docs: short description of endpoints

Author: nicolst

README.md

@@ -1,7 +1,72 @@
-# labid (very WIP)
+# LabID
 
 Signed JWT token exchange service
 
+## Usage
+
+LabID exposes 3 endpoints: `/token`, `/jwks` and
+`/.well-known/openid-configuration`.
+
+### Servers
+
+LabID is available in all three Dapla Lab environments (dev, test, prod).
+Internally it is always available at `http://labid.labid.svc.cluster.local`.
+External endpoints are available at 
+
+| Environment | URL |
+| - | - |
+| dev | https://labid.lab.dapla-dev-external.ssb.no |
+| test | https://labid.lab.dapla-test-external.ssb.no |
+| prod | https://labid.lab.dapla-external.ssb.no |
+
+The external URLs are available from SSB IPs (VPN) and the corresponding Nais
+clusters (nais-test for dev and test, nais-prod for prod).
+
+### Endpoints
+
+#### `/token` (cluster-internal only)
+Lets you exchange a Kubernetes service account token for a LabID token.
+It is only available internally from Dapla Lab services. Python users can use
+[dapla-auth-client](https://github.com/statisticsnorway/dapla-auth-client) to
+perform the exchange. The endpoint follows the 
+[RFC8693](https://datatracker.ietf.org/doc/html/rfc8693) standard, but in a very
+restricted way. An example request is as follows:
+
+```sh
+curl -vvv 'http://labid.labid.svc.cluster.local/token' \
+--header 'Content-Type: application/x-www-form-urlencoded' \
+--data-urlencode 'grant_type=urn:ietf:params:oauth:grant-type:token-exchange' \
+--data-urlencode 'subject_token_type=urn:ietf:params:oauth:grant-type:id_token' \
+--data-urlencode 'subject_token=<kubernetes-token>' \
+--data-urlencode 'scope=current_group,all_groups' \
+--data-urlencode 'audience=my-audience'
+```
+
+`Content-Type`, `grant_type` and `subject_token` must use the values above, no
+other values are supported.
+
+`scope` is an optional field and supports two values: 
+1. `current_group` adds a `dapla.group` claim to the LabID token, whose value is
+   the Dapla group the service is started as, e.g. `dapla-felles-developers`
+2. `all_groups` adds a `dapla.groups` claim to the LabID token, which is a list
+   of all the Dapla groups the user is a member of.
+
+`audience` is an optional field and can be whatever you want.
+
+`subject_token` must be the content of the file 
+`/var/run/secrets/kubernetes.io/serviceaccount/token`, which is the service' SA
+token.
+
+#### `/jwks` (externally available)
+Exposes the public part of LabID's signing keys, which can be used to validate 
+the issued tokens. Usually this is handled automatically by auth libraries
+through `/.well-known/openid-configuration`.
+
+#### `/.well-known/openid-configuration` (externally available)
+Exposes necessary information about LabID as an Authorization Server as
+specified by [RFC8414](https://datatracker.ietf.org/doc/html/rfc8414). Usually
+handled automatically be auth libraries.
+
 
 ```mermaid
 sequenceDiagram