Implement OpenID Connect-based login (#7256)

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diff --git a/docs/dev/oidc.md b/docs/dev/oidc.md
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@@ -0,0 +1,175 @@
+# How to test OpenID Connect
+
+Any OpenID Connect Provider (OP) should work with Synapse, as long as it supports the authorization code flow.
+There are a few options for that:
+
+ - start a local OP. Synapse has been tested with [Hydra][hydra] and [Dex][dex-idp].
+   Note that for an OP to work, it should be served under a secure (HTTPS) origin.
+   A certificate signed with a self-signed, locally trusted CA should work. In that case, start Synapse with a `SSL_CERT_FILE` environment variable set to the path of the CA.
+ - use a publicly available OP. Synapse has been tested with [Google][google-idp].
+ - setup a SaaS OP, like [Auth0][auth0] and [Okta][okta]. Auth0 has a free tier which has been tested with Synapse.
+
+[google-idp]: https://developers.google.com/identity/protocols/OpenIDConnect#authenticatingtheuser
+[auth0]: https://auth0.com/
+[okta]: https://www.okta.com/
+[dex-idp]: https://github.com/dexidp/dex
+[hydra]: https://www.ory.sh/docs/hydra/
+
+
+## Sample configs
+
+Here are a few configs for providers that should work with Synapse.
+
+### [Dex][dex-idp]
+
+[Dex][dex-idp] is a simple, open-source, certified OpenID Connect Provider.
+Although it is designed to help building a full-blown provider, with some external database, it can be configured with static passwords in a config file.
+
+Follow the [Getting Started guide](https://github.com/dexidp/dex/blob/master/Documentation/getting-started.md) to install Dex.
+
+Edit `examples/config-dev.yaml` config file from the Dex repo to add a client:
+
+```yaml
+staticClients:
+- id: synapse
+  secret: secret
+  redirectURIs:
+  - '[synapse base url]/_synapse/oidc/callback'
+  name: 'Synapse'
+```
+
+Run with `dex serve examples/config-dex.yaml`
+
+Synapse config:
+
+```yaml
+oidc_config:
+   enabled: true
+   skip_verification: true # This is needed as Dex is served on an insecure endpoint
+   issuer: "http://127.0.0.1:5556/dex"
+   discover: true
+   client_id: "synapse"
+   client_secret: "secret"
+   scopes:
+     - openid
+     - profile
+   user_mapping_provider:
+     config:
+       localpart_template: '{{ user.name }}'
+       display_name_template: '{{ user.name|capitalize }}'
+```
+
+### [Auth0][auth0]
+
+1. Create a regular web application for Synapse
+2. Set the Allowed Callback URLs to `[synapse base url]/_synapse/oidc/callback`
+3. Add a rule to add the `preferred_username` claim.
+   <details>
+    <summary>Code sample</summary>
+
+    ```js
+    function addPersistenceAttribute(user, context, callback) {
+      user.user_metadata = user.user_metadata || {};
+      user.user_metadata.preferred_username = user.user_metadata.preferred_username || user.user_id;
+      context.idToken.preferred_username = user.user_metadata.preferred_username;
+
+      auth0.users.updateUserMetadata(user.user_id, user.user_metadata)
+        .then(function(){
+            callback(null, user, context);
+        })
+        .catch(function(err){
+            callback(err);
+        });
+    }
+    ```
+
+  </details>
+
+
+```yaml
+oidc_config:
+   enabled: true
+   issuer: "https://your-tier.eu.auth0.com/" # TO BE FILLED
+   discover: true
+   client_id: "your-client-id" # TO BE FILLED
+   client_secret: "your-client-secret" # TO BE FILLED
+   scopes:
+     - openid
+     - profile
+   user_mapping_provider:
+     config:
+       localpart_template: '{{ user.preferred_username }}'
+       display_name_template: '{{ user.name }}'
+```
+
+### GitHub
+
+GitHub is a bit special as it is not an OpenID Connect compliant provider, but just a regular OAuth2 provider.
+The `/user` API endpoint can be used to retrieve informations from the user.
+As the OIDC login mechanism needs an attribute to uniquely identify users and that endpoint does not return a `sub` property, an alternative `subject_claim` has to be set.
+
+1. Create a new OAuth application: https://github.com/settings/applications/new
+2. Set the callback URL to `[synapse base url]/_synapse/oidc/callback`
+
+```yaml
+oidc_config:
+   enabled: true
+   issuer: "https://github.com/"
+   discover: false
+   client_id: "your-client-id" # TO BE FILLED
+   client_secret: "your-client-secret" # TO BE FILLED
+   authorization_endpoint: "https://github.com/login/oauth/authorize"
+   token_endpoint: "https://github.com/login/oauth/access_token"
+   userinfo_endpoint: "https://api.github.com/user"
+   scopes:
+     - read:user
+   user_mapping_provider:
+     config:
+       subject_claim: 'id'
+       localpart_template: '{{ user.login }}'
+       display_name_template: '{{ user.name }}'
+```
+
+### Google
+
+1. Setup a project in the Google API Console
+2. Obtain the OAuth 2.0 credentials (see <https://developers.google.com/identity/protocols/oauth2/openid-connect>)
+3. Add this Authorized redirect URI: `[synapse base url]/_synapse/oidc/callback`
+
+```yaml
+oidc_config:
+   enabled: true
+   issuer: "https://accounts.google.com/"
+   discover: true
+   client_id: "your-client-id" # TO BE FILLED
+   client_secret: "your-client-secret" # TO BE FILLED
+   scopes:
+     - openid
+     - profile
+   user_mapping_provider:
+     config:
+       localpart_template: '{{ user.given_name|lower }}'
+       display_name_template: '{{ user.name }}'
+```
+
+### Twitch
+
+1. Setup a developer account on [Twitch](https://dev.twitch.tv/)
+2. Obtain the OAuth 2.0 credentials by [creating an app](https://dev.twitch.tv/console/apps/)
+3. Add this OAuth Redirect URL: `[synapse base url]/_synapse/oidc/callback`
+
+```yaml
+oidc_config:
+   enabled: true
+   issuer: "https://id.twitch.tv/oauth2/"
+   discover: true
+   client_id: "your-client-id" # TO BE FILLED
+   client_secret: "your-client-secret" # TO BE FILLED
+   client_auth_method: "client_secret_post"
+   scopes:
+     - openid
+   user_mapping_provider:
+     config:
+       localpart_template: '{{ user.preferred_username }}'
+       display_name_template: '{{ user.name }}'
+```
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index 98ead7dc0e..1e397f7734 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -1470,6 +1470,94 @@ saml2_config:
   #template_dir: "res/templates"
 
 
+# Enable OpenID Connect for registration and login. Uses authlib.
+#
+oidc_config:
+    # enable OpenID Connect. Defaults to false.
+    #
+    #enabled: true
+
+    # use the OIDC discovery mechanism to discover endpoints. Defaults to true.
+    #
+    #discover: true
+
+    # the OIDC issuer. Used to validate tokens and discover the providers endpoints. Required.
+    #
+    #issuer: "https://accounts.example.com/"
+
+    # oauth2 client id to use. Required.
+    #
+    #client_id: "provided-by-your-issuer"
+
+    # oauth2 client secret to use. Required.
+    #
+    #client_secret: "provided-by-your-issuer"
+
+    # auth method to use when exchanging the token.
+    # Valid values are "client_secret_basic" (default), "client_secret_post" and "none".
+    #
+    #client_auth_method: "client_auth_basic"
+
+    # list of scopes to ask. This should include the "openid" scope. Defaults to ["openid"].
+    #
+    #scopes: ["openid"]
+
+    # the oauth2 authorization endpoint. Required if provider discovery is disabled.
+    #
+    #authorization_endpoint: "https://accounts.example.com/oauth2/auth"
+
+    # the oauth2 token endpoint. Required if provider discovery is disabled.
+    #
+    #token_endpoint: "https://accounts.example.com/oauth2/token"
+
+    # the OIDC userinfo endpoint. Required if discovery is disabled and the "openid" scope is not asked.
+    #
+    #userinfo_endpoint: "https://accounts.example.com/userinfo"
+
+    # URI where to fetch the JWKS. Required if discovery is disabled and the "openid" scope is used.
+    #
+    #jwks_uri: "https://accounts.example.com/.well-known/jwks.json"
+
+    # skip metadata verification. Defaults to false.
+    # Use this if you are connecting to a provider that is not OpenID Connect compliant.
+    # Avoid this in production.
+    #
+    #skip_verification: false
+
+
+    # An external module can be provided here as a custom solution to mapping
+    # attributes returned from a OIDC provider onto a matrix user.
+    #
+    user_mapping_provider:
+      # The custom module's class. Uncomment to use a custom module.
+      # Default is 'synapse.handlers.oidc_handler.JinjaOidcMappingProvider'.
+      #
+      #module: mapping_provider.OidcMappingProvider
+
+      # Custom configuration values for the module. Below options are intended
+      # for the built-in provider, they should be changed if using a custom
+      # module. This section will be passed as a Python dictionary to the
+      # module's `parse_config` method.
+      #
+      # Below is the config of the default mapping provider, based on Jinja2
+      # templates. Those templates are used to render user attributes, where the
+      # userinfo object is available through the `user` variable.
+      #
+      config:
+        # name of the claim containing a unique identifier for the user.
+        # Defaults to `sub`, which OpenID Connect compliant providers should provide.
+        #
+        #subject_claim: "sub"
+
+        # Jinja2 template for the localpart of the MXID
+        #
+        localpart_template: "{{ user.preferred_username }}"
+
+        # Jinja2 template for the display name to set on first login. Optional.
+        #
+        #display_name_template: "{{ user.given_name }} {{ user.last_name }}"
+
+
 
 # Enable CAS for registration and login.
 #
@@ -1554,6 +1642,13 @@ sso:
     #
     #   This template has no additional variables.
     #
+    # * HTML page to display to users if something goes wrong during the
+    #   OpenID Connect authentication process: 'sso_error.html'.
+    #
+    #   When rendering, this template is given two variables:
+    #     * error: the technical name of the error
+    #     * error_description: a human-readable message for the error
+    #
     # You can see the default templates at:
     # https://github.com/matrix-org/synapse/tree/master/synapse/res/templates
     #