diff --git a/cryptography/bindings/openssl/api.py b/cryptography/bindings/openssl/api.py
index 0707941..720a6ad 100644
--- a/cryptography/bindings/openssl/api.py
+++ b/cryptography/bindings/openssl/api.py
@@ -68,7 +68,7 @@
         ctx = self._ffi.gc(ctx, self._lib.EVP_CIPHER_CTX_cleanup)
         # TODO: compute name using a better algorithm
         ciphername = "{0}-{1}-{2}".format(
-            cipher.name, len(cipher.key) * 8, mode.name
+            cipher.name, cipher.key_size, mode.name
         )
         evp_cipher = self._lib.EVP_get_cipherbyname(ciphername.encode("ascii"))
         if evp_cipher == self._ffi.NULL:
diff --git a/cryptography/primitives/block/ciphers.py b/cryptography/primitives/block/ciphers.py
index 3bd5daa..f40fc2a 100644
--- a/cryptography/primitives/block/ciphers.py
+++ b/cryptography/primitives/block/ciphers.py
@@ -18,3 +18,7 @@
     def __init__(self, key):
         super(AES, self).__init__()
         self.key = key
+
+    @property
+    def key_size(self):
+        return len(self.key) * 8
diff --git a/tests/primitives/test_ciphers.py b/tests/primitives/test_ciphers.py
new file mode 100644
index 0000000..31b4275
--- /dev/null
+++ b/tests/primitives/test_ciphers.py
@@ -0,0 +1,29 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import binascii
+
+import pytest
+
+from cryptography.primitives.block.ciphers import AES
+
+
+class TestAES(object):
+    @pytest.mark.parametrize(("key", "keysize"), [
+        (b"0" * 32, 128),
+        (b"0" * 48, 192),
+        (b"0" * 64, 256),
+    ])
+    def test_key_size(self, key, keysize):
+        cipher = AES(binascii.unhexlify(key))
+        assert cipher.key_size == keysize