Support async payments in BOLT 12.

This builds on prior commits by adding the ability to fetch an invoice from an always-online node on behalf of an often-offline recipient, e.g. a mobile node. The idea is that often-offline recipients will supply some always-online node such as their wallet vendor with a static (i.e. `payment_hash`-less) invoice to return on its behalf. The recipient will then publish an offer containing blinded paths that terminate at this always-online node, who payers can request the invoice from if the recipient is offline at the time. After receiving the static invoice, payers will commence the protocol outlined in [1] to send the HTLC asynchronously. [1]: https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-October/003307.html
lightning · Jun 3, 2024 · edc93aa · edc93aa
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diff --git a/04-onion-routing.md b/04-onion-routing.md
@@ -213,6 +213,9 @@ This is formatted according to the Type-Length-Value format defined in [BOLT #1]
     1. type: 18 (`total_amount_msat`)
     2. data:
         * [`tu64`:`total_msat`]
+    1. type: 5482373484 (`sender_provided_payment_preimage`)
+    2. data:
+        * [`32*byte`:`payment_preimage`]
 
 `short_channel_id` is the ID of the outgoing channel used to route the
 message; the receiving peer should operate the other end of this channel.
@@ -239,6 +242,9 @@ The requirements ensure consistency in responding to an unexpected
 `outgoing_cltv_value`, whether it is the final node or not, to avoid
 leaking its position in the route.
 
+`sender_provided_payment_preimage` is set in the case that the recipient is 
+often-offline and another node provided a static BOLT 12 invoice on their behalf.
+
 ### Requirements
 
 The creator of `encrypted_recipient_data` (usually, the recipient of payment):
@@ -269,6 +275,12 @@ The writer of the TLV `payload`:
         - MUST use the current block height as a baseline value. 
         - if a [random offset](07-routing-gossip.md#recommendations-for-routing) was added to improve privacy:
           - SHOULD add the offset to the baseline value.
+      - if paying to a static BOLT 12 invoice:
+        - MUST set `sender_provided_payment_preimage` to randomly generated unique bytes.
+        - MUST set `update_add_htlc.payment_hash` to match the SHA256 hash of
+          `sender_provided_payment_preimage`.
+      - otherwise:
+        - MUST NOT set `sender_provided_payment_preimage`.
     - MUST NOT include any other tlv field.
   - For every node outside of a blinded route:
     - MUST include `amt_to_forward` and `outgoing_cltv_value`.
@@ -319,6 +331,7 @@ The reader:
       - 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`.
+      - MUST use `sender_provided_payment_preimage` when claiming the HTLC, if present
   - 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.

diff --git a/09-features.md b/09-features.md
@@ -27,6 +27,7 @@ The Context column decodes as follows:
 * `C+`: presented in the `channel_announcement` message, but always even (required).
 * `9`: presented in [BOLT 11](11-payment-encoding.md) invoices.
 * `B`: presented in the `allowed_features` field of a blinded path.
+* `R`: presented in [BOLT 12](12-offers.md) invoice requests.
 
 | Bits  | Name                              | Description                                               | Context  | Dependencies              | Link                                                                  |
 |-------|-----------------------------------|-----------------------------------------------------------|----------|---------------------------|-----------------------------------------------------------------------|
@@ -51,11 +52,17 @@ The Context column decodes as follows:
 | 48/49 | `option_payment_metadata`         | Payment metadata in tlv record                            | 9        |                           | [BOLT #11](11-payment-encoding.md#tagged-fields)                      |
 | 50/51 | `option_zeroconf`                 | Understands zeroconf channel types                        | IN       | `option_scid_alias`       | [BOLT #2][bolt02-channel-ready]                                       |
 | 52/53 | `option_htlc_hold`               | Hold HTLCs and forward on receipt of an onion message     | IN       | `option_onion_messages`                                   |
+| 56/57 | `option_om_mailbox`               | Store-and-forward onion messages for often-offline peers  | IN       | `option_onion_messages`   | [BOLT #12](bolt12-offers.md)                                          |
+| 59    | `static_invoice_pay`              | Supports paying BOLT 12 static invoices                   | R        | `option_onion_messages`   | [BOLT #12](bolt12-offers.md)                                          |
 
 ## Definitions
 
 We define `option_anchors` as `option_anchor_outputs || option_anchors_zero_fee_htlc_tx`.
 
+We define `option_om_mailbox` as the ability to store an onion message on behalf
+of an offline peer, and forward it once the peer comes online (subject to rate
+limiting).
+
 ## Requirements
 
 The origin node:

diff --git a/12-offer-encoding.md b/12-offer-encoding.md
@@ -42,7 +42,7 @@ Here we use "user" as shorthand for the individual user's lightning
 node and "merchant" as the shorthand for the node of someone who is
 selling or has sold something.
 
-There are two basic payment flows supported by BOLT 12:
+There are three basic payment flows supported by BOLT 12:
 
 The general user-pays-merchant flow is:
 1. A merchant publishes an *offer*, such as on a web page or a QR code.
@@ -59,6 +59,19 @@ The merchant-pays-user flow (e.g. ATM or refund):
 3. The merchant confirms the *invoice_node_id* to ensure it's about to pay the correct
    person, and makes a payment to the invoice.
 
+The pay-mobile-user flow (e.g. paying a friend back to their mobile node):
+1. The mobile user supplies some always-online node with a static (i.e. 
+   `payment_hash`-less) invoice to return on its behalf. This always-online node may 
+   be the mobile user's channel counterparty, wallet vendor, or another node on the 
+   network that it has an out-of-band relationship with.
+2. The mobile user publishes an offer that contains blinded paths that terminate
+   at the always-online node.
+3. The payer sends an `invoice_request` to the always-online node, who replies
+   with the static invoice previously provided by the mobile user if the mobile user 
+   is offline. If they are online, the `invoice_request` is forwarded to the mobile 
+   user as usual.
+4. The payer makes a payment to the mobile user as indicated by the invoice.
+
 ## Payment Proofs and Payer Proofs
 
 Note that the normal lightning "proof of payment" can only demonstrate that an
@@ -71,6 +84,9 @@ to request the invoice.  In addition, the Merkle construction of the BOLT 12
 invoice signature allows the user to reveal invoice fields in case
 of a dispute selectively.
 
+Payers will not get proofs in the case that they received a static invoice from the 
+payee, see the pay-mobile-user flow above.
+
 # Encoding
 
 Each of the forms documented here are in
@@ -255,8 +271,9 @@ A writer of an offer:
       after midnight 1 January 1970, UTC that invoice_request should not be
       attempted.
   - if it is connected only by private channels:
-    - MUST include `offer_paths` containing one or more paths to the node from
-      publicly reachable nodes.
+    - MUST include `offer_paths` containing one or more paths to the node
+      that will reply to the `invoice_request`, using introduction nodes that are
+      publicly reachable.
   - otherwise:
     - MAY include `offer_paths`.
   - if it includes `offer_paths`:
@@ -274,6 +291,8 @@ A writer of an offer:
       - MUST set `offer_quantity_max` to 0.
   - otherwise:
     - MUST NOT set `offer_quantity_max`.
+  - if it is often-offline and the invoice may be provided by another node on their behalf:
+    - MUST NOT include more than 1 chain in `offer_chains`.
 
 A reader of an offer:
   - if the offer contains any TLV fields greater or equal to 80:
@@ -409,6 +428,20 @@ for [Signature Calculation](#signature-calculation).
     2. data:
         * [`bip340sig`:`sig`]
 
+## Invoice Request Features
+
+| Bits | Description                      | Name                        |
+|------|----------------------------------|-----------------------------|
+| 59   | Supports paying static invoices  | static_invoice_pay/optional |
+
+Setting `static_invoice_pay` indicates that the payer supports receiving a
+`payment_hash`-less invoice in response to their `invoice_request`, and
+subsequently setting `sender_provided_payment_preimage` in their payment onion.
+
+Useful if the payee is often offline and the invoice is being returned on
+their behalf by another node, to avoid trusting that other node to not reuse a
+`payment_hash`.
+
 ## Requirements for Invoice Requests
 
 The writer:
@@ -486,7 +519,14 @@ The reader:
     - MUST fail the request if bitcoin is not a supported chain.
   - otherwise:
     - MUST fail the request if `invreq_chain`.`chain` is not a supported chain.
-
+  - if receiving the `invoice_request` on behalf of an often-offline payee:
+    - if the payee is online:
+      - MUST forward the `invoice_request` to the payee
+    - otherwise (payee is offline):
+      - if `invreq_features` supports `static_invoice_pay`:
+        - MUST reply with the static invoice previously provided by the payee
+      - otherwise:
+        - MUST reply with `invoice_error`
 
 ## Rationale
 
@@ -515,10 +555,11 @@ informative for the payer to know how the sender claims
 
 # Invoices
 
-Invoices are a payment request, and when the payment is made, 
-it can be combined with the invoice to form a cryptographic receipt.
+Invoices are a payment request. If `invoice_payment_hash` is set, then when the
+payment is made, the payment preimage can be combined with the invoice to form a
+cryptographic receipt.
 
-The recipient sends an `invoice` in response to an `invoice_request` using
+The recipient creates an `invoice` for responding to an `invoice_request` using
 the `onion_message` `invoice` field.
 
 1. `tlv_stream`: `invoice`
@@ -604,6 +645,9 @@ the `onion_message` `invoice` field.
     1. type: 176 (`invoice_node_id`)
     2. data:
         * [`point`:`node_id`]
+    1. type: 178 (`invoice_message_paths`)
+    2. data:
+        * [`...*blinded_path`:`paths`]
     1. type: 240 (`signature`)
     2. data:
         * [`bip340sig`:`sig`]
@@ -642,21 +686,25 @@ may (due to capacity limits on a single channel) require it.
 A writer of an invoice:
   - MUST set `invoice_created_at` to the number of seconds since Midnight 1
     January 1970, UTC when the invoice was created.
-  - MUST set `invoice_amount` to the minimum amount it will accept, in units of 
-    the minimal lightning-payable unit (e.g. milli-satoshis for bitcoin) for
-    `invreq_chain`.
-  - if the invoice is in response to an `invoice_request`:
+  - if `invoice_payment_hash` is set and the invoice is in response to an `invoice_request`:
     - MUST copy all non-signature fields from the `invoice_request` (including unknown fields).
     - if `invreq_amount` is present:
       - MUST set `invoice_amount` to `invreq_amount`
     - otherwise:
       - MUST set `invoice_amount` to the *expected amount*.
-  - otherwise (invoice not requested, e.g. for user to scan directly):
+  - otherwise if the invoice was not requested (e.g. for user to scan directly):
     - MUST set `invreq_chain` as it would for an invoice_request.
     - MUST set `offer_description` as it would for an offer.
     - MUST NOT set `invreq_payer_id` or `offer_node_id`.
-  - MUST set `invoice_payment_hash` to the SHA256 hash of the
-    `payment_preimage` that will be given in return for payment.
+  - if the invoice is intended to be provided by a node other than the recipient:
+      - MUST NOT set `invoice_payment_hash`.
+      - MUST NOT set `invoice_amount`.
+      - MUST include `invoice_message_paths` containing at least two paths to
+        the recipient, where the penultimate hop supports `option_om_mailbox`.
+      - MUST NOT set any `invoice_request` TLV fields (i.e. 0 or in the 80-159 range)
+  - otherwise:
+      - MUST set `invoice_payment_hash` to the SHA256 hash of the
+        `payment_preimage` that will be given in return for payment.
   - if `offer_node_id` is present:
     - MUST set `invoice_node_id` to `offer_node_id`.
   - otherwise:
@@ -682,15 +730,16 @@ A writer of an invoice:
     - MUST include `invoice_blindedpay` with exactly one `blinded_payinfo` for each `blinded_path` in `paths`, in order.
     - MUST set `features` in each `blinded_payinfo` to match `encrypted_data_tlv`.`allowed_features` (or empty, if no `allowed_features`).
     - SHOULD ignore any payment which does not use one of the paths.
-  - if `offer_node_id` is present, and `invreq_payer_id` is identical to a previous `invoice_request`:
-    - MAY simply reuse the previous invoice.
+  - if `offer_node_id` is present, and `invreq_payer_id` is identical to a previous `invoice_request`, or if providing invoices on behalf of an often offline recipient:
+    - MAY reuse the previous invoice.
   - otherwise:
     - MUST NOT reuse a previous invoice.
 
 A reader of an invoice:
-  - MUST reject the invoice if `invoice_amount` is not present.
   - MUST reject the invoice if `invoice_created_at` is not present.
-  - MUST reject the invoice if `invoice_payment_hash` is not present.
+  - if `static_invoice_pay` was not supported in `invreq_features`:
+    - MUST reject the invoice if `invoice_payment_hash` is not present.
+    - MUST reject the invoice if `invoice_amount` is not present.
   - MUST reject the invoice if `invoice_node_id` is not present.
   - if `invoice_features` contains unknown _odd_ bits that are non-zero:
     - MUST ignore the bit.
@@ -707,7 +756,8 @@ A reader of an invoice:
     - MUST NOT use the corresponding `invoice_paths`.`path` if `payinfo`.`features` has any unknown even bits set.
     - MUST reject the invoice if this leaves no usable paths.
   - if the invoice is a response to an `invoice_request`:
-    - MUST reject the invoice if all fields less than type 160 do not exactly match the `invoice_request`.
+    - if `invoice_payment_hash` is set:
+      - MUST reject the invoice if all fields less than type 160 do not exactly match the `invoice_request`.
     - if `offer_node_id` is present (invoice_request for an offer):
       - MUST reject the invoice if `invoice_node_id` is not equal to `offer_node_id`.
     - otherwise (invoice_request without an offer):
@@ -727,6 +777,10 @@ A reader of an invoice:
     - MUST ignore any `fallback_address` for which `version` is greater than 16.
     - MUST ignore any `fallback_address` for which `address` is less than 2 or greater than 40 bytes.
     - MUST ignore any `fallback_address` for which `address` does not meet known requirements for the given `version`
+  - if `invoice_payment_hash` is unset:
+    - MUST reject the invoice if `invoice_message_paths` is not present or is empty.
+    - MUST pay asynchronously using the `held_htlc_available` onion message
+      flow, where the onion message is sent over `invoice_message_paths`.
 
 ## Rationale