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Attempts at common terminology for Digital Identity systems

By Christian Lundkvist @ChrisLundkvist <christian.lundkvist@consensys.net>

Introduction

This note is an attempt to come to consensus about common terminology when discussing digital identity systems. Often discussions about digital identity suffers from a lack of precise arguments where the people discussing have different internal definitions of what identity means in the context. Thus there is a lot of conflation of identity vs reputation vs biometric IDs etc. I invite everyone to critizise the terminology used here and suggest better terms where appropriate, or to point out errors or other areas that should be covered.

I will try to be general and to not introduce language specific to any particular implementation or technology stack. In the examples I will cover specific implementation stacks such as PGP, Ethereum, Blockstack or a CA-based centralized approach.

Names and keys

A cryptographic name (or just name) is a cryptographic hash, number or (potentially human-meaningful) string that acts as a global, unique, persistent identifier for the purpose of interactions with cryptographic systems such as blockchains, messaging platforms or authentication systems. The well-known Zooko's triangle states that such names can have two of the three properties Decentralized, Secure, and Human-meaningful. Blockchains such as Namecoin showed that it was possible to have systems where all three properties hold up to byzantine fault tolerance.

The cryptographic name is publicly tied to one or more private/public key pairs - the private keys are the core of the identity and are used to sign messages such as transactions on a blockchain, encrypted point-to-point messages, or messages or certificates enabling access control. We denote the private key holder the agent or user of the name, or simply the keyholder. The keys associated to the name do not neccessarily need to be persistent, they could be swapped out using a key revocation or key rotation system that links the name to the keys.

The public key(s) are tied to the name through a publicly accessible secure registry. This registry could be a centrally managed trusted system such as a certificate authority or a physically decentralized (but logically centralized) system like a blockchain. Names can also be tied to keys through a non-centralized system of certificates, as advocated in the SPKI proposal. Note that in the SPKI proposal global unique names are achieved by adding namespaces onto local names.

Examples

Public keys

The simplest form of a name is the public key corresponding to a private key (or the hash of a public key). In this case no registry or certificate is needed to bind the name to the key, the public key itself is the name. One downside of this scheme is that since the name is the public key it's not possible to switch out the corresponding private key while keeping the name persistent in the case of key loss or compromise.

PGP

Using our terminology the names in the PGP system are public keys. It is tempting to say that names in PGP are email addresses. However, anyone can generate a PGP keypair and sign a PGP certificate tying a given email address to that key, and this makes email addresses not unique in the PGP system. Using the terminology below, an email address is instead an attribute connected to a specific key.

Blockchain ID

In Blockchain ID the cryptographic name is unique but also human-meaningful. A longer description of the system is here.

Ethereum-based identity systems

The flexibility of the Ethereum smart contract system allows most kinds of naming systems to be utilized (public-key, human-meaningful, random hash, etc). So the question becomes one of selecting desirable properties. Human-meaningful naming systems have the problem of namesquatting, that Namecoin tries to solve by paying fees for names which is difficult to do in a decentralized fashion. Instead a non-human-meaningful random hash cryptographically linked to public keys could be a better cryptographic name to use.

Other systems

  • X.509?
  • PEP?

Attributes

A name can be linked to a set of attributes. An attribute is a piece of data that's cryptographically associated with a name. Attributes tie a name together with (mainly) external data and anchors the name into a context outside of the cryptographic realm of the name and its associated private and public keys. Attributes can refer to data like First Name, Last Name, and photo for a name representing a person, or they can be a tax identification number or accounting information for a name representing a corporation. Attributes allow humans to put cryptographic names in a social context and is the main way humans will relate to identities. As an example, the Facebook system identifies users by their Facebook ID (the number acting as the unique identifier of their profile). Very few users would use this number to identify their friends, they instead relate to their friends on Facebook by their Name, Profile Picture etc. Attributes can be either public, or stored encrypted and be selectively revealed.

Global vs Local attributes

Many attributes will only be relevant in certain contexts. Thus attribute data could be external to the name and possibly maintained by third parties. The name could have links to externally maintained attribute stores. Even basic things like Name, Profile picture etc could potentially be tied to external entities, and the user can choose to update the information either by changing the data held by the external attribute store, or by changing which attribute store is linked to.

Nyms/Usernames as attributes

Usernames/Nyms are special attributes as they are supposed to map onto identities in a one-to-one fashion in the context where they're used. Here there may be Nym registries that are localized, for instance tied to a company, or decentralized like Namecoin or registries on a blockchain like Ethereum. If the cryptographic name of the identity is a human-meaningful one this can be used directly as a username.

Examples

Schema.org

The Schemas outlined in schema.org/Person give a good example of attribute data for a human person:

{
"@type": "Person",
"name": "Christian Lundkvist",
"givenName": "Christian",
"familyName": "Lundkvist",
"description": "Blockchain nerd",
"address": [
        {
            "@type": "PostalAddress",
            "streetAddress": "49 Bogart St",
            "addressLocality": "Brooklyn, NY",
            "postalCode": "11206",
            "addressCountry": "United States"
        }
    ]
}

PGP

In the basic implementation of PGP the attributes are email addresses and attached User ID strings.

Blockchain ID

The Blockchain ID attributes are based on the schema.org attributes above but enhance them with additional attributes for things like social media accounts etc.

Claims/Attestations

A claim or attestation is a statement about a name or an attribute of a name, signed by (a key associated to) another name. This is a very general notion that can apply in many different contexts. Some attestations could be about relationships between identities: Company ABC makes an attestation that Person XYZ is employed there, and Person XYZ can make the corresponding attestation about Company ABC. There will need to be some standardization around how to format these attestations in order to maintain compatibility. The [JSON Web Token][jwt] standard could be a good starting point for this. In this standard the name making the claim is called the issuer, and the subject of the claim is called the subject.

Attestations are very close to the "web-of-trust" of PGP but could be more granular and contain more information. A simple example of an attestation is a "link", i.e. a simple two-way mutual connection between two identities, signed by both of the identities. One could also imagine a one-way simple attestation similar to the "follow" used by Twitter.

Attestations could also be used for reputation systems, e.g. you can have an attestation giving 1-5 stars in an online shopping context.

Examples

PGP

PGP attestations embody the original notion of a web-of-trust. The attestation is of the unidirectional type where one PGP identity signs the public key of another PGP identity. The implied sentiment in the attestation is "I attest that the user of this key is also the user of this email address.".

schema.org attestations

The schema.org/Person specs have various attestations regarding relationships between people. In the spec there are the fields knows, follows, relatedTo, sibling, spouse, parent.

In the spec these attestations are also attributes, i.e. my identity has an attribute that I know someone.

{
"follows": [person0, person1],
"knows": [person2]
}

The schema.org spec does not have any cryptographic anchoring of the relationship, so it needs to be augmented with cryptographic signatures. This can be done either by adding signatures to the attestation schema (using JSON web tokens), making it in effect a certificate as is done by the BlockchainID Token. Another way that can be used in Ethereum-based systems is to embed the attestation in a smart contract directly on the blockchain. The transactions that put the data in the smart contract provides the cryptographic anchoring in this case.

JSON Web Token (JWT)

A JWT is a JSON data structure containing a claim, together with a subject which is the subject of the claim and an issuer, which is the entity that issued and signed the token. Often in our discussions around user-owned identitiy the subject and issuer will be the same entity. The JWT can be digitally signed and/or encrypted. An example from the Blockchain ID spec:

  {
        "token": "eyJ0eXAiOiJKV1QiLCJhbGciOiJFUzI1NiJ9.eyJjbGFpbSI6eyJrbm93cyI6W3siQHR5cGUiOiJQZXJzb24iLCJpZCI6Im11bmVlYi5pZCJ9XX0sInN1YmplY3QiOnsiQHR5cGUiOiJQZXJzb24iLCJwdWJsaWNLZXkiOiIwMzJlY2RlNDM5N2ZhNDczZjFjZTQ4MmNlYTYyMzVlYjBjMjBiM2NlNGMwMTBhNjY5MDM0NDIyODAwYmJjZDQ5MWQifSwiaXNzdWVyIjp7IkB0eXBlIjoiUGVyc29uIiwicHVibGljS2V5IjoiMDMyZWNkZTQzOTdmYTQ3M2YxY2U0ODJjZWE2MjM1ZWIwYzIwYjNjZTRjMDEwYTY2OTAzNDQyMjgwMGJiY2Q0OTFkIn19.snFhetKhj3uwoVDXXKk5w7wxVpkeTBFjhE-J0s9zkqM9-ZcG7oD1_hi7fBXiLdvgwgbqQJ9VvbYQfTukaiedaQ",
        "data": {
            "header": {
                "typ": "JWT",
                "alg": "ES256"
            },
            "payload": {
                "claim": {
                    "knows": [
                        {
                            "@type": "Person",
                            "id": "muneeb.id"
                        }
                    ]
                },
                "subject": {
                    "@type": "Person",
                    "publicKey": "032ecde4397fa473f1ce482cea6235eb0c20b3ce4c010a669034422800bbcd491d"
                },
                "issuer": {
                    "@type": "Person",
                    "publicKey": "032ecde4397fa473f1ce482cea6235eb0c20b3ce4c010a669034422800bbcd491d"
                }
            },
            "signature": "snFhetKhj3uwoVDXXKk5w7wxVpkeTBFjhE-J0s9zkqM9-ZcG7oD1_hi7fBXiLdvgwgbqQJ9VvbYQfTukaiedaQ"
        },
        "chainPath": "2643afc402f51102d49ee70496354b53f753978c127656ab10fb24a699a842e6",
        "encrypted": false
    }

Identity

An identity is the totality of a cryptographic name together with its associated attributes and attestations.

Examples

PGP

A PGP identity is a public key together with the associated email address(es) and corresponding web-of-trust attestations from other keys.

Blockchain ID

The Blockchain ID system uses a human-meaningful name together with attributes and attestations stored in a distributed storage system like a DHT and hashed into a blockchain.

The uPort Identity system (Ethereum-based)

The uPort system now being built at ConsenSys will use a non-human-meningful random hash name embedded in an Ethereum contract. This contract contains hashes of attribute information stored in IPFS, along with reputational attestations stored in the contract itself. This gives the flexibility to do key revocation, key rotation and decentralized key resets while maintaining a persistent cryptographic name. The reputational attestations can be read and processed by other smart contracts.

Other systems

  • X.509
  • PEP
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