Document the usage of refresh tokens. (#11427)

Co-authored-by: David Robertson <davidr@element.io>

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+# Refresh Tokens
+
+Synapse supports refresh tokens since version 1.49 (some earlier versions had support for an earlier, experimental draft of [MSC2918] which is not compatible).
+
+
+[MSC2918]: https://github.com/matrix-org/matrix-doc/blob/main/proposals/2918-refreshtokens.md#msc2918-refresh-tokens
+
+
+## Background and motivation
+
+Synapse users' sessions are identified by **access tokens**; access tokens are
+issued to users on login. Each session gets a unique access token which identifies
+it; the access token must be kept secret as it grants access to the user's account.
+
+Traditionally, these access tokens were eternally valid (at least until the user
+explicitly chose to log out).
+
+In some cases, it may be desirable for these access tokens to expire so that the
+potential damage caused by leaking an access token is reduced.
+On the other hand, forcing a user to re-authenticate (log in again) often might
+be too much of an inconvenience.
+
+**Refresh tokens** are a mechanism to avoid some of this inconvenience whilst
+still getting most of the benefits of short access token lifetimes.
+Refresh tokens are also a concept present in OAuth 2 — further reading is available
+[here](https://datatracker.ietf.org/doc/html/rfc6749#section-1.5).
+
+When refresh tokens are in use, both an access token and a refresh token will be
+issued to users on login. The access token will expire after a predetermined amount
+of time, but otherwise works in the same way as before. When the access token is
+close to expiring (or has expired), the user's client should present the homeserver
+(Synapse) with the refresh token.
+
+The homeserver will then generate a new access token and refresh token for the user
+and return them. The old refresh token is invalidated and can not be used again*.
+
+Finally, refresh tokens also make it possible for sessions to be logged out if they
+are inactive for too long, before the session naturally ends; see the configuration
+guide below.
+
+
+*To prevent issues if clients lose connection half-way through refreshing a token,
+the refresh token is only invalidated once the new access token has been used at
+least once. For all intents and purposes, the above simplification is sufficient.
+
+
+## Caveats
+
+There are some caveats:
+
+* If a third party gets both your access token and refresh token, they will be able to
+  continue to enjoy access to your session.
+  * This is still an improvement because you (the user) will notice when *your*
+    session expires and you're not able to use your refresh token.
+    That would be a giveaway that someone else has compromised your session.
+    You would be able to log in again and terminate that session.
+    Previously (with long-lived access tokens), a third party that has your access
+    token could go undetected for a very long time.
+* Clients need to implement support for refresh tokens in order for them to be a
+  useful mechanism.
+  * It is up to homeserver administrators if they want to issue long-lived access
+    tokens to clients not implementing refresh tokens.
+    * For compatibility, it is likely that they should, at least until client support
+      is widespread.
+      * Users with clients that support refresh tokens will still benefit from the
+        added security; it's not possible to downgrade a session to using long-lived
+        access tokens so this effectively gives users the choice.
+    * In a closed environment where all users use known clients, this may not be
+      an issue as the homeserver administrator can know if the clients have refresh
+      token support. In that case, the non-refreshable access token lifetime
+      may be set to a short duration so that a similar level of security is provided.
+
+
+## Configuration Guide
+
+The following configuration options, in the `registration` section, are related:
+
+* `session_lifetime`: maximum length of a session, even if it's refreshed.
+  In other words, the client must log in again after this time period.
+  In most cases, this can be unset (infinite) or set to a long time (years or months).
+* `refreshable_access_token_lifetime`: lifetime of access tokens that are created
+  by clients supporting refresh tokens.
+  This should be short; a good value might be 5 minutes (`5m`).
+* `nonrefreshable_access_token_lifetime`: lifetime of access tokens that are created
+  by clients which don't support refresh tokens.
+  Make this short if you want to effectively force use of refresh tokens.
+  Make this long if you don't want to inconvenience users of clients which don't
+  support refresh tokens (by forcing them to frequently re-authenticate using
+  login credentials).
+* `refresh_token_lifetime`: lifetime of refresh tokens.
+  In other words, the client must refresh within this time period to maintain its session.
+  Unless you want to log inactive sessions out, it is often fine to use a long
+  value here or even leave it unset (infinite).
+  Beware that making it too short will inconvenience clients that do not connect
+  very often, including mobile clients and clients of infrequent users (by making
+  it more difficult for them to refresh in time, which may force them to need to
+  re-authenticate using login credentials).
+
+**Note:** All four options above only apply when tokens are created (by logging in or refreshing).
+Changes to these settings do not apply retroactively.
+
+
+### Using refresh token expiry to log out inactive sessions
+
+If you'd like to force sessions to be logged out upon inactivity, you can enable
+refreshable access token expiry and refresh token expiry.
+
+This works because a client must refresh at least once within a period of
+`refresh_token_lifetime` in order to maintain valid credentials to access the
+account.
+
+(It's suggested that `refresh_token_lifetime` should be longer than
+`refreshable_access_token_lifetime` and this section assumes that to be the case
+for simplicity.)
+
+Note: this will only affect sessions using refresh tokens. You may wish to
+set a short `nonrefreshable_access_token_lifetime` to prevent this being bypassed
+by clients that do not support refresh tokens.
+
+
+#### Choosing values that guarantee permitting some inactivity
+
+It may be desirable to permit some short periods of inactivity, for example to
+accommodate brief outages in client connectivity.
+
+The following model aims to provide guidance for choosing `refresh_token_lifetime`
+and `refreshable_access_token_lifetime` to satisfy requirements of the form:
+
+1. inactivity longer than `L` **MUST** cause the session to be logged out; and
+2. inactivity shorter than `S` **MUST NOT** cause the session to be logged out.
+
+This model makes the weakest assumption that all active clients will refresh as
+needed to maintain an active access token, but no sooner.
+*In reality, clients may refresh more often than this model assumes, but the
+above requirements will still hold.*
+
+To satisfy the above model,
+* `refresh_token_lifetime` should be set to `L`; and
+* `refreshable_access_token_lifetime` should be set to `L - S`.