SOLID-OIDC

1. Introduction

This section is non-normative

The Solid project aims to change the way web applications work today to improve privacy and user control of personal data by utilizing current standards, protocols, and tools, to facilitate building extensible and modular decentralized applications based on Linked Data principles.

This specification is written for Authorization and Resource Server owners intending to implement Solid-OIDC. It is also useful to Solid application developers charged with implementing a Solid-OIDC client.

The OAuth 2.0 and OpenID Connect Core 1.0 web standards were published in October 2012 and November 2014, respectively. Since publication they’ve seen rapid and widespread adoption across the industry, in turn gaining extensive "real-world" data and experience. The strengths of the protocols are now clear; however, in a changing eco-system where privacy and control of digital identities are becoming more pressing concerns, it is also clear that additional functionality is required.

The additional functionality documented herein aims to address:

1. Resource servers having no existing trust relationship with identity providers.

2. Ephemeral Clients as a first-order use-case.

1.1. Out of Scope

This section is non-normative

At the time of writing, there is no demonstrated use case for a strongly asserted identity; however, it is likely that authorization requirements will necessitate it.

2. Terminology

This section is non-normative

This specification uses the terms "access token", "authorization server", "resource server" (RS), "authorization endpoint", "token endpoint", "grant type", "access token request", "access token response", and "client" defined by The OAuth 2.0 Authorization Framework [RFC6749].

Throughout this specification, we will use the term Identity Provider (IdP) in line with the terminology used in the Open ID Connect Core 1.0 specification (OIDC). It should be noted that The OAuth 2.0 Authorization Framework (OAuth) refers to this same entity as an Authorization Server.

This specification also uses the following terms:

WebID as defined in the WebID 1.0 Editors Draft

A WebID is a URI with an HTTP or HTTPS scheme which denotes an Agent (Person, Organization, Group, Device, etc.)

JSON Web Token (JWT) as defined by [RFC7519]

A string representing a set of claims as a JSON object that is encoded in a JWS or JWE, enabling the claims to be digitally signed or MACed and/or encrypted.

JSON Web Key (JWK) as defined by [RFC7517]

A JSON object that represents a cryptographic key. The members of the object represent properties of the key, including its value.

Demonstration of Proof-of-Possession at the Application Layer (DPoP) as defined in the DPoP Internet-Draft

A mechanism for sender-constraining OAuth tokens via a proof-of-possession mechanism on the application level.

DPoP Proof as defined by [DPOP]

A DPoP proof is a JWT that is signed (using JWS) using a private key chosen by the client.

Proof Key for Code Exchange (PKCE) as defined by [RFC7636]

An extension to the Authorization Code flow which mitigates the risk of an authorization code interception attack.

3. Core Concepts

This section is non-normative

In a decentralized ecosystem, such as Solid, an IdP may be an identity-as-a-service vendor or, at the other end of the spectrum, a user-controlled IdP. In any case, the user may be authenticating from a browser or an application.

Therefore, this specification assumes the use of the Authorization Code Flow with PKCE, in accordance with OAuth and OIDC best practices. It is also assumed that there are no preexisting trust relationships with the IdP. This means that client registration, whether dynamic, or static, is entirely optional.

3.1. WebIDs

This section is non-normative

In line with Linked Data principles, a WebID is a HTTP URI that, when dereferenced, resolves to a profile document that is structured data in an RDF 1.1 format. This profile document allows people to link with others to grant access to identity resources as they see fit. WebIDs underpin Solid and are used as a primary identifier for Users and Client applications in this specification.

4. Basic Flow

This section is non-normative

The basic authentication and authorization flow is as follows:

1. The Client requests a non-public resource from the RS.

2. The RS returns a 401 with a WWW-Authenticate HTTP header containing parameters that inform the Client that a DPoP-bound Access Token is required.

3. The Client presents its Client Identifier and the associated Secret to the IdP and requests an Authorization Code.

4. If granted, the Client presents the Authorization Code and a DPoP proof, to the Token Endpoint.

5. The Token Endpoint returns a DPoP-bound Access Token and OIDC ID Token, to the Client.

6. The Client presents the DPoP-bound Access Token and DPoP proof, to the RS.

7. The RS gets the public key from the IdP and uses it to validate the signature on the DPoP-bound Access Token (JWS).

8. If the DPoP proof and Access Token are valid, then the RS returns the requested resource.

5. Client Identifiers

OAuth and OIDC require the Client application to identify itself to the IdP and RS by presenting a client identifier (Client ID). Solid applications SHOULD use a WebID as their Client ID.

5.1. WebID

A client’s WebID resource MUST be serialized as an application/ld+json document unless content negotiation requires a different outcome.

The serialized JSON form of a client WebID SHOULD use the normative JSON-LD @context provided at https://www.w3.org/ns/solid/oidc-context.jsonld such that the resulting document produces a JSON serialization of an OIDC client registration, per the definition of client registration metadata from [RFC7591] section 2.

Related Issue Solid-OIDC client description in WebID document

Also, the IdP MUST dereference the Client’s WebID document and match any Client-supplied parameters, with the values in the Client’s WebID document.

Further, the redirect_uri provided by the Client MUST be included in the registration redirect_uris list.

NOTE: the method by which the IdP resolves the WebID to an RDF document, is defined in WebID 1.0. This example uses JSON-LD :

{
  "@context": "https://www.w3.org/ns/solid/oidc-context.jsonld",

  "client_id": "https://app.example/webid#id",
  "client_name": "Solid Application Name",
  "redirect_uris": ["https://app.example/callback"],
  "client_uri": "https://app.example/",
  "logo_uri" : "https://app.example/logo.png",
  "tos_uri" : "https://app.example/tos.html",
  "scope" : "openid profile offline_access",
  "grant_types" : ["refresh_token","authorization_code"],
  "response_types" : ["code"],
  "default_max_age" : 3600,
  "require_auth_time" : true
}

5.1.1. JSON-LD context

This specification defines a JSON-LD context for use with OIDC client identifiers. This context is available at https://www.w3.org/ns/solid/oidc-context.jsonld. Client identifier documents that reference this JSON-LD context MUST use the HTTPS scheme.

NOTE: the oidc vocabulary that is part of this context uses the HTTP scheme.

The JSON-LD context is defined as:

{
  "@context": {
    "@version": 1.1,
    "@protected": true,
    "oidc": "http://www.w3.org/ns/solid/oidc#",
    "xsd": "http://www.w3.org/2001/XMLSchema#",
    "client_id": {
      "@id": "@id",
      "@type": "@id"
    },
    "client_uri": {
      "@id": "oidc:client_uri",
      "@type": "@id"
    },
    "logo_uri": {
      "@id": "oidc:logo_uri",
      "@type": "@id"
    },
    "policy_uri": {
      "@id": "oidc:policy_uri",
      "@type": "@id"
    },
    "tos_uri": {
      "@id": "oidc:tos_uri",
      "@type": "@id"
    },
    "redirect_uris": {
      "@id": "oidc:redirect_uris",
      "@type": "@id",
      "@container": [
        "@id",
        "@set"
      ]
    },
    "require_auth_time": {
      "@id": "oidc:require_auth_time",
      "@type": "xsd:boolean"
    },
    "default_max_age": {
      "@id": "oidc:default_max_age",
      "@type": "xsd:integer"
    },
    "application_type": {
      "@id": "oidc:application_type"
    },
    "client_name": {
      "@id": "oidc:client_name"
    },
    "contacts": {
      "@id": "oidc:contacts"
    },
    "grant_types": {
      "@id": "oidc:grant_types"
    },
    "response_types": {
      "@id": "oidc:response_types"
    },
    "scope": {
      "@id": "oidc:scope"
    },
    "token_endpoint_auth_method": {
      "@id": "oidc:token_endpoint_auth_method"
    }
  }
}

5.2. The Public WebID

Ephemeral Clients MAY use the identifier http://www.w3.org/ns/solid/terms#PublicOidcClient. If the Client uses this identifier then the IdP MAY accept any redirect_uri as valid. Since it is public, the Client is effectively anonymous to the RS.

5.3. OIDC Registration

If the Client does not use a WebID as the client identifier, then it MUST present a client identifier registered with the IdP via either OIDC dynamic or static registration. See also [OpenIDConnectDynamicClientRegistration].

6. Token Instantiation

Assuming one of the following options

• Client ID and Secret, and valid DPoP Proof (for dynamic and static registration)

• Client WebID with a proper registration and valid DPoP Proof (for a client webid)

• A client id of http://www.w3.org/ns/solid/terms#PublicOidcClient (for a public WebId)

the IdP MUST return two tokens to the Client:

1. A DPoP-bound Access Token

2. An OIDC ID Token

6.1. DPoP-bound Access Token

The DPoP-bound Access Token MUST be a valid JWT. See also: [RFC7519].

When requesting a DPoP-bound Access Token, the Client MUST send a DPoP proof JWT that is valid according to the OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer (DPoP) §section-5. The DPoP proof JWT is used to bind the access token to a public key. See also: [DPOP].

The DPoP-bound Access Token payload MUST contain these claims:

• webid — The WebID claim MUST be the user’s WebID.

• iss — The issuer claim MUST be a valid URL of the IdP instantiating this token.

• aud — The audience claim MUST either be the string solid or be an array of values, one of which is the string solid. In the decentralized world of Solid OIDC, the principal of an access token is not a specific endpoint, but rather the Solid API; that is, any Solid server at any accessible address on the world wide web. See also: JSON Web Token (JWT) §section-4.1.3.

• iat — The issued-at claim is the time at which the DPoP-bound Access Token was issued.

• exp — The expiration claim is the time at which the DPoP-bound Access Token becomes invalid.

• cnf — The confirmation claim is used to identify the DPoP Public Key bound to the Access Token. See also: OAuth 2.0 Demonstration of Proof-of-Possession at the Application Layer (DPoP) §section-7.

• client_id - The ClientID claim is used to identify the client. See also: section 5. Client Identifiers.

{
    "webid": "https://janedoe.com/web#id",
    "iss": "https://idp.example.com",
    "aud": "solid",
    "iat": 1541493724,
    "exp": 1573029723,
    "cnf":{
      "jkt":"0ZcOCORZNYy-DWpqq30jZyJGHTN0d2HglBV3uiguA4I"
    },
    "client_id": "https://client.example.com/web#id"
}

6.2. OIDC ID Token

The user’s WebID MUST be present in the ID Token as the webid claim.

{
    "webid": "https://janedoe.com/web#id",
    "iss": "https://idp.example.com",
    "sub": "janedoe",
    "aud": "https://client.example.com/web#id",
    "nonce": "n-0S6_WzA2Mj",
    "exp": 1311281970,
    "iat": 1311280970,
}

7. Resource Access

7.1. DPoP Proof Validation

A DPoP Proof that is valid according to DPoP Internet-Draft, Section 4.3, MUST be present when a DPoP-bound Access Token is used.

7.2. Access Token Validation

The DPoP-bound Access Token MUST be validated according to DPoP Internet-Draft, Section 6, but the RS MAY perform additional verification in order to determine whether to grant access to the requested resource.

The user’s WebID in the webid claim MUST be dereferenced and checked against the iss claim in the Access Token. If the iss claim is different from the domain of the WebID, then the RS MUST check the WebID document for the existence of a statement matching ?webid <http://www.w3.org/ns/solid/terms#oidcIssuer> ?iss., where ?webid and ?iss are the values of the webid and iss claims respectively. This prevents a malicious identity provider from issuing valid Access Tokens for arbitrary WebIDs.

Unless the RS acquires IdP keys through some other means, or the RS chooses to reject tokens issued by this IdP, the RS MUST follow OpenID Connect Discovery 1.0 [OpenID.Discovery] to find an IdP’s signing keys (JWK).

8. Solid-OIDC Conformance Discovery

An Identity Provider that conforms to the Solid-OIDC specification MUST advertise this in the OpenID Connect Discovery 1.0 [OpenID.Discovery] resource. An Identity Provider would indicate this support by using the solid_oidc_supported metadata property, referencing the Solid-OIDC specification URL.

{
    "solid_oidc_supported": "https://solidproject.org/TR/solid-oidc"
}

9. Security Considerations

This section is non-normative

As this specification builds upon existing web standards, security considerations from OAuth, OIDC, PKCE, and the DPoP specifications may also apply unless otherwise indicated. The following considerations should be reviewed by implementors and system/s architects of this specification.

Some of the references within this specification point to documents with a Living Standard or Draft status, meaning their contents can still change over time. It is advised to monitor these documents, as such changes might have security implications.

In addition to above considerations, implementors should consider the Security Considerations in context of the Solid Protocol.

9.1. TLS Requirements

All TLS requirements outlined in [BCP195] apply to this specification.

All tokens, Client, and User credentials MUST only be transmitted over TLS.

9.2. Client IDs

An RS SHOULD assign a fixed set of low trust policies to any client identified as anonymous.

Implementors SHOULD expire Client IDs that are kept in server storage to mitigate the potential for a bad actor to fill server storage with unexpired or otherwise useless Client IDs.

9.3. Client Secrets

Client secrets SHOULD NOT be stored in browser local storage. Doing so will increase the risk of data leaks should an attacker gain access to Client credentials.

9.4. Client Trust

This section is non-normative

Clients are ephemeral, client registration is optional, and most Clients cannot keep secrets. These, among other factors, are what makes Client trust challenging.

10. Privacy Considerations

10.1. Access Token Reuse

This section is non-normative

With JWTs being extendable by design, there is potential for a privacy breach if Access Tokens get reused across multiple resource servers. It is not unimaginable that a custom claim is added to the Access Token on instantiation. This addition may unintentionally give other resource servers consuming the Access Token information about the user that they may not wish to share outside of the intended RS.

11. Acknowledgments

This section is non-normative

The Solid Community Group would like to thank the following individuals for reviewing and providing feedback on the specification (in alphabetical order):

Tim Berners-Lee, Justin Bingham, Sarven Capadisli, Aaron Coburn, Matthias Evering, Jamie Fiedler, Michiel de Jong, Ted Thibodeau Jr, Kjetil Kjernsmo, Mitzi László, Pat McBennett, Adam Migus, Jackson Morgan, Davi Ottenheimer, Justin Richer, severin-dsr, Henry Story, Michael Thornburgh, Emmet Townsend, Ruben Verborgh, Ricky White, Paul Worrall, Dmitri Zagidulin.