Stateless offer and refund builder utilities

Add utility functions to ChannelManager for creating OfferBuilder and
RefundBuilder such that derived keys are used for the signing pubkey and
payer id, respectively. This allows for stateless verification of any
InvoiceRequest and Invoice messages. Later, blinded paths can be
included in the returned builders.

lightning/src/ln/channelmanager.rs

@@ -56,6 +56,10 @@ use crate::ln::outbound_payment;
 use crate::ln::outbound_payment::{OutboundPayments, PaymentAttempts, PendingOutboundPayment, Retry};
 use crate::ln::wire::Encode;
 use crate::chain::keysinterface::{EntropySource, KeysManager, NodeSigner, Recipient, SignerProvider, ChannelSigner};
+use crate::offers::offer::OfferBuilder;
+use crate::offers::parse::SemanticError;
+use crate::offers::refund::RefundBuilder;
+use crate::offers::signer::DerivedPubkey;
 use crate::util::config::{UserConfig, ChannelConfig};
 use crate::util::events::{Event, EventHandler, EventsProvider, MessageSendEvent, MessageSendEventsProvider, ClosureReason, HTLCDestination};
 use crate::util::events;
@@ -5318,6 +5322,36 @@ where
 		Ok(payment_secret)
 	}
 
+	/// Creates an [`OfferBuilder`] such that the [`Offer`] it builds is recognized by the
+	/// [`OnionMessenger`] when handling [`InvoiceRequest`] messages for the offer.
+	///
+	/// [`Offer`]: crate::offers::offer::Offer
+	/// [`OnionMessenger`]: crate::onion_message::OnionMessenger
+	/// [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
+	pub fn create_offer_builder(&self, description: String) -> OfferBuilder {
+		let nonce = inbound_payment::Nonce::from_entropy_source(&*self.entropy_source);
+		let signing_pubkey = DerivedPubkey::new(&self.inbound_payment_key, nonce);
+
+		// TODO: Set blinded paths
+		OfferBuilder::deriving_signing_pubkey(description, signing_pubkey)
+	}
+
+	/// Creates a [`RefundBuilder`] such that the [`Refund`] it builds is recognized by the
+	/// [`OnionMessenger`] when handling [`Invoice`] messages for the refund.
+	///
+	/// [`Refund`]: crate::offers::refund::Refund
+	/// [`OnionMessenger`]: crate::onion_message::OnionMessenger
+	/// [`Invoice`]: crate::offers::invoice::Invoice
+	pub fn create_refund_builder(
+		&self, description: String, amount_msats: u64
+	) -> Result<RefundBuilder, SemanticError> {
+		let nonce = inbound_payment::Nonce::from_entropy_source(&*self.entropy_source);
+		let payer_id = DerivedPubkey::new(&self.inbound_payment_key, nonce);
+
+		// TODO: Set blinded paths
+		RefundBuilder::deriving_payer_id(description, payer_id, amount_msats)
+	}
+
 	/// Gets a payment secret and payment hash for use in an invoice given to a third party wishing
 	/// to pay us.
 	///

lightning/src/ln/inbound_payment.rs

@@ -109,7 +109,10 @@ impl Nonce {
 	pub const LENGTH: usize = 16;
 
 	/// Creates a `Nonce` from the given [`EntropySource`].
-	pub fn from_entropy_source<ES: EntropySource>(entropy_source: &ES) -> Self {
+	pub fn from_entropy_source<ES: Deref>(entropy_source: ES) -> Self
+	where
+		ES::Target: EntropySource,
+	{
 		let mut bytes = [0u8; Self::LENGTH];
 		let rand_bytes = entropy_source.get_secure_random_bytes();
 		bytes.copy_from_slice(&rand_bytes[..Self::LENGTH]);

lightning/src/offers/mod.rs

@@ -19,7 +19,6 @@ pub mod offer;
 pub mod parse;
 mod payer;
 pub mod refund;
-#[allow(unused)]
 pub(crate) mod signer;
 #[cfg(test)]
 mod test_utils;

lightning/src/offers/offer.rs

@@ -13,6 +13,8 @@
 //! published as a QR code to be scanned by a customer. The customer uses the offer to request an
 //! invoice from the merchant to be paid.
 //!
+//! # Example
+//!
 //! ```
 //! extern crate bitcoin;
 //! extern crate core;
@@ -65,6 +67,14 @@
 //! # Ok(())
 //! # }
 //! ```
+//!
+//! # Note
+//!
+//! If constructing an [`Offer`] for use with a [`ChannelManager`], use
+//! [`ChannelManager::create_offer_builder`] instead of [`OfferBuilder::new`].
+//!
+//! [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
+//! [`ChannelManager::create_offer_builder`]: crate::ln::channelmanager::ChannelManager::create_offer_builder
 
 use bitcoin::blockdata::constants::ChainHash;
 use bitcoin::network::constants::Network;
@@ -106,6 +116,14 @@ impl OfferBuilder {
 	/// while the offer is valid.
 	///
 	/// Use a different pubkey per offer to avoid correlating offers.
+	///
+	/// # Note
+	///
+	/// If constructing an [`Offer`] for use with a [`ChannelManager`], use
+	/// [`ChannelManager::create_offer_builder`] instead of [`OfferBuilder::new`].
+	///
+	/// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
+	/// [`ChannelManager::create_offer_builder`]: crate::ln::channelmanager::ChannelManager::create_offer_builder
 	pub fn new(description: String, signing_pubkey: PublicKey) -> Self {
 		let offer = OfferContents {
 			chains: None, metadata: None, amount: None, description,
@@ -123,7 +141,6 @@ impl OfferBuilder {
 	///
 	/// [`InvoiceRequest::verify`]: crate::offers::invoice_request::InvoiceRequest::verify
 	/// [`ExpandedKey`]: crate::ln::inbound_payment::ExpandedKey
-	#[allow(unused)]
 	pub(crate) fn deriving_signing_pubkey(
 		description: String, signing_pubkey: DerivedPubkey
 	) -> Self {

lightning/src/offers/refund.rs

@@ -18,6 +18,8 @@
 //! [`InvoiceRequest`]: crate::offers::invoice_request::InvoiceRequest
 //! [`Offer`]: crate::offers::offer::Offer
 //!
+//! # Example
+//!
 //! ```
 //! extern crate bitcoin;
 //! extern crate core;
@@ -70,6 +72,14 @@
 //! # Ok(())
 //! # }
 //! ```
+//!
+//! # Note
+//!
+//! If constructing a [`Refund`] for use with a [`ChannelManager`], use
+//! [`ChannelManager::create_refund_builder`] instead of [`RefundBuilder::new`].
+//!
+//! [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
+//! [`ChannelManager::create_refund_builder`]: crate::ln::channelmanager::ChannelManager::create_refund_builder
 
 use bitcoin::blockdata::constants::ChainHash;
 use bitcoin::network::constants::Network;
@@ -114,6 +124,14 @@ impl RefundBuilder {
 	///
 	/// Additionally, sets the required [`Refund::description`], [`Refund::metadata`], and
 	/// [`Refund::amount_msats`].
+	///
+	/// # Note
+	///
+	/// If constructing a [`Refund`] for use with a [`ChannelManager`], use
+	/// [`ChannelManager::create_refund_builder`] instead of [`RefundBuilder::new`].
+	///
+	/// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
+	/// [`ChannelManager::create_refund_builder`]: crate::ln::channelmanager::ChannelManager::create_refund_builder
 	pub fn new(
 		description: String, metadata: Vec<u8>, payer_id: PublicKey, amount_msats: u64
 	) -> Result<Self, SemanticError> {
@@ -138,7 +156,6 @@ impl RefundBuilder {
 	///
 	/// [`Invoice::verify`]: crate::offers::invoice::Invoice::verify
 	/// [`ExpandedKey`]: crate::ln::inbound_payment::ExpandedKey
-	#[allow(unused)]
 	pub(crate) fn deriving_payer_id(
 		description: String, payer_id: DerivedPubkey, amount_msats: u64
 	) -> Result<Self, SemanticError> {