matrix-doc/proposals/2858-Multiple-SSO-Identity-Providers.md

MSC2858: Multiple SSO Identity Providers

Matrix already has generic SSO support, but it does not yield the best user experience especially for instances which wish to offer multiple identity providers (IdPs). This MSC provides a simple and fully backwards compatible way to extend the current spec which would allow clients to give users options like Continue with Google and Continue with Github side-by-side.

Currently, Matrix supports m.login.sso, m.login.token and /login/sso/redirect for clients to pass their user to the configured Identity provider and for them to come back with something which is exchangeable for a Matrix access token. This flow offers no insight to the user as to what Identity providers are available: clients can offer only a very generic Sign in with SSO button. With the currently possible solutions and workarounds the experience is far from great and users have to blindly click Sign in with SSO without any clue as to what's hiding on the other side of the door. Some users will definitely not be familiar with SSO but will be with the concept of "Continue with Google" or similar.

Proposal

We extend the login flow to allow clients to choose an SSO Identity provider before control is handed over to the server. The following sequence diagram illustrates the proposed, updated, login flow:

Extensions to login flow discovery

The response to GET /_matrix/client/r0/login is extended to optionally include an identity_providers property for flows whose type m.login.sso. This would look like this:

{
    "flows": [
        {
            "type": "m.login.sso",
            "identity_providers": [
                {
                    "id": "google",
                    "name": "Google",
                    "icon": "mxc://...",
                    "brand": "google"
                },
                {
                    "id": "github",
                    "name": "Github",
                    "icon": "mxc://...",
                    "brand": "github"
                }
            ]
        },
        {
            "type": "m.login.token"
        }
    ]
}

The value of the identity_providers property is a list, each entry consisting of an object with the following fields:

• The id field is required. It is an opaque string chosen by the homeserver implementation, and uniquely identifies the identity provider on that server. Clients should not infer any semantic meaning from the identifier. The identifier should be between 1 and 255 characters in length, and should consist of the characters matching unreserved URI characters as defined in RFC3986:

  ALPHA  DIGIT  "-" / "." / "_" / "~"
  

• The name field is required. It should be a human readable string intended for printing by the client. No explicit length limit or grammar is specified.

• The icon field is optional. It should point to an icon representing the IdP. If present then it must be an MXC URI to an image resource.

• The brand field is optional. It allows the client to style the login button to suit a particular brand. It should be a string using the following grammar:

  • Must be at least one character and no more than 255 characters in length.
  • Must start with one of the characters [a-z], and be entirely composed of the characters [a-z], [0-9], -, _ and ..

  To reduce confusion over which identifier should be used for each brand (for example: should "Sign in with Microsoft" be microsoft or azure?), it is proposed to maintain a registry of identifiers outside the core specification document, avoiding the need for a full MSC to add entries to the list. An initial list of proposed identifiers is given below.

  [Rationale: this grammar is based on the MSC2758, removing the requirements for a namespaced hierarchy. In discussion, it was agreed that a separate registry was seen as important for a lightweight process by which implementations can agree on identifiers. The registry makes the namespacing of MSC2758 redundant; the namespacing system was also somewhat confusing.]

  Server implementations are free to add additional brands, though they should be mindful of clients which do not recognise any given brand.

  Clients are free to implement any set of brands they wish, including all or any of the brands listed in the registry, but are expected to apply a sensible unbranded fallback for any brand they do not recognise/support.

  Where icon and brand are both present, it is recommended that clients which support the brand give precedence to brand over icon.

Extend the /login/sso/redirect endpoint

A new endpoint is added to support redirecting directly to one of the IdPs:

GET /_matrix/client/r0/login/sso/redirect/{idp_id}

This would behave identically to the existing endpoint without the last argument except would allow the server to forward the user directly to the correct IdP.

For the case of backwards compatibility the existing endpoint is to remain, and if the server supports multiple SSO IdPs it should offer the user a page which lets them choose between the available IdP options as a fallback.

If the idp_id is unrecognised, the server should display some sort of error page to the user. (A protocol whereby an error can be returned to the original client could be a matter for a future improvement, but is out of scope for now.)

Notes on user-interactive auth

No change is proposed to the SSO flow for User-Interactive Authentication.

For a reauthentication operation, the server implementation is free to choose any suitable IdP to authenticate the user. (Often, this will simply be the IdP that the user logged in with.)

Proposed initial identifiers for the brand identifier

The following identifiers are proposed for the initial content of the brand identifier registry. The descriptions are guidelines to help server administrators pick a suitable brand identifier, and to help client authors style buttons in their clients.

• Identifier: apple

  Description: Suitable for "Sign in with Apple": see https://developer.apple.com/design/human-interface-guidelines/sign-in-with-apple/overview/buttons/.

• Identifier: facebook

  Description: "Continue with Facebook": see https://developers.facebook.com/docs/facebook-login/web/login-button/.

• Identifier: github

  Description: Logos available at https://github.com/logos.

• Identifier: gitlab

  Description: Logos available at https://about.gitlab.com/press/press-kit/.

• Identifier: google

  Description: Suitable for "Google Sign-In": see https://developers.google.com/identity/branding-guidelines.

• Identifier: twitter

  Description: Suitable for "Log in with Twitter": see https://developer.twitter.com/en/docs/authentication/guides/log-in-with-twitter#tab1.

When considering a new identifier for private use, administrators should pick some sensible name following the advice of RFC6648 sec 3.

Alternatives

An alternative to the whole approach would be to allow m.login.sso.$idp but this forces treating an opaque identifier as hierarchical and offers worse backwards compatibility.

An alternative to the proposed backwards compatibility plan where the server offers a fallback page which fills the gap and lets the user choose which SSO IdP they need is for the server to deterministically always pick one, maybe the first option and let old clients only auth via that one but that means potentially locking users out of their accounts.

MSC2964 proposes replacing much of Matrix's authentication mechanism with OAuth2.0. If that is adopted, then the Matrix client would not be able to specify an authentication mechanism; rather it is left up to the server to host pages allowing the user to choose their authentication mechanism.

Styling information as an alternative to brand

The brand field is intended to allow clients to style "login" buttons according to the identity provider in question. For example, a mobile application might show:

Some identity providers have very specific rules about how such buttons should be presented, so a fine level of control is important.

An alternative way to achieve this would be for the server to give full details about the styling: icon, font colour, border colour, background colour, etc. However, this soon becomes unscalable. For example, it might be desirable to offer each logo at a range of resolutions to suit different screen sizes. Likewise, some brands need different styling depending on the background colour, so a complete second set of colours must be specified to account for dark or light themes.

Potential issues

• New Identity Providers added by server administators will be unbranded until clients adopt support for the new brand.

Security considerations

This could potentially aid phishing attacks by bad homeservers, where if the app says Continue with Google and then they are taken to a page which is styled to look like the Google login page they might be a tiny bit more susceptible to being phished as opposed as to when they click a more generic Sign in with SSO button, but this attack was possible anyhow using a different vector of a controlled Element/client instance which modifies the text.

Unstable prefix

Whilst in development use org.matrix.msc2858.identity_providers for the flow discovery and /_matrix/client/unstable/org.matrix.msc2858/login/sso/redirect/{idp_id} for the new endpoints.

When identity providers are listed under the experimental org.matrix.msc2858.identity_providers field of the response to /login, (instead of identity_providers), different values for the brand field are used. In particular the following were defined:

• org.matrix.gitlab (now gitlab).
• org.matrix.github (now github).
• org.matrix.apple (now apple).
• org.matrix.google (now google).
• org.matrix.facebook (now facebook).
• org.matrix.twitter (now twitter).