blip42: Add contact secret and payer offer support to invoice requests

Implements BLIP-42 contact management for the sender side:

- Add contact_secret and payer_offer fields to InvoiceRequestContents
- Add builder methods: contact_secrets(), payer_offer()
- Add accessor methods: contact_secret(), payer_offer()
- Add OptionalOfferPaymentParams fields for contact_secrects and payer_offer
- Update ChannelManager::pay_for_offer to pass contact information
- Add create_compact_offer_builder to OffersMessageFlow for small payer offers
- Update tests to include new InvoiceRequestFields

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

Author: vincenzopalazzo

lightning/src/ln/channelmanager.rs

@@ -92,6 +92,7 @@ use crate::ln::outbound_payment::{
 };
 use crate::ln::types::ChannelId;
 use crate::offers::async_receive_offer_cache::AsyncReceiveOfferCache;
+use crate::offers::contacts::ContactSecrets;
 use crate::offers::flow::{HeldHtlcReplyPath, InvreqResponseInstructions, OffersMessageFlow};
 use crate::offers::invoice::{Bolt12Invoice, UnsignedBolt12Invoice};
 use crate::offers::invoice_error::InvoiceError;
@@ -728,6 +729,34 @@ pub struct OptionalOfferPaymentParams {
 	/// will ultimately fail once all pending paths have failed (generating an
 	/// [`Event::PaymentFailed`]).
 	pub retry_strategy: Retry,
+	/// Contact secrets to include in the invoice request for BLIP-42 contact management.
+	/// If provided, these secrets will be used to establish a contact relationship with the recipient.
+	pub contact_secrects: Option<ContactSecrets>,
+	/// A custom payer offer to include in the invoice request for BLIP-42 contact management.
+	///
+	/// If provided, this offer will be included in the invoice request, allowing the recipient to
+	/// contact you back. If `None`, **no payer offer will be included** in the invoice request.
+	///
+	/// You can create custom offers using [`OffersMessageFlow::create_compact_offer_builder`]:
+	/// - Pass `None` for no blinded path (smallest size, ~70 bytes)
+	/// - Pass `Some(intro_node_id)` for a single blinded path (~200 bytes)
+	///
+	/// # Example
+	/// ```rust,ignore
+	/// // Include a compact offer with a single blinded path
+	/// let payer_offer = flow.create_compact_offer_builder(
+	///     &entropy_source,
+	///     Some(trusted_peer_pubkey)
+	/// )?.build()?;
+	///
+	/// let params = OptionalOfferPaymentParams {
+	///     payer_offer: Some(payer_offer),
+	///     ..Default::default()
+	/// };
+	/// ```
+	///
+	/// [`OffersMessageFlow::create_compact_offer_builder`]: crate::offers::flow::OffersMessageFlow::create_compact_offer_builder
+	pub payer_offer: Option<Offer>,
 }
 
 impl Default for OptionalOfferPaymentParams {
@@ -739,6 +768,8 @@ impl Default for OptionalOfferPaymentParams {
 			retry_strategy: Retry::Timeout(core::time::Duration::from_secs(2)),
 			#[cfg(not(feature = "std"))]
 			retry_strategy: Retry::Attempts(3),
+			contact_secrects: None,
+			payer_offer: None,
 		}
 	}
 }
@@ -12944,6 +12975,8 @@ where
 			payment_id,
 			None,
 			create_pending_payment_fn,
+			optional_params.contact_secrects,
+			optional_params.payer_offer,
 		)
 	}
 
@@ -12973,6 +13006,8 @@ where
 			payment_id,
 			Some(offer.hrn),
 			create_pending_payment_fn,
+			optional_params.contact_secrects,
+			optional_params.payer_offer,
 		)
 	}
 
@@ -13015,6 +13050,8 @@ where
 			payment_id,
 			None,
 			create_pending_payment_fn,
+			optional_params.contact_secrects,
+			optional_params.payer_offer,
 		)
 	}
 
@@ -13023,6 +13060,7 @@ where
 		&self, offer: &Offer, quantity: Option<u64>, amount_msats: Option<u64>,
 		payer_note: Option<String>, payment_id: PaymentId,
 		human_readable_name: Option<HumanReadableName>, create_pending_payment: CPP,
+		contacts: Option<ContactSecrets>, payer_offer: Option<Offer>,
 	) -> Result<(), Bolt12SemanticError> {
 		let entropy = &*self.entropy_source;
 		let nonce = Nonce::from_entropy_source(entropy);
@@ -13048,6 +13086,20 @@ where
 			Some(hrn) => builder.sourced_from_human_readable_name(hrn),
 		};
 
+		let builder = if let Some(secrets) = contacts.as_ref() {
+			builder.contact_secrets(secrets.clone())
+		} else {
+			builder
+		};
+
+		// Add payer offer only if provided by the user.
+		// If the user explicitly wants to include an offer, they should provide it via payer_offer parameter.
+		let builder = if let Some(offer) = payer_offer {
+			builder.payer_offer(&offer)
+		} else {
+			builder
+		};
+
 		let invoice_request = builder.build_and_sign()?;
 		let _persistence_guard = PersistenceNotifierGuard::notify_on_drop(self);
 
@@ -15649,7 +15701,7 @@ where
 								self.pending_outbound_payments
 									.received_offer(payment_id, Some(retryable_invoice_request))
 									.map_err(|_| Bolt12SemanticError::DuplicatePaymentId)
-						});
+						}, None, None);
 					if offer_pay_res.is_err() {
 						// The offer we tried to pay is the canonical current offer for the name we
 						// wanted to pay. If we can't pay it, there's no way to recover so fail the

lightning/src/ln/offers_tests.rs

@@ -683,6 +683,8 @@ fn creates_and_pays_for_offer_using_two_hop_blinded_path() {
 			quantity: None,
 			payer_note_truncated: None,
 			human_readable_name: None,
+			contact_secret: None,
+			payer_offer: None,
 		},
 	});
 	assert_eq!(invoice_request.amount_msats(), Some(10_000_000));
@@ -841,6 +843,8 @@ fn creates_and_pays_for_offer_using_one_hop_blinded_path() {
 			quantity: None,
 			payer_note_truncated: None,
 			human_readable_name: None,
+			contact_secret: None,
+			payer_offer: None,
 		},
 	});
 	assert_eq!(invoice_request.amount_msats(), Some(10_000_000));
@@ -962,6 +966,8 @@ fn pays_for_offer_without_blinded_paths() {
 			quantity: None,
 			payer_note_truncated: None,
 			human_readable_name: None,
+			contact_secret: None,
+			payer_offer: None,
 		},
 	});
 
@@ -1229,6 +1235,8 @@ fn creates_and_pays_for_offer_with_retry() {
 			quantity: None,
 			payer_note_truncated: None,
 			human_readable_name: None,
+			contact_secret: None,
+			payer_offer: None,
 		},
 	});
 	assert_eq!(invoice_request.amount_msats(), Some(10_000_000));
@@ -1294,6 +1302,8 @@ fn pays_bolt12_invoice_asynchronously() {
 			quantity: None,
 			payer_note_truncated: None,
 			human_readable_name: None,
+			contact_secret: None,
+			payer_offer: None,
 		},
 	});
 
@@ -1391,6 +1401,8 @@ fn creates_offer_with_blinded_path_using_unannounced_introduction_node() {
 			quantity: None,
 			payer_note_truncated: None,
 			human_readable_name: None,
+			contact_secret: None,
+			payer_offer: None,
 		},
 	});
 	assert_ne!(invoice_request.payer_signing_pubkey(), bob_id);

lightning/src/offers/flow.rs

@@ -574,6 +574,46 @@ where
 		Ok((builder.into(), nonce))
 	}
 
+	/// Creates a minimal [`OfferBuilder`] with derived metadata and an optional blinded path.
+	///
+	/// If `intro_node_id` is `None`, creates an offer with no blinded paths (~70 bytes) suitable
+	/// for scenarios like BLIP-42 where the payer intentionally shares their contact info.
+	///
+	/// If `intro_node_id` is `Some`, creates an offer with a single blinded path (~200 bytes)
+	/// providing privacy/routability for unannounced nodes. The intro node must be a public
+	/// peer (routable via gossip) with an outbound channel.
+	///
+	/// # Privacy
+	///
+	/// - `None`: Exposes the derived signing pubkey directly without blinded path privacy
+	/// - `Some`: Intro node learns payer identity (choose trusted/routable peer)
+	///
+	/// This is not exported to bindings users as builder patterns don't map outside of move semantics.
+	pub fn create_compact_offer_builder<ES: Deref>(
+		&self, entropy_source: ES, intro_node_id: Option<PublicKey>,
+	) -> Result<OfferBuilder<'_, DerivedMetadata, secp256k1::All>, Bolt12SemanticError>
+	where
+		ES::Target: EntropySource,
+	{
+		match intro_node_id {
+			None => {
+				// Use the internal builder but don't add any paths
+				self.create_offer_builder_intern(
+					&*entropy_source,
+					|_, _, _| Ok(core::iter::empty()),
+				)
+				.map(|(builder, _)| builder)
+			},
+			Some(node_id) => {
+				// Delegate to create_offer_builder with a single-peer list to reuse the router logic
+				self.create_offer_builder(
+					entropy_source,
+					vec![MessageForwardNode { node_id, short_channel_id: None }],
+				)
+			},
+		}
+	}
+
 	/// Creates an [`OfferBuilder`] such that the [`Offer`] it builds is recognized by the
 	/// [`OffersMessageFlow`], and any corresponding [`InvoiceRequest`] can be verified using
 	/// [`Self::verify_invoice_request`]. The offer will expire at `absolute_expiry` if `Some`,