From: Sergey Matveev <stargrave@stargrave.org>
Date: Sun, 2 Dec 2018 17:17:39 +0000 (+0300)
Subject: 28147-89 CBC key meshing ability
X-Git-Tag: 3.15~12
X-Git-Url: http://www.git.cypherpunks.ru/?p=pygost.git;a=commitdiff_plain;h=dd0805888217ef624c159baad44c2488101d5750

28147-89 CBC key meshing ability
---

diff --git a/NEWS b/NEWS
index e2e8679..2655265 100644
--- a/NEWS
+++ b/NEWS
@@ -1,6 +1,7 @@
 3.13:
     * Ability to explicitly specify used 28147-89 Sbox in
       pygost.wrap.* functions
+    * Ability to use key meshing in 28147-89 CBC mode
 
 3.12:
     * Added mode argument to pygost.gost3410_vko.kek_34102012256,
diff --git a/news.texi b/news.texi
index 7841bef..e87f637 100644
--- a/news.texi
+++ b/news.texi
@@ -8,6 +8,7 @@
     @itemize
     @item Ability to explicitly specify used 28147-89 Sbox in
         @code{pygost.wrap.*} functions
+    @item Ability to use key meshing in 28147-89 CBC mode
     @end itemize
 
 @anchor{Release 3.12}
diff --git a/pygost/gost28147.py b/pygost/gost28147.py
index 585d0e7..78a0e37 100644
--- a/pygost/gost28147.py
+++ b/pygost/gost28147.py
@@ -281,7 +281,7 @@ ecb_encrypt = partial(ecb, action=encrypt)
 ecb_decrypt = partial(ecb, action=decrypt)
 
 
-def cbc_encrypt(key, data, iv=8 * b"\x00", pad=True, sbox=DEFAULT_SBOX):
+def cbc_encrypt(key, data, iv=8 * b"\x00", pad=True, sbox=DEFAULT_SBOX, mesh=False):
     """ CBC encryption mode of operation
 
     :param bytes key: encryption key
@@ -291,6 +291,7 @@ def cbc_encrypt(key, data, iv=8 * b"\x00", pad=True, sbox=DEFAULT_SBOX):
     :type bool pad: perform ISO/IEC 7816-4 padding
     :param sbox: S-box parameters to use
     :type sbox: str, SBOXES'es key
+    :param bool mesh: enable key meshing
     :returns: ciphertext
     :rtype: bytes
 
@@ -307,13 +308,15 @@ def cbc_encrypt(key, data, iv=8 * b"\x00", pad=True, sbox=DEFAULT_SBOX):
         raise ValueError("Data is not blocksize aligned")
     ciphertext = [iv]
     for i in xrange(0, len(data), BLOCKSIZE):
+        if mesh and i >= MESH_MAX_DATA and i % MESH_MAX_DATA == 0:
+            key, _ = meshing(key, iv, sbox=sbox)
         ciphertext.append(ns2block(encrypt(sbox, key, block2ns(
             strxor(ciphertext[-1], data[i:i + BLOCKSIZE])
         ))))
     return b"".join(ciphertext)
 
 
-def cbc_decrypt(key, data, pad=True, sbox=DEFAULT_SBOX):
+def cbc_decrypt(key, data, pad=True, sbox=DEFAULT_SBOX, mesh=False):
     """ CBC decryption mode of operation
 
     :param bytes key: encryption key
@@ -321,6 +324,7 @@ def cbc_decrypt(key, data, pad=True, sbox=DEFAULT_SBOX):
     :type bool pad: perform ISO/IEC 7816-4 unpadding after decryption
     :param sbox: S-box parameters to use
     :type sbox: str, SBOXES'es key
+    :param bool mesh: enable key meshing
     :returns: plaintext
     :rtype: bytes
     """
@@ -330,8 +334,15 @@ def cbc_decrypt(key, data, pad=True, sbox=DEFAULT_SBOX):
         raise ValueError("Data is not blocksize aligned")
     if len(data) < 2 * BLOCKSIZE:
         raise ValueError("There is no either data, or IV in ciphertext")
+    iv = data[:BLOCKSIZE]
     plaintext = []
     for i in xrange(BLOCKSIZE, len(data), BLOCKSIZE):
+        if (
+                mesh and
+                (i - BLOCKSIZE) >= MESH_MAX_DATA and
+                (i - BLOCKSIZE) % MESH_MAX_DATA == 0
+        ):
+            key, _ = meshing(key, iv, sbox=sbox)
         plaintext.append(strxor(
             ns2block(decrypt(sbox, key, block2ns(data[i:i + BLOCKSIZE]))),
             data[i - BLOCKSIZE:i],
diff --git a/pygost/test_gost28147.py b/pygost/test_gost28147.py
index f28618d..037b7b8 100644
--- a/pygost/test_gost28147.py
+++ b/pygost/test_gost28147.py
@@ -338,6 +338,13 @@ class CBCTest(TestCase):
         iv = urandom(8)
         cbc_decrypt(key, cbc_encrypt(key, 8 * b"x", iv))
 
+    def test_meshing(self):
+        pt = urandom(MESH_MAX_DATA * 3)
+        key = urandom(32)
+        ct = cbc_encrypt(key, pt)
+        dt = cbc_decrypt(key, ct)
+        self.assertEqual(pt, dt)
+
 
 class CFBMeshingTest(TestCase):
     def setUp(self):