Make token endpoint configurable and align with OAUTH

IETF-RFC.md

@@ -645,7 +645,7 @@ contain the following information about its OCM API:
   to the more secure (and possibly required) invite flow.
   _ `"receive-code"` - to indicate that this OCM Server can receive a
   `code` as part of a Share Creation Notification, and exchange it
-  for a bearer token at the Sending Server's `/token` API endpoint.
+  for a bearer token at the Sending Server's tokenEndPoint.
   _ `"invite-wayf"` - to indicate that this OCM Server exposes a WAYF
   Page to facilitate the Invite flow.
 * OPTIONAL: criteria (array of string) - The criteria for accepting a
@@ -687,6 +687,11 @@ contain the following information about its OCM API:
   `"/index.php/apps/sciencemesh/accept"` is specified here then a WAYF
   Page SHOULD redirect the end-user to
 `/index.php/apps/sciencemesh/accept?token=zi5kooKu3ivohr9a&providerDomain=example.com`.
+* OPTIONAL: tokenEndPoint (string) - URL of the token endpoint where
+  the Sending Server can exchange a `code` for a bearer token.
+  Implementations that offer the `"receive-code"` capability MUST
+  provide this URL as well.
+  Example: `"https://my-cloud-storage.org/ocm/token"`.
 
 # Share Creation Notification
 
@@ -759,7 +764,7 @@ To create a Share, the Sending Server SHOULD make a HTTP POST request
   that the share does not expire.
 * OPTIONAL code (string)
   A nonce to be exchanged for a (potentially short-lived)
-  bearer token at the Sending Server's /token endpoint.
+  bearer token at the Sending Server's tokenEndPoint [RFC6749]
 * REQUIRED protocol (object)
   JSON object with specific options for each protocol.
   The supported protocols are: - `webdav`, to access the data -
@@ -961,9 +966,10 @@ is as follows:
     `resourceTypes[0].protocols.webdav` value is the
     `<sender-ocm-path>` to be used in step 3.
 2.  If `code` is not empty, the receiver SHOULD make a signed POST
-    request to the `/token` path inside the Sending Server's OCM API, to
+    request to the path in the Sending Server’s tokenEndPoint, to
     exchange the code for a short-lived bearer token, and then use that
-    bearer token to access the Resource.
+    bearer token to access the Resource (See the [Code Flow](
+    #code-flow) section).
 3.  If `protocol.name` = `webdav`, the receiver SHOULD inspect the
     `protocol.options` property.  If it contains a `sharedSecret`, as in
     the [legacy example](
@@ -994,6 +1000,85 @@ Additionally, if `protocol.<protocolname>.requirements` includes
 Party has been authenticated with MFA, or prompt the consumer in order
 to elevate their session, if applicable.
 
+
+# Code Flow
+
+This section defines the procedure for issuing short-lived bearer access
+tokens for use by the Receiving Server when accessing a resource shared
+through OCM.  The mechanism is aligned with the OAuth 2.0
+*authorization_code* grant type but is performed entirely as a
+server to server interaction between the Sending and Receiving Servers.
+No user interaction or redirect is involved. [RFC6749]
+
+##  Token Request
+
+To obtain an access token, the Receiving Server MUST send an HTTP POST
+request to the Sending Server’s tokenEndPoint as discovered in the
+OCM provider metadata, following section 4.4.2 of [RFC6749].  Here
+follows an example of such POST request:
+
+```
+POST {tokenEndPoint} HTTP/1.1
+Host: my-cloud-storage.org
+Date: Wed, 05 Nov 2025 14:00:00 GMT
+Content-Type: application/x-www-form-urlencoded
+Digest: SHA-256=ok6mQ3WZzKc8nb7s/Jt2yY1uK7d2n8Zq7dhl3Q0s1xk=
+Content-Length: 101
+Signature-Input: 
+    sig1=("@method" "@target-uri" "content-digest" "date"); \
+    created=1730815200; keyid="receiver.example.org#2025"; \
+    alg="rsa-sha256"
+Signature: sig1=:
+    bM2sV2a4oM8pWc4Q8r9Zb8bQ7a2vH1kR9xT0yJ3uE4wO5lV6bZ1cP2rN3qD4tR5hC=:
+
+grant_type=authorization_code&
+client_id=receiver.example.org&
+code=my_secret_code
+```
+
+The request MUST be signed using an HTTP Message Signature
+[RFC9421].  The `client_id` identifies the Receiving Server and MUST be
+set to its fully qualified domain name.  The `code` parameter carries
+the authorization code that was issued by the Sending Server in the
+Share Creation Notification.  It is allowed to send the additional
+parameters defined in [RFC6749] for the `authorization_code` grant type,
+but they MUST be ignored.
+
+##  Token Response
+
+If the request is valid and the code is accepted, the Sending Server
+MUST respond with HTTP 200 OK and a OAuth-compliant JSON object
+containing the issued token:
+
+```
+{
+  "access_token": "8f3d3f26-f1e6-4b47-9e3e-9af6c0d4ad8b",
+  "token_type": "Bearer",
+  "expires_in": 300
+}
+```
+
+The `access_token` is an opaque bearer credential with no internal
+structure visible to the Receiving Server.  The token authorizes the
+Receiving Server to access the shared resource using the appropriate
+transport protocol (e.g., WebDAV).  The `expires_in` value indicates
+the token lifetime in seconds.  No `refresh_token` is issued, instead
+the same request to the tokenEndPoint MUST be repeated before the
+`access_token` has expired, to recieve a new `access_token` that can
+then be used in the same manner.
+
+##  Error Responses
+
+If the request is invalid, the Sending Server MUST return an HTTP 400
+response with a JSON object containing an OAuth 2.0 error code
+[RFC6749]:
+```
+{ "error": "invalid_request" }
+```
+
+Permitted error codes are `invalid_request`, `invalid_client`,
+`invalid_grant`, `unauthorized_client` and `unsupported_grant_type`.
+
 # Share Deletion
 
 A `"SHARE_ACCEPTED"` notification followed by a `"SHARE_UNSHARED"`
@@ -1067,6 +1152,15 @@ The legacy format of an OCM Share Notification with shared secrets is
 only provided for backwards compatibility with existing implementations.
 Implementers SHOULD NOT use it and prefer short-lived tokens instead.
 
+##  Code Flow
+
+All `{tokenEndPoint}` requests MUST be transmitted over HTTPS and
+signed using HTTP Signatures.  Bearer tokens MUST be treated as 
+confidential and never logged, persisted beyond their lifetime, or
+transmitted over unsecured channels.
+
+
+
 # References
 
 ## Normative References
@@ -1089,15 +1183,13 @@ Signatures](https://tools.ietf.org/html/rfc9421)", February 2024.
 "[Uniform Resource Identifier (URI): Generic Syntax
 ](https://datatracker.ietf.org/doc/html/rfc3986)", January 2005
 
+[RFC6749] Hardt, D. (ed), "[The OAuth 2.0 Authorization Framework](
+https://datatracker.ietf.org/html/rfc6749)", October 2012.
+
 [RFC8615] Nottingham, M. "[Well-Known Uniform Resource Identifiers 
 (URIs)](https://datatracker.ietf.org/doc/html/rfc8615)", May 2019
 
 
-## Informative References
-
-[RFC6749] Hardt, D. (ed), "[The OAuth 2.0 Authorization Framework](
-https://datatracker.ietf.org/html/rfc6749)", October 2012.
-
 # Appendix A: Multi-factor Authentication
 
 If a Receiving Server exposes the capability `enforce-mfa`, it

spec.yaml

@@ -231,19 +231,20 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/Error"
-  /token:
+  /{tokenEndPoint}:
     post:
       summary: Token Exchange endpoint
       description: >
         This optional endpoint allows to obtain a (potentially short-lived) bearer token in exchange for a code.
         See [Resource Access](https://github.com/cs3org/OCM-API/blob/develop/IETF-RFC.md#resource-access)
-        for more details.
+        for more details. The actual endpoint URL is discovered via OCM provider metadata
+        (tokenEndPoint).
       requestBody:
         content:
-          application/json:
+          application/x-www-form-urlencoded:
             schema:
               $ref: "#/components/schemas/TokenRequest"
-        description: The JSON request body.
+        description: Form-encoded request body.
         required: true
       responses:
         "200":
@@ -252,7 +253,7 @@ paths:
             application/json:
               schema:
                 $ref: "#/components/schemas/TokenResponse"
-        "403":
+        "400":
           description: Token denied.
           content:
             application/json:
@@ -466,6 +467,12 @@ components:
                 -----BEGIN PUBLIC KEY-----
                 MII...QDD
                 -----END PUBLIC KEY-----
+        tokenEndPoint:
+          type: string
+          description: >
+            Optional URL path of the Token Exchange endpoint to obtain bearer tokens in exchange for codes.
+            If the `receive-code` capability is exposed, the tokenEndPoint MUST be advertised in the discovery response.
+          example: /index.php/apps/sciencemesh/token
         inviteAcceptDialog:
           type: string
           description: >
@@ -554,7 +561,7 @@ components:
           type: string
           description: |
             A nonce to be exchanged for a (potentially short-lived) bearer token
-            at the Sending Server's `/token` endpoint.
+            at the Sending Server's {tokenEndPoint}.
         protocol:
           type: object
           description: |
@@ -637,9 +644,9 @@ components:
                         MFA-authenticated. This requirement MAY be used if the
                         recipient provider exposes the `enforce-mfa` capability.
                       - `use-code` requires the recipient to exchange the given
-                        `code` via a signed HTTPS request to `/token` at the Sending
-                        Server, in order to get a short-lived token to be used for
-                        subsequent access. This requirement MAY be used if the
+                        `code` via a signed HTTPS request to {tokenEndPoint} at the
+                        Sending Server, in order to get a short-lived token to be used
+                        for subsequent access. This requirement MAY be used if the
                         recipient provider exposes the `receive-code` capability.
                     enum:
                       - mfa-enforced
@@ -842,6 +849,10 @@ components:
           example: John Doe
     TokenRequest:
       type: object
+      required:
+        - grant_type
+        - client_id
+        - code
       properties:
         client_id:
           type: string
@@ -854,8 +865,14 @@ components:
           example: xyz
         grant_type:
           type: string
-          description: Must be set to 'ocm_authorization_code'
-          example: ocm_authorization_code
+          description: Must be set to 'authorization_code'
+          enum: ["authorization_code"]
+          example: authorization_code
+        redirect_uri:
+          type: string
+          description: >
+            Optional parameter that MUST be ignored by the server.
+          example: https://receiver.org/ocm/callback
     TokenResponse:
       type: object
       properties:
@@ -865,13 +882,9 @@ components:
           example: asdfgh
         token_type:
           type: string
-          description: Must be set to 'bearer'
-          example: bearer
+          description: Must be set to 'Bearer'
+          example: Bearer
         expires_in:
           type: number
           description: Number of seconds before this access_token will need to be refreshed.
-          example: 3600
-        refresh_token:
-          type: string
-          description: A refresh token
-          example: qwertyuiop
+          example: 300