Documentation/proposals: Add a proposal for refresh token revocation.

Documentation/proposals/token-revocation.md

+# Proposal: design for revoking refresh tokens.
+
+Refresh tokens are issued to the client by the authorization server and are used
+to request a new access token when the current access token becomes invalid or expires.
+It is a common usecase for the end users to revoke client access to their identity.
+This proposal defines the changes needed in Dex v2 to support refresh token revocation.
+
+## Motivation
+
+1. Currently refresh tokens are not associated with the user. Need a new "session object" for this.
+2. Need an API to list refresh tokens based on the UserID.
+3. We need a way for users to login to dex and revoke a client.
+4. Limit the number refresh tokens for each user-client pair to 1.
+
+## Details
+
+Currently in Dex when an end user successfully logs in via a connector and has the OfflineAccess
+scope set to true, a refresh token is created and stored in the backing datastore. There is no
+association between the end user and the refresh token. Hence if we want to support the functionality
+of users being able to revoke refresh tokens, the first step is to have a structure in place that allows
+us retrieve a list of refresh tokens depending on the authenticated user.
+
+```go
+// Reference object for RefreshToken containing only metadata.
+type RefreshTokenRef struct {
+	// ID of the RefreshToken
+	ID string
+	CreatedAt time.Time
+	LastUsed  time.Time
+}
+
+// Session objects pertaining to users with refresh tokens.
+//
+// Will have to handle garbage collection i.e. if no refresh token exists for a user,
+// this object must be cleaned up.
+type OfflineSession struct {
+        // UserID of an end user who has logged in to the server.
+        UserID        string
+        // The ID of the connector used to login the user.
+        ConnID     string
+        // List of pointers to RefreshTokens issued for SessionID
+        Refresh         []*RefreshTokenRef
+}
+
+// Retrieve OfflineSession obj for given userId and connID
+func getOfflineSession (userId string, connID string)
+
+```
+
+### Changes in Dex CodeFlows
+
+1. Client requests a refresh token:
+   Try to retrieve the `OfflineSession` object for the User with the given `UserID + ConnID`.
+   This leads to two possibilities:   
+	* Object exists: This means a Refresh token already exists for the user.
+          Update the existing `OffilineSession` object with the newly received token as follows:
+		* CreateRefresh() will create a new `RefreshToken` obj in the storage.
+		* Update the `Refresh` list with the new `RefreshToken` pointer.
+		* Delete the old refresh token in storage.
+
+	* No object found: This implies that this will be the first refresh token for the user.
+ 		* CreateRefresh() will create a new `RefreshToken` obj in the storage.
+		* Create an OfflineSession for the user and add the new `RefreshToken` pointer to
+		  the `Refresh` list.
+                
+2. Refresh token rotation:
+   There will be no change to this codeflow. When the client refreshes a refresh token, the `TokenID`
+   still remains intact and only the `RefreshToken` obj gets updated with a new nonce. We do not need
+   any additional checks in the OfflineSession objects as the `RefreshToken` pointers still remain intact.
+
+3. User revokes a refresh token (New functionality):
+   A user that has been authenticated externally will have the ability to revoke their refresh tokens.
+   Please note that Dex's API does not perform the authentication, this will have to be done by an
+   external app.
+   Steps involved:
+	* Get `OfflineSession` obj with given UserID + ConnID. 
+	* If a refresh token exists in `Refresh`, delete the `RefreshToken` (handle this in storage)
+	  and its pointer value in `Refresh`. Clean up the OfflineSession object.
+	* If there is no refresh token found, handle error case.
+
+NOTE: To avoid race conditions between “requesting a refresh token” and “revoking a refresh token”, use
+locking mechanism when updating an `OfflineSession` object.