Identity is a term that is heavily debated. For our purposes, it is useful to see the identity of a person as the union of all characteristics, judgements and other knowledge that parties have about that person. Generalizing this idea, we say that the identity of any entity consists of the combined knowledge (that is: of all parties that know) about this entity.
Inspired by Pfitzmann and Hansen (2010), we define a partial identity (of an entity) as all the knowledge that a single, specific party has about that entity (= the 'subject' of the partial identity). The identity of an entity is then the union/collection of all of its partial identities.
The purpose of defining the term Identity is to make people aware that within the context of our framework SSI as we see it
An Identity is a character string that is being used for identification purposes by a specific party.
The following strings are examples: 'localhost', 'https://localhost/', 'Trust over IP community', 'the mayor of New York', 'guardianship', 'my mother', 'did:sov:2wJPyULfLLnYTEFYzByfUR', 'did:sov:2wJ', 'issue #24', etc., etc.
Discussion - Scope of Identification
RFC 3986, Section 1.1. states "an identity embodies the information required to distinguish what is being identified from all other things within its scope of identification". This statement suggests that identitys (URIs) have a single scope, supposedly specified by "the URI schemes and naming authority (if any)". However, there is no such requirement, and there is nothing in place to guarantee this (apart from IANA, many other (sometimes even very commonly used) URI schemes exist). Pfitzmann and Hansen (2010) (section 13.2) use the term 'identifiability sets' rather than 'scope of identification', and describe how 'attackers' - but that could equally well have been regular users - each have, or construct their own scope, and use contextual information to do so.
The criterion that makes a text string qualify as an identity doesn't seem to cut it, as only using a text for identification purposes doesn't make it have (what we will call) the 'identification property', i.e. the property that it actually identifies something. It may only have that property in combination with an associated (single) scope of identification, which may depend on the context in which it is being used. RFC 2986, page 6 illustrates this using the identity "http://lcoalhost/".
The lack of (identifying) scopes of identification becomes an issue when a party (say Alice) sends the identity (e.g.
my car) to another party (say Bob), expecting that Bob will then be able to identify the same entity that she identifies with it (presumably some specific car).
If Bob had just met Alice for the first time, and hadn't seen her coming in a car, then Alice must acquaint Bob with the existence of the entity that she refers to with
my car, e.g. by pointing her finger to it, or describing the make, brand and license plate or some other characteristic that allows Bob to single out her car (in the context of their meeting one another). Then, Bob can 'register' the existence of that car in his knowledge (optionally tagging it with an identity of his own, e.g.
Alice's car), and associate it with the attribute (party='Alice', identity='
my car'). It is important to have the "party='Alice'" part in there, because other parties, (e.g. Carol) may also use an identity
my car, which would and should then refer to another car. This shows that the scope of interpretation for an identity has to do with the (knowledge of) parties that use it, and that understanding the intended meaning requires a proper identification of that scope.