diff --git a/calibre-plugin/libadobe.py b/calibre-plugin/libadobe.py
index 0032aa2..8e264ad 100644
--- a/calibre-plugin/libadobe.py
+++ b/calibre-plugin/libadobe.py
@@ -217,8 +217,8 @@ def makeSerial(random):
             
         sha_out = hashlib.sha1(dataToHash.encode('latin-1')).hexdigest().lower()
     else: 
-        import binascii
-        sha_out = binascii.hexlify(Random.get_random_bytes(20)).lower()
+        # SHA1 of a bunch of random bytes
+        sha_out = hashlib.sha1(Random.get_random_bytes(256)).hexdigest().lower()
 
     return sha_out
 
diff --git a/tests/main.py b/tests/main.py
index f8a88a9..4eee315 100755
--- a/tests/main.py
+++ b/tests/main.py
@@ -77,6 +77,10 @@ class TestAdobe(unittest.TestCase):
         self.assertEqual((len(libadobe.makeSerial(False))), 40, "SHA1 hash for device serial invalid (device-based)")
         self.assertEqual((len(libadobe.makeSerial(True))), 40,  "SHA1 hash for device serial invalid (random)")
         self.assertEqual(libadobe.makeSerial(False), libadobe.makeSerial(False), "Two non-random serials not identical")
+        self.assertNotEqual(libadobe.makeSerial(True), libadobe.makeSerial(True), "Two random serials are identical ...")
+
+        self.assertIsInstance(libadobe.makeSerial(True), str, "Serial must be a string, not bytes!")
+        self.assertIsInstance(libadobe.makeSerial(False), str, "Serial must be a string, not bytes!")
 
     def test_fingerprintGeneration(self): 
         '''Check if fingerprint generation works'''