fix: use 'int.to_bytes' and 'int.from_bytes' for py3 (#904)

Author: arithmetic1728

google/auth/_helpers.py

@@ -17,6 +17,7 @@
 import base64
 import calendar
 import datetime
+import sys
 
 import six
 from six.moves import urllib
@@ -233,3 +234,12 @@ def unpadded_urlsafe_b64encode(value):
         Union[str|bytes]: The encoded value
     """
     return base64.urlsafe_b64encode(value).rstrip(b"=")
+
+
+def is_python_3():
+    """Check if the Python interpreter is Python 2 or 3.
+
+    Returns:
+        bool: True if the Python interpreter is Python 3 and False otherwise.
+    """
+    return sys.version_info > (3, 0)

google/auth/crypt/es256.py

@@ -53,8 +53,16 @@ def verify(self, message, signature):
         sig_bytes = _helpers.to_bytes(signature)
         if len(sig_bytes) != 64:
             return False
-        r = utils.int_from_bytes(sig_bytes[:32], byteorder="big")
-        s = utils.int_from_bytes(sig_bytes[32:], byteorder="big")
+        r = (
+            int.from_bytes(sig_bytes[:32], byteorder="big")
+            if _helpers.is_python_3()
+            else utils.int_from_bytes(sig_bytes[:32], byteorder="big")
+        )
+        s = (
+            int.from_bytes(sig_bytes[32:], byteorder="big")
+            if _helpers.is_python_3()
+            else utils.int_from_bytes(sig_bytes[32:], byteorder="big")
+        )
         asn1_sig = encode_dss_signature(r, s)
 
         message = _helpers.to_bytes(message)
@@ -121,7 +129,11 @@ def sign(self, message):
 
         # Convert ASN1 encoded signature to (r||s) raw signature.
         (r, s) = decode_dss_signature(asn1_signature)
-        return utils.int_to_bytes(r, 32) + utils.int_to_bytes(s, 32)
+        return (
+            (r.to_bytes(32, byteorder="big") + s.to_bytes(32, byteorder="big"))
+            if _helpers.is_python_3()
+            else (utils.int_to_bytes(r, 32) + utils.int_to_bytes(s, 32))
+        )
 
     @classmethod
     def from_string(cls, key, key_id=None):