diff --git a/lib/tfw/crypto.py b/lib/tfw/crypto.py
index be2d54e..a40cf5c 100644
--- a/lib/tfw/crypto.py
+++ b/lib/tfw/crypto.py
@@ -19,16 +19,20 @@ def message_checksum(message):
 
 
 def sign_message(key, message):
-    signature = HMAC(key, message_bytes(message)).signature
+    signature = message_signature(key, message)
     message['signature'] = b64encode(signature).decode()
 
 
+def message_signature(key, message):
+    return HMAC(key, message_bytes(message)).signature
+
+
 def verify_message(key, message):
     message = deepcopy(message)
     try:
         signature_b64 = message.pop('signature')
         signature = b64decode(signature_b64)
-        actual_signature = HMAC(key, message_bytes(message)).signature
+        actual_signature = message_signature(key, message)
         return signature == actual_signature
     except KeyError:
         return False