From 9ab3b1d03f734f4aae28364f7a6c7c7c223b30a9 Mon Sep 17 00:00:00 2001
From: Sergey Matveev <stargrave@stargrave.org>
Date: Tue, 26 Jan 2021 14:45:36 +0300
Subject: [PATCH] gost3410 private key % Q

---
 README                           |  2 ++
 news.texi                        |  5 +++++
 pygost/__init__.py               |  2 +-
 pygost/gost3410.py               | 16 ++++++++++++++++
 pygost/stubs/pygost/gost3410.pyi |  3 +++
 pygost/test_gost3410.py          |  8 +++++---
 6 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/README b/README
index 5795233..acabfb1 100644
--- a/README
+++ b/README
@@ -39,7 +39,9 @@ Example 34.10-2012 keypair generation, signing and verifying:
     >>> from os import urandom
     >>> prv_raw = urandom(64)
     >>> from pygost.gost3410 import prv_unmarshal
+    >>> from pygost.gost3410 import prv_marshal
     >>> prv = prv_unmarshal(prv_raw)
+    >>> prv_raw = prv_marshal(curve, prv)
     >>> from pygost.gost3410 import public_key
     >>> pub = public_key(curve, prv)
     >>> from pygost.gost3410 import pub_marshal
diff --git a/news.texi b/news.texi
index 564df7b..324c05c 100644
--- a/news.texi
+++ b/news.texi
@@ -3,6 +3,11 @@
 
 @table @strong
 
+@anchor{Release 5.4}
+@item 5.4
+@item @code{gost3410.prv_marshal} helper can make private keys that are
+in curve's Q field, for better compatibility with some implementations.
+
 @anchor{Release 5.3}
 @item 5.3
     @itemize
diff --git a/pygost/__init__.py b/pygost/__init__.py
index f1f6ad6..21fbf59 100644
--- a/pygost/__init__.py
+++ b/pygost/__init__.py
@@ -3,4 +3,4 @@
 PyGOST is free software: see the file COPYING for copying conditions.
 """
 
-__version__ = "5.3"
+__version__ = "5.4"
diff --git a/pygost/gost3410.py b/pygost/gost3410.py
index 88cbfbe..58a20e0 100644
--- a/pygost/gost3410.py
+++ b/pygost/gost3410.py
@@ -324,10 +324,26 @@ def prv_unmarshal(prv):
 
     :param bytes prv: serialized private key
     :rtype: long
+
+    It is advisable to use :py:func:`pygost.gost3410.prv_marshal` to
+    assure that key i in curve's Q field for better compatibility with
+    some implementations.
     """
     return bytes2long(prv[::-1])
 
 
+def prv_marshal(curve, prv):
+    """Marshal little-endian private key
+
+    :param GOST3410Curve curve: curve to use
+    :param long prv: serialized private key
+    :rtype: bytes
+
+    Key is in curve's Q field.
+    """
+    return long2bytes(prv % curve.q, point_size(prv))[::-1]
+
+
 def pub_marshal(pub):
     """Marshal public key
 
diff --git a/pygost/stubs/pygost/gost3410.pyi b/pygost/stubs/pygost/gost3410.pyi
index 4be9bbb..345a3e6 100644
--- a/pygost/stubs/pygost/gost3410.pyi
+++ b/pygost/stubs/pygost/gost3410.pyi
@@ -55,6 +55,9 @@ def verify(curve: GOST3410Curve, pub: PublicKey, digest: bytes, signature: bytes
 def prv_unmarshal(prv: bytes) -> int: ...
 
 
+def prv_marshal(curve: GOST3410Curve, prv: int) -> bytes: ...
+
+
 def pub_marshal(pub: PublicKey) -> bytes: ...
 
 
diff --git a/pygost/test_gost3410.py b/pygost/test_gost3410.py
index 51624db..677968d 100644
--- a/pygost/test_gost3410.py
+++ b/pygost/test_gost3410.py
@@ -19,6 +19,8 @@ from unittest import TestCase
 
 from pygost.gost3410 import CURVES
 from pygost.gost3410 import GOST3410Curve
+from pygost.gost3410 import prv_marshal
+from pygost.gost3410 import prv_unmarshal
 from pygost.gost3410 import public_key
 from pygost.gost3410 import sign
 from pygost.gost3410 import uv2xy
@@ -81,8 +83,8 @@ class Test341001(TestCase):
 
     def test_sequence(self):
         c = CURVES["id-GostR3410-2001-TestParamSet"]
-        prv = bytes2long(urandom(32))
-        pubX, pubY = public_key(c, prv)
+        prv = prv_unmarshal(urandom(32))
+        pubX, pubY = public_key(c, prv_unmarshal(prv_marshal(c, prv)))
         for _ in range(20):
             digest = urandom(32)
             s = sign(c, prv, digest)
@@ -255,7 +257,7 @@ class Test34102012(TestCase):
     def test_sequence(self):
         c = CURVES["id-tc26-gost-3410-12-512-paramSetA"]
         prv = bytes2long(urandom(64))
-        pubX, pubY = public_key(c, prv)
+        pubX, pubY = public_key(c, prv_unmarshal(prv_marshal(c, prv)))
         for _ in range(20):
             digest = urandom(64)
             s = sign(c, prv, digest)
-- 
2.44.0