docs: add coment in test.tool

re6st/tests/tools.py

@@ -104,10 +104,17 @@ def create_ca_file(pkey_file, cert_file, serial=0x120010db80042):
 
 
 def prefix2cn(prefix):
+    """ prefix binary format
+        cn, common name, serial/length
+    """
     return "%u/%u" % (int(prefix, 2), len(prefix))
 
-def serial2prefix(serial):
-    return bin(serial)[2:].rjust(16, '0')
+def serial2prefix(serial, length = 16):
+    """serial, decimal of prefix
+       prefix binary
+    """
+    # return bin(serial)[2:].rjust(16, '0')
+    return "{:0{}b}".format(serial, length)
 
 # pkey: private key
 def decrypt(pkey, incontent):