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@@ -2,6 +2,7 @@ Changes in RiotX 0.21.0 (2020-XX-XX)
 ===================================================
 
 Features ✨:
+ - Identity server support (#607)
  - Switch language support (#41)
 
 Improvements 🙌:
diff --git a/docs/identity_server.md b/docs/identity_server.md
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+# Identity server
+
+Issue: #607
+PR: #1354
+
+## Introduction
+Identity Servers support contact discovery on Matrix by letting people look up Third Party Identifiers to see if the owner has publicly linked them with their Matrix ID.
+
+## Implementation
+
+The current implementation was Inspired by the code from Riot-Android.
+
+Difference though (list not exhaustive):
+- Only API v2 is supported (see https://matrix.org/docs/spec/identity_service/latest)
+- Homeserver has to be up to date to support binding (Versions.isLoginAndRegistrationSupportedBySdk() has to return true)
+- The SDK managed the session and client secret when binding ThreePid. Those data are not exposed to the client.
+- The SDK supports incremental sendAttempt (this is not used by RiotX)
+- The "Continue" button is now under the information, and not as the same place that the checkbox
+- The app can cancel a binding. Current data are erased from DB.
+- The API (IdentityService) is improved.
+- A new DB to store data related to the identity server management.
+
+Missing features (list not exhaustive):
+- Invite by 3Pid (will be in a dedicated PR)
+- Add email or phone to account (not P1, can be done on Riot-Web)
+- List email and phone of the account (could be done in a dedicated PR)
+- Search contact (not P1)
+- Logout from identity server when user sign out or deactivate his account (pending question in the issue).
+
+## Related MSCs
+The list can be found here: https://matrix.org/blog/2019/09/27/privacy-improvements-in-synapse-1-4-and-riot-1-4
+
+## Steps and requirements
+
+- Only one identity server by account can be set. The user's choice is stored in account data with key `m.identity_server`. But every clients will managed its own token to log in to the identity server
+```json
+{
+  "type": "m.identity_server",
+  "content": {
+    "base_url": "https://matrix.org"
+  }
+}
+```
+- The accepted terms are stored in the account data:
+```json
+{
+  "type": "m.accepted_terms",
+  "content": {
+    "accepted": [
+      "https://vector.im/identity-server-privacy-notice-1"
+    ]
+  }
+}
+```
+
+- Default identity server URL, from Wellknown data is proposed to the user.
+- Identity server can be set
+- Identity server can be changed on another user's device, so when the change is detected (thanks to account data sync) RiotX should properly disconnect from a previous identity server (I think it was not the case in Riot-Android, where we keep the token forever)
+- Registration to the identity server is managed with an openId token
+- Terms of service can be accepted when configuring the identity server.
+- Terms of service can be accepted after, if they change.
+- Identity server can be modified
+- Identity server can be disconnected with a warning dialog, with special content if there are current bound 3pid on this identity server.
+- Email can be bound
+- Email can be unbound
+- Phone can be bound
+- Phone can be unbound
+- Look up can be performed, to get matrixIds from local contact book (phone and email): Android permission correctly handled (not done yet)
+- Look up pepper can be updated if it is rotated on the identity server
+- Invitation using 3PID can be done (See #548) (not done yet)
+- Homeserver access-token will never be sent to an identity server
+- When user sign-out: logout from the identity server if any?
+- When user deactivate account: logout from the identity server if any?
+
+## Screens
+
+### Settings
+
+Identity server settings can be accessed from the internal setting of the application, both from "Discovery" section and from identity detail section.
+
+### Discovery screen
+
+This screen displays the identity server configuration and the binding of the user's ThreePid (email and msisdn). This is the main screen of the feature.
+
+### Set identity server screen
+
+This screen is a form to set a new identity server URL
+
+## Ref:
+- https://matrix.org/blog/2019/09/27/privacy-improvements-in-synapse-1-4-and-riot-1-4 is a good summary of the role of an Identity server and the proper way to configure and use it in respect to the privacy and the consent of the user.
+- API documentation: https://matrix.org/docs/spec/identity_service/latest
+- vector.im TOS: https://vector.im/identity-server-privacy-notice