diff --git a/daemon/peer.c b/daemon/peer.c
index 941246f58..c72073f57 100644
--- a/daemon/peer.c
+++ b/daemon/peer.c
@@ -911,11 +911,18 @@ static void their_htlc_added(struct peer *peer, struct htlc *htlc,
 
 	//FIXME: dirty trick to retrieve unexported state
 	memcpy(&pk, peer->dstate->secret, sizeof(pk));
+
 	packet = parse_onionpacket(peer,
 				   htlc->routing, tal_count(htlc->routing));
-	if (packet)
-		step = process_onionpacket(packet, packet, &pk, htlc->rhash.u.u8,
-					   sizeof(htlc->rhash));
+	if (packet) {
+		u8 shared_secret[32];
+
+		if (onion_shared_secret(shared_secret, packet, &pk))
+			step = process_onionpacket(packet, packet,
+						   shared_secret,
+						   htlc->rhash.u.u8,
+						   sizeof(htlc->rhash));
+	}
 
 	if (!step) {
 		log_unusual(peer->log, "Bad onion, failing HTLC %"PRIu64,
diff --git a/daemon/sphinx.c b/daemon/sphinx.c
index 216429702..3d1485659 100644
--- a/daemon/sphinx.c
+++ b/daemon/sphinx.c
@@ -247,6 +247,15 @@ static bool create_shared_secret(
 	return true;
 }
 
+bool onion_shared_secret(
+	u8 *secret,
+	const struct onionpacket *packet,
+	const struct privkey *privkey)
+{
+	return create_shared_secret(secret, &packet->ephemeralkey,
+				    privkey->secret);
+}
+
 void pubkey_hash160(
 	u8 *dst,
 	const struct pubkey *pubkey)
@@ -267,7 +276,8 @@ void pubkey_hash160(
 	memcpy(dst, r.u.u8, sizeof(r));
 }
 
-static void generate_key_set(u8 secret[SHARED_SECRET_SIZE], struct keyset *keys)
+static void generate_key_set(const u8 secret[SHARED_SECRET_SIZE],
+			     struct keyset *keys)
 {
 	generate_key(keys->rho, "rho", 3, secret);
 	generate_key(keys->pi, "pi", 2, secret);
@@ -417,13 +427,12 @@ struct onionpacket *create_onionpacket(
 struct route_step *process_onionpacket(
 	const tal_t *ctx,
 	const struct onionpacket *msg,
-	struct privkey *hop_privkey,
+	const u8 *shared_secret,
 	const u8 *assocdata,
 	const size_t assocdatalen
 	)
 {
 	struct route_step *step = talz(ctx, struct route_step);
-	u8 secret[SHARED_SECRET_SIZE];
 	u8 hmac[20];
 	struct keyset keys;
 	u8 paddedhoppayloads[TOTAL_HOP_PAYLOAD_SIZE + HOP_PAYLOAD_SIZE];
@@ -434,8 +443,7 @@ struct route_step *process_onionpacket(
 
 	step->next = talz(step, struct onionpacket);
 	step->next->version = msg->version;
-	create_shared_secret(secret, &msg->ephemeralkey, hop_privkey->secret);
-	generate_key_set(secret, &keys);
+	generate_key_set(shared_secret, &keys);
 
 	compute_packet_hmac(msg, assocdata, assocdatalen, keys.mu, hmac);
 
@@ -461,7 +469,7 @@ struct route_step *process_onionpacket(
 	memcpy(&step->next->hoppayloads, paddedhoppayloads + HOP_PAYLOAD_SIZE,
 	       TOTAL_HOP_PAYLOAD_SIZE);
 
-	compute_blinding_factor(&msg->ephemeralkey, secret, blind);
+	compute_blinding_factor(&msg->ephemeralkey, shared_secret, blind);
 	if (!blind_group_element(&step->next->ephemeralkey, &msg->ephemeralkey, blind))
 		return tal_free(step);
 	memcpy(&step->next->nexthop, paddedheader, SECURITY_PARAMETER);
diff --git a/daemon/sphinx.h b/daemon/sphinx.h
index 65813f3bf..ab373c079 100644
--- a/daemon/sphinx.h
+++ b/daemon/sphinx.h
@@ -69,13 +69,25 @@ struct onionpacket *create_onionpacket(
 	const size_t assocdatalen
 	);
 
+/**
+ * onion_shared_secret - calculate ECDH shared secret between nodes.
+ *
+ * @secret: the shared secret (32 bytes long)
+ * @pubkey: the public key of the other node
+ * @privkey: the private key of this node (32 bytes long)
+ */
+bool onion_shared_secret(
+	u8 *secret,
+	const struct onionpacket *packet,
+	const struct privkey *privkey);
+
 /**
  * process_onionpacket - process an incoming packet by stripping one
  * onion layer and return the packet for the next hop.
  *
  * @ctx: tal context to allocate from
  * @packet: incoming packet being processed
- * @hop_privkey: the processing node's private key to decrypt the packet
+ * @shared_secret: the result of onion_shared_secret.
  * @hoppayload: the per-hop payload destined for the processing node.
  * @assocdata: associated data to commit to in HMACs
  * @assocdatalen: length of the assocdata
@@ -83,7 +95,7 @@ struct onionpacket *create_onionpacket(
 struct route_step *process_onionpacket(
 	const tal_t * ctx,
 	const struct onionpacket *packet,
-	struct privkey *hop_privkey,
+	const u8 *shared_secret,
 	const u8 *assocdata,
 	const size_t assocdatalen
 	);
diff --git a/test/test_sphinx.c b/test/test_sphinx.c
index 8ae231647..de3016af2 100644
--- a/test/test_sphinx.c
+++ b/test/test_sphinx.c
@@ -78,6 +78,7 @@ int main(int argc, char **argv)
 		u8 serialized[TOTAL_PACKET_SIZE];
 		char hextemp[2 * sizeof(serialized) + 1];
 		memset(hextemp, 0, sizeof(hextemp));
+		u8 shared_secret[32];
 
 		if (argc != 2)
 			opt_usage_exit_fail("Expect a privkey with --decode");
@@ -91,7 +92,10 @@ int main(int argc, char **argv)
 		if (!msg)
 			errx(1, "Error parsing message.");
 
-		step = process_onionpacket(ctx, msg, &seckey, assocdata,
+		if (!onion_shared_secret(shared_secret, msg, &seckey))
+			errx(1, "Error creating shared secret.");
+		
+		step = process_onionpacket(ctx, msg, shared_secret, assocdata,
 					   sizeof(assocdata));
 
 		if (!step->next)