f - sha256::Hash instead of tweak for signing key

Author: jkczyz

lightning/src/ln/inbound_payment.rs

@@ -13,9 +13,8 @@ use alloc::string::ToString;
 use bitcoin::hashes::{Hash, HashEngine};
 use bitcoin::hashes::cmp::fixed_time_eq;
 use bitcoin::hashes::hmac::{Hmac, HmacEngine};
-use bitcoin::hashes::sha256::Hash as Sha256;
+use bitcoin::hashes::sha256::{Hash as Sha256, self};
 use bitcoin::secp256k1::{PublicKey, Secp256k1, SecretKey};
-use bitcoin::secp256k1::scalar::Scalar;
 use crate::chain::keysinterface::{KeyMaterial, EntropySource};
 use crate::ln::{PaymentHash, PaymentPreimage, PaymentSecret};
 use crate::ln::msgs;
@@ -79,15 +78,13 @@ impl ExpandedKey {
 	///
 	/// [`Offer::signing_pubkey`]: crate::offers::offer::Offer::signing_pubkey
 	pub(crate) fn signing_pubkey_for_offer(&self, nonce: Nonce) -> PublicKey {
+		let mut engine = sha256::Hash::engine();
+		engine.input(&self.ldk_pmt_hash_key);
+		engine.input(&nonce.0);
+
+		let hash = sha256::Hash::from_engine(engine);
 		let secp_ctx = Secp256k1::new();
-		let mut tweak = [0; 32];
-		tweak[..Nonce::LENGTH].copy_from_slice(&nonce.0);
-
-		SecretKey::from_slice(&self.ldk_pmt_hash_key)
-			.unwrap()
-			.mul_tweak(&Scalar::from_be_bytes(tweak).unwrap())
-			.unwrap()
-			.public_key(&secp_ctx)
+		SecretKey::from_slice(&hash).unwrap().public_key(&secp_ctx)
 	}
 }