onion_utils: add next_hop_packet_pubkey method

To get the next hop's packet's pubkey. This will be used to DRY onion message
forwarding in the upcoming Onion Messages PR lightningdevkit#1503

lightning/src/ln/channelmanager.rs

@@ -24,7 +24,7 @@ use bitcoin::blockdata::transaction::Transaction;
 use bitcoin::blockdata::constants::genesis_block;
 use bitcoin::network::constants::Network;
 
-use bitcoin::hashes::{Hash, HashEngine};
+use bitcoin::hashes::Hash;
 use bitcoin::hashes::sha256::Hash as Sha256;
 use bitcoin::hashes::sha256d::Hash as Sha256dHash;
 use bitcoin::hash_types::{BlockHash, Txid};
@@ -2164,22 +2164,10 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 				}
 			},
 			onion_utils::Hop::Forward { next_hop_data, next_hop_hmac, new_packet_bytes } => {
-				let mut new_pubkey = msg.onion_routing_packet.public_key.unwrap();
-
-				let blinding_factor = {
-					let mut sha = Sha256::engine();
-					sha.input(&new_pubkey.serialize()[..]);
-					sha.input(&shared_secret);
-					Sha256::from_engine(sha).into_inner()
-				};
-
-				let public_key = if let Err(e) = new_pubkey.mul_assign(&self.secp_ctx, &blinding_factor[..]) {
-					Err(e)
-				} else { Ok(new_pubkey) };
-
+				let new_pubkey = msg.onion_routing_packet.public_key.unwrap();
 				let outgoing_packet = msgs::OnionPacket {
 					version: 0,
-					public_key,
+					public_key: onion_utils::next_hop_packet_pubkey(&self.secp_ctx, new_pubkey, &shared_secret),
 					hop_data: new_packet_bytes,
 					hmac: next_hop_hmac.clone(),
 				};

lightning/src/ln/onion_utils.rs

@@ -74,6 +74,21 @@ pub(super) fn gen_ammag_from_shared_secret(shared_secret: &[u8]) -> [u8; 32] {
 	Hmac::from_engine(hmac).into_inner()
 }
 
+pub(super) fn next_hop_packet_pubkey<T: secp256k1::Signing + secp256k1::Verification>(secp_ctx: &Secp256k1<T>, mut packet_pubkey: PublicKey, packet_shared_secret: &[u8; 32]) -> Result<PublicKey, secp256k1::Error> {
+	let blinding_factor = {
+		let mut sha = Sha256::engine();
+		sha.input(&packet_pubkey.serialize()[..]);
+		sha.input(packet_shared_secret);
+		Sha256::from_engine(sha).into_inner()
+	};
+
+	if let Err(e) = packet_pubkey.mul_assign(secp_ctx, &blinding_factor[..]) {
+		Err(e)
+	} else {
+		Ok(packet_pubkey)
+	}
+}
+
 // can only fail if an intermediary hop has an invalid public key or session_priv is invalid
 #[inline]
 pub(super) fn construct_onion_keys_callback<T: secp256k1::Signing, FType: FnMut(SharedSecret, [u8; 32], PublicKey, &RouteHop, usize)> (secp_ctx: &Secp256k1<T>, path: &Vec<RouteHop>, session_priv: &SecretKey, mut callback: FType) -> Result<(), secp256k1::Error> {