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+Design Doc 007: Specification of the Payment Flow
+#################################################
+
+Summary
+=======
+
+This design document describes how the payment flow works in the browser, and how
+features like session IDs, re-purchase detection and refunds interact.
+
+Requirements
+============
+
+* The payment flow must both support wallets that are integrated in the browser,
+  as well as external wallets (mobile phone, command line)
+* The initiator of the payment can be a Website or another channel,
+  such as an e-mail or a messaging service.
+* For paid digital content, there should be a reasonable technical barrier to
+  sharing the content with unauthorized users
+* A simple API should be offered to shops
+* Sharing of links or re-visiting of bookmarks should result in well-defined,
+  behavior instead of random, ugly error messages.
+
+Proposed Solution
+=================
+
+Session-bound payment flow for Web resources
+--------------------------------------------
+
+In this payment flow, the user initiates the payment by navigating to a
+paywalled Web resource.  Let *resource-URL* be the URL of the paywalled resource.
+
+When *resource-URL* is requested, the storefront runs the following steps:
+
+1. Extract the the *order-ID* (or null) and *resource name* from the *resource-URL*.
+2. Extract the *session-ID* (or null) from the request's signed cookie
+3. If *session-ID* and *order-ID* is non-null and the storefront's
+   *session-payment-cache* contains the tuple (*order-ID*, *resource-name*, *session-ID*),
+   return to the client the content for *resource name*.  **Terminate.**
+4. If the *session-ID* is null, assign a fresh session ID and set it in a cookie to be sent with the response
+5. If *order-ID* is null, create a new order for *resource-name* by doing a ``POST /private/orders`` to
+   the merchant backend.  Store the new order ID as *order-ID*.
+6. Check the status of the payment for *order-ID* under *session-ID* by doing a ``GET /private/orders/{order-ID}?session_id={session-ID}``.
+   This results in the *order-status*, *refund-amount* and the *client-order-status-URL*.
+7. If the *order-status* is paid and *refund-amount* is non-zero, 
+   return to the client the refund info page for *resource name*.  **Terminate.**
+8. If the *order-status* is paid, store the tuple (*order-ID*, *resource-name*, *session-ID*) in *session-payment-cache*
+   and return to the client the content for *resource name*.  **Terminate.**
+9. Otherwise, the *order-status* is unpaid.  Redirect the client to *client-order-status-URL*. **Terminate.**
+
+
+The merchant backend runs the following steps to generate the
+*client-order-status-URL* when processing a request for ``GET
+/private/orders/{order-ID}?session_id={session-ID}``:
+
+1. Let *session-ID* be the session ID of the request (note: **not** the last paid session ID)
+2. If *order-ID* does not identify an existing order, return an error.  **Terminate**.
+3. If *order-ID* identifies an order that is *unclaimed* and has claim token *claim-token*, return the URL
+
+   ::
+
+     {backendBaseUrl}/orders/{order-ID}?token={claim-token}&session_id={session-ID}
+
+   **Terminate.**
+4. If *order-ID* identifies an order that is either *claimed* or *paid* and has contract terms hash *contract-hash*, return the URL
+
+   ::
+
+     {backendBaseUrl}/orders/{order-ID}?h_contract={contract-hash}&session_id={session-ID}
+
+The merchant backend runs the following steps to generate the HTML page for
+``GET /orders/{order-ID}?session_id={session-ID}&claim_token={claim-token}&h_contract={contract-hash}``:
+
+1. If *order-ID* does not identify an existing order, render an error page.  **Terminate.**
+2. If *order-ID* identifies a paid order *ord*, run these steps:
+
+   1. If the *contract-hash* request parameter does not match the contract terms hash of *ord*,
+      return a 403 Forbidden response. **Terminate.**
+   2. If *ord* has refunds that are not picked up, prompt the URI
+
+      ::
+
+        taler{proto_suffix}://refund/{/merchant_prefix*}/{order-id}/{session-id}
+
+      Redirect to the *fulfillment-URL* of *ord* once the refund is picked up and
+      payment has been proven under *session-ID*.
+
+      **Terminate.**
+
+   3. Prompt the URI
+
+      ::
+
+        taler{proto_suffix}://pay/{/merchant_prefix*}/{order-id}/{session-id}
+
+      Redirect to the *fulfillment-URL* of *ord* once
+      payment has been proven under *session-ID*.
+
+      **Terminate.**
+
+
+3. If *order-ID* identifies an unpaid order *ord*, run these steps:
+  
+   1. If the the *claim-token* request parameter does not matches the claim token of
+      *ord*,  return a 403 Forbidden response. **Terminate**.
+   2. Prompt the URI
+
+      ::
+
+        taler{proto_suffix}://pay/{/merchant_prefix*}/{order-id}/{session-ID}?c={claim-token}
+
+      Redirect to the *fulfillment-URL* of *ord* once
+      payment has been proven under *session-ID*.
+
+      If there is a non-null *already-paid-order-ID* for *session-ID*, redirect
+      to the *fulfillment-URL* of *already-paid-order-ID*
+
+      **Terminate.**
+
+
+Discussion / Q&A
+================
+
+Covered Scenarios
+-----------------
+
+* **Re-purchase detection**. Let's say a detached wallet has already successfully paid for a resource URL.
+  A browser navigates to the resource URL.  The storefront will generate a new order and assign a session ID.
+  Upon scanning the QR code, the wallet will detect that it already has puchased the resource (checked via the fulfillment URL).
+  It will then prove the payment of the **old** order ID under the **new** session ID.
+
+
+Problematic Scenarios
+---------------------
+
+Let's say I bought some article a few months ago and I lost my wallet. I still have the augmented fulfillment URL
+for the article bookmarked.  When I re-visit the URL, I will be prompted via QR code, but I can *never* prove
+that I already paid, because I lost my wallet!
+
+In this case, it might make sense to include some "make new purchase" link on the client order status page.
+It's not clear if this is a common/important scenario though.
+
+But we might want to make clear on the client order status page that it's showing a QR code for something
+that was already paid.