lnonion: factor out next_blinding_from_shared_secret

diff --git a/electrum/lnonion.py b/electrum/lnonion.py
index 25d2686..e3deb45 100644
--- a/electrum/lnonion.py
+++ b/electrum/lnonion.py
@@ -204,6 +204,14 @@ def blinding_privkey(privkey: bytes, blinding: bytes) -> bytes:
     return our_privkey
 
 
+def next_blinding_from_shared_secret(pubkey: bytes, shared_secret: bytes) -> bytes:
+    # E_i+1=SHA256(E_i||ss_i) * E_i
+    blinding_factor = sha256(pubkey + shared_secret)
+    blinding_factor_int = int.from_bytes(blinding_factor, byteorder="big")
+    next_public_key_int = ecc.ECPubkey(pubkey) * blinding_factor_int
+    return next_public_key_int.get_public_key_bytes()
+
+
 def new_onion_packet(
     payment_path_pubkeys: Sequence[bytes],
     session_key: bytes,
@@ -454,10 +462,7 @@ def process_onion_packet(
         trampoline_onion_packet = trampoline_onion_packet['trampoline_onion_packet']
         trampoline_onion_packet = OnionPacket.from_bytes(trampoline_onion_packet)
     # calc next ephemeral key
-    blinding_factor = sha256(onion_packet.public_key + shared_secret)
-    blinding_factor_int = int.from_bytes(blinding_factor, byteorder="big")
-    next_public_key_int = ecc.ECPubkey(onion_packet.public_key) * blinding_factor_int
-    next_public_key = next_public_key_int.get_public_key_bytes()
+    next_public_key = next_blinding_from_shared_secret(onion_packet.public_key, shared_secret)
     next_onion_packet = OnionPacket(
         public_key=next_public_key,
         hops_data=next_hops_data_fd.read(data_size),
diff --git a/electrum/onion_message.py b/electrum/onion_message.py
index 85b69f2..c291f99 100644
--- a/electrum/onion_message.py
+++ b/electrum/onion_message.py
@@ -39,7 +39,8 @@ from electrum.crypto import sha256, get_ecdh
 from electrum.lnmsg import OnionWireSerializer
 from electrum.lnonion import (get_bolt04_onion_key, OnionPacket, process_onion_packet, blinding_privkey,
                               OnionHopsDataSingle, decrypt_onionmsg_data_tlv, encrypt_onionmsg_data_tlv,
-                              get_shared_secrets_along_route, new_onion_packet, encrypt_hops_recipient_data)
+                              get_shared_secrets_along_route, new_onion_packet, encrypt_hops_recipient_data,
+                              next_blinding_from_shared_secret)
 from electrum.lnutil import LnFeatures, MIN_FINAL_CLTV_DELTA_ACCEPTED, MAXIMUM_REMOTE_TO_SELF_DELAY_ACCEPTED
 from electrum.util import OldTaskGroup, log_exceptions, random_shuffled_copy
 
@@ -297,11 +298,7 @@ def send_onion_message_to(
                 if next_path_key_override:
                     next_path_key = next_path_key_override.get('path_key')
                 else:
-                    # E_i+1=SHA256(E_i||ss_i) * E_i
-                    blinding_factor = sha256(our_blinding + shared_secret)
-                    blinding_factor_int = int.from_bytes(blinding_factor, byteorder="big")
-                    next_public_key_int = ecc.ECPubkey(our_blinding) * blinding_factor_int
-                    next_path_key = next_public_key_int.get_public_key_bytes()
+                    next_path_key = next_blinding_from_shared_secret(our_blinding, shared_secret)
 
                 path_key = next_path_key