the values are opaque, there is no meaning attached to any particular value beyond its identification of the user. Therefore particular values created by an identity provider must not be reassigned such that the same value given to a particular

An opaque string of no more than 256 characters.

The format comprises the entity name of the identity provider, the entity name of the service provider, and the opaque string value. These strings are separated by “!” symbols.

Notes on usage
Service Providers should use this attribute to support aspects of its service that depend on recognising the same user from session to session. The most common use is to enable service personalisation, to record user preferences such as stored search expressions across user sessions. A secondary use is to enable tracking of user activity, to make it easier to detect systematic downloading of content or other suspected breaches of licence conditions.

The attribute enables an organisation to provide a persistent, opaque, user identifier to a service provider. For each user, the identity provider presents a different value to each service provider to which the attribute is released.

Notes on privacy
This attribute is intended to be a privacy-preserving attribute.