Clarify failure requirements

And make `total_amount_msat` mandatory.

02-peer-protocol.md

@@ -1141,6 +1141,16 @@ A node:
   commitment transactions:
     - MUST NOT send an `update_fulfill_htlc`, `update_fail_htlc`, or
 `update_fail_malformed_htlc`.
+  - When failing an incoming HTLC:
+    - If `current_blinding_point` is set in the onion payload:
+      - MUST send an `update_fail_htlc` error using the
+        `invalid_onion_blinding` failure code with the `sha256_of_onion`
+        of the onion it received, for any local or downstream errors.
+      - SHOULD add a random delay before sending `update_fail_htlc`.
+    - If `blinding_point` is set in the incoming `update_add_htlc`:
+      - MUST send an `update_fail_malformed_htlc` error using the
+        `invalid_onion_blinding` failure code with the `sha256_of_onion`
+        of the onion it received, for any local or downstream errors.
 
 A receiving node:
   - if the `id` does not correspond to an HTLC in its current commitment transaction:
@@ -1157,23 +1167,10 @@ A receiving node:
   - if the `sha256_of_onion` in `update_fail_malformed_htlc` doesn't match the
   onion it sent:
     - MAY retry or choose an alternate error response.
-  - a receiving node which has an outgoing HTLC canceled by `update_fail_htlc`:
-    - if `current_blinding_point` is set in the onion payload of the corresponding incoming HTLC:
-      - MUST return an `update_fail_malformed_htlc` error using the
-        `invalid_onion_blinding` failure code, with the `sha256_of_onion`
-        of the onion it received.
-      - SHOULD add a random delay before sending `update_fail_malformed_htlc`.
-  - a receiving node which has an outgoing HTLC canceled by `update_fail_malformed_htlc`:
-    - if it is part of a blinded route:
-      - MUST return an `update_fail_malformed_htlc` error using the
-        `invalid_onion_blinding` failure code, with the `sha256_of_onion`
-        of the onion it received.
-      - If its onion payload contains `current_blinding_point`:
-        - SHOULD add a random delay before sending `update_fail_malformed_htlc`.
-    - otherwise:
-      - MUST return an error in the `update_fail_htlc` sent to the link which
-        originally sent the HTLC, using the `failure_code` given and setting the
-        data to `sha256_of_onion`.
+  - otherwise, a receiving node which has an outgoing HTLC canceled by `update_fail_malformed_htlc`:
+    - MUST return an error in the `update_fail_htlc` sent to the link which
+      originally sent the HTLC, using the `failure_code` given and setting the
+      data to `sha256_of_onion`.
 
 #### Rationale
 
@@ -1193,7 +1190,7 @@ errors. However, without re-checking the actual encrypted packet sent,
 it won't know whether the error was its own or the remote's; so
 such detection is left as an option.
 
-Nodes inside a blinded route must use `update_fail_malformed_htlc` to avoid
+Nodes inside a blinded route must use `invalid_onion_blinding` to avoid
 leaking information to senders trying to probe the blinded route.
 
 ### Committing Updates So Far: `commitment_signed`

04-onion-routing.md

@@ -263,8 +263,7 @@ The writer of `tlv_payload`:
     - For the first node in the blinded route:
       - MUST include the `blinding_point` provided by the recipient in `current_blinding_point`
     - If it is the final node:
-      - MUST include `amt_to_forward` and `outgoing_cltv_value`.
-      - MUST include `total_amount_msat` when using `basic_mpp`.
+      - MUST include `amt_to_forward`, `outgoing_cltv_value` and `total_amount_msat`.
     - MUST NOT include any other tlv field.
   - For every node outside of a blinded route:
     - MUST include `amt_to_forward` and `outgoing_cltv_value`.
@@ -312,11 +311,12 @@ The reader:
         - `outgoing_cltv_value = cltv_expiry - payment_relay.cltv_expiry_delta`
     - If it is the final node:
       - MUST return an error if the payload contains other tlv fields than `encrypted_recipient_data`, `current_blinding_point`, `amt_to_forward`, `outgoing_cltv_value` and `total_amount_msat`.
-      - MUST return an error if `amt_to_forward` or `outgoing_cltv_value` are not present.
+      - MUST return an error if `amt_to_forward`, `outgoing_cltv_value` or `total_amount_msat` are not present.
       - MUST return an error if `amt_to_forward` is below what it expects for the payment.
       - MUST return an error if incoming `cltv_expiry` < `outgoing_cltv_value`.
       - MUST return an error if incoming `cltv_expiry` < `current_block_height` + `min_final_cltv_expiry_delta`.
   - Otherwise (it is not part of a blinded route):
+    - MUST return an error if `blinding_point` is set in the incoming `update_add_htlc` or `current_blinding_point` is present.
     - MUST return an error if `amt_to_forward` or `outgoing_cltv_value` are not present.
     - if it is not the final node:
       - MUST return an error if:
@@ -1048,8 +1048,10 @@ The _erring node_:
   - SHOULD set `pad` such that the `failure_len` plus `pad_len` is equal to
     256. Deviating from this may cause older nodes to be unable to parse the
     return message.
-  - If `blinding_point` is set in the incoming `update_add_htlc`:
-    - MUST return `invalid_onion_blinding` for any local error or other downstream errors.
+  - If `blinding_point` is set in the incoming `update_add_htlc` or
+    `current_blinding_point` is set in the onion payload:
+    - MUST return `invalid_onion_blinding` for any local error or other
+      downstream errors.
 
 The _origin node_:
   - once the return message has been decrypted: