add test to verify AES CTR is always available in 0.9.8+, comment updates

2 files changed, 12 insertions, 5 deletions

diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index d9165492..c66f0452 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -132,14 +132,14 @@ class Backend(object):
         return _HashContext(self, algorithm)
 
     def cipher_supported(self, cipher, mode):
-        if self._cipher_supported(cipher, mode):
+        if self._evp_cipher_supported(cipher, mode):
             return True
         elif isinstance(mode, CTR) and isinstance(cipher, AES):
             return True
         else:
             return False
 
-    def _cipher_supported(self, cipher, mode):
+    def _evp_cipher_supported(self, cipher, mode):
         try:
             adapter = self._cipher_registry[type(cipher), type(mode)]
         except KeyError:
@@ -856,10 +856,12 @@ class _CipherContext(object):
         return self._tag
 
 
-# This is needed to provide support for AES CTR mode in OpenSSL 0.9.8. It can
-# be removed when we drop 0.9.8 support (RHEL5 extended life ends 2020).
 @utils.register_interface(interfaces.CipherContext)
 class _AESCTRCipherContext(object):
+    """
+    This is needed to provide support for AES CTR mode in OpenSSL 0.9.8. It can
+    be removed when we drop 0.9.8 support (RHEL5 extended life ends 2020).
+    """
     def __init__(self, backend, cipher, mode):
         self._backend = backend
 
diff --git a/tests/hazmat/backends/test_openssl.py b/tests/hazmat/backends/test_openssl.py
index 154af00f..fb2ca19f 100644
--- a/tests/hazmat/backends/test_openssl.py
+++ b/tests/hazmat/backends/test_openssl.py
@@ -22,7 +22,7 @@ from cryptography.hazmat.primitives import hashes, interfaces
 from cryptography.hazmat.primitives.asymmetric import dsa, padding, rsa
 from cryptography.hazmat.primitives.ciphers import Cipher
 from cryptography.hazmat.primitives.ciphers.algorithms import AES
-from cryptography.hazmat.primitives.ciphers.modes import CBC
+from cryptography.hazmat.primitives.ciphers.modes import CBC, CTR
 from cryptography.hazmat.primitives.interfaces import BlockCipherAlgorithm
 
 from ...utils import raises_unsupported_algorithm
@@ -64,6 +64,11 @@ class TestOpenSSL(object):
     def test_supports_cipher(self):
         assert backend.cipher_supported(None, None) is False
 
+    def test_aes_ctr_always_available(self):
+        # AES CTR should always be available in both 0.9.8 and 1.0.0+
+        assert backend.cipher_supported(AES(b"\x00" * 16),
+                                        CTR(b"\x00" * 16)) is True
+
     def test_register_duplicate_cipher_adapter(self):
         with pytest.raises(ValueError):
             backend.register_cipher_adapter(AES, CBC, None)