From ade04d6c0456bb39793e8b3547ac99aeffe8af73 Mon Sep 17 00:00:00 2001
From: Sergey Matveev <stargrave@stargrave.org>
Date: Thu, 25 Nov 2021 21:11:25 +0300
Subject: [PATCH] Fix MGM tag_size usage

---
 news.texi          |  4 ++++
 pygost/__init__.py |  2 +-
 pygost/mgm.py      |  2 +-
 pygost/test_mgm.py | 33 ++++++++++++++++++++++-----------
 4 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/news.texi b/news.texi
index 7e21260..f5752e6 100644
--- a/news.texi
+++ b/news.texi
@@ -3,6 +3,10 @@
 
 @table @strong
 
+@anchor{Release 5.7}
+@item 5.7
+Fixed MGM ignoring of the set tag size.
+
 @anchor{Release 5.6}
 @item 5.6
 Fixed lint errors for previous release.
diff --git a/pygost/__init__.py b/pygost/__init__.py
index 6fc85a3..12d2bad 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__ = "5.6"
+__version__ = "5.7"
diff --git a/pygost/mgm.py b/pygost/mgm.py
index 49c478d..aabce95 100644
--- a/pygost/mgm.py
+++ b/pygost/mgm.py
@@ -58,7 +58,7 @@ class MGM(object):
         """
         if bs not in (8, 16):
             raise ValueError("Only 64/128-bit blocksizes allowed")
-        self.tag_size = bs if tag_size is None else bs
+        self.tag_size = bs if tag_size is None else tag_size
         if self.tag_size < 4 or self.tag_size > bs:
             raise ValueError("Invalid tag_size")
         self.encrypter = encrypter
diff --git a/pygost/test_mgm.py b/pygost/test_mgm.py
index a2c5e5e..c84be2b 100644
--- a/pygost/test_mgm.py
+++ b/pygost/test_mgm.py
@@ -39,7 +39,7 @@ class TestVector(TestCase):
 
 
 class TestSymmetric(TestCase):
-    def _itself(self, mgm, bs):
+    def _itself(self, mgm, bs, tag_size):
         for _ in range(1000):
             nonce = nonce_prepare(urandom(bs))
             ad = urandom(randint(0, 20))
@@ -47,18 +47,29 @@ class TestSymmetric(TestCase):
             if len(ad) + len(pt) == 0:
                 continue
             ct = mgm.seal(nonce, pt, ad)
+            self.assertEqual(len(ct) - tag_size, len(pt))
             self.assertSequenceEqual(mgm.open(nonce, ct, ad), pt)
 
     def test_magma(self):
-        mgm = MGM(
-            GOST3412Magma(urandom(KEYSIZE)).encrypt,
-            GOST3412Magma.blocksize,
-        )
-        self._itself(mgm, GOST3412Magma.blocksize)
+        for tag_size in (
+                GOST3412Magma.blocksize,
+                GOST3412Magma.blocksize - 2,
+        ):
+            mgm = MGM(
+                GOST3412Magma(urandom(KEYSIZE)).encrypt,
+                GOST3412Magma.blocksize,
+                tag_size,
+            )
+            self._itself(mgm, GOST3412Magma.blocksize, tag_size)
 
     def test_kuznechik(self):
-        mgm = MGM(
-            GOST3412Kuznechik(urandom(KEYSIZE)).encrypt,
-            GOST3412Kuznechik.blocksize,
-        )
-        self._itself(mgm, GOST3412Kuznechik.blocksize)
+        for tag_size in (
+                GOST3412Kuznechik.blocksize,
+                GOST3412Kuznechik.blocksize - 2,
+        ):
+            mgm = MGM(
+                GOST3412Kuznechik(urandom(KEYSIZE)).encrypt,
+                GOST3412Kuznechik.blocksize,
+                tag_size,
+            )
+            self._itself(mgm, GOST3412Kuznechik.blocksize, tag_size)
-- 
2.44.0