add support for update_into on CipherContext (#3190)

* add support for update_into on CipherContext

This allows you to provide your own buffer (like recv_into) to improve
performance when repeatedly calling encrypt/decrypt on large payloads.

* another skip_if

* more skip_if complexity

* maybe do this right

* correct number of args

* coverage for the coverage gods

* add a cffi minimum test tox target and travis builder

This tests against macOS so we capture some commoncrypto branches

* extra arg

* need to actually install py35

* fix

* coverage for GCM decrypt in CC

* no longer relevant

* 1.8 now

* pep8

* dramatically simplify

* update docs

* remove unneeded test

* changelog entry

* test improvements

* coverage fix

* add some comments to example

* move the comments to their own line

* fix and move comment

1 files changed, 20 insertions, 0 deletions

diff --git a/tests/hazmat/primitives/test_block.py b/tests/hazmat/primitives/test_block.py
index eb0a2c3b..11a70195 100644
--- a/tests/hazmat/primitives/test_block.py
+++ b/tests/hazmat/primitives/test_block.py
@@ -6,6 +6,8 @@ from __future__ import absolute_import, division, print_function
 
 import binascii
 
+import cffi
+
 import pytest
 
 from cryptography.exceptions import (
@@ -15,6 +17,7 @@ from cryptography.hazmat.backends.interfaces import CipherBackend
 from cryptography.hazmat.primitives.ciphers import (
     Cipher, algorithms, base, modes
 )
+from cryptography.utils import _version_check
 
 from .utils import (
     generate_aead_exception_test, generate_aead_tag_exception_test
@@ -70,6 +73,23 @@ class TestCipherContext(object):
         with pytest.raises(AlreadyFinalized):
             decryptor.finalize()
 
+    @pytest.mark.skipif(
+        not _version_check(cffi.__version__, '1.7'),
+        reason="cffi version too old"
+    )
+    def test_use_update_into_after_finalize(self, backend):
+        cipher = Cipher(
+            algorithms.AES(binascii.unhexlify(b"0" * 32)),
+            modes.CBC(binascii.unhexlify(b"0" * 32)),
+            backend
+        )
+        encryptor = cipher.encryptor()
+        encryptor.update(b"a" * 16)
+        encryptor.finalize()
+        with pytest.raises(AlreadyFinalized):
+            buf = bytearray(31)
+            encryptor.update_into(b"b" * 16, buf)
+
     def test_unaligned_block_encryption(self, backend):
         cipher = Cipher(
             algorithms.AES(binascii.unhexlify(b"0" * 32)),