Add support for AES XTS (#3900)

* Add support for AES XTS

We drop the non-byte aligned test vectors because according to NIST
http://csrc.nist.gov/groups/STM/cavp/documents/aes/XTSVS.pdf
"An implementation may support a data unit length that is not a
multiple of 8 bits." OpenSSL does not support this, so we can't
use those test vectors.

* fix docs and pep8

* docs fix

* the spellchecker is so frustrating

* add note about AES 192 for XTS (it's not supported)

* docs work

* enforce key length on ECB mode in AES as well (thanks XTS)

* a few more words about why we exclude some test vectors for XTS

1 files changed, 24 insertions, 0 deletions

diff --git a/tests/hazmat/primitives/test_ciphers.py b/tests/hazmat/primitives/test_ciphers.py
index f1718c07..2f58c9fc 100644
--- a/tests/hazmat/primitives/test_ciphers.py
+++ b/tests/hazmat/primitives/test_ciphers.py
@@ -37,6 +37,30 @@ class TestAES(object):
             AES(binascii.unhexlify(b"0" * 12))
 
 
+class TestAESXTS(object):
+    @pytest.mark.requires_backend_interface(interface=CipherBackend)
+    @pytest.mark.parametrize(
+        "mode",
+        (modes.CBC, modes.CTR, modes.CFB, modes.CFB8, modes.OFB)
+    )
+    def test_invalid_key_size_with_mode(self, mode, backend):
+        with pytest.raises(ValueError):
+            ciphers.Cipher(AES(b"0" * 64), mode(b"0" * 16), backend)
+
+    def test_xts_tweak_not_bytes(self):
+        with pytest.raises(TypeError):
+            modes.XTS(32)
+
+    def test_xts_tweak_too_small(self):
+        with pytest.raises(ValueError):
+            modes.XTS(b"0")
+
+    @pytest.mark.requires_backend_interface(interface=CipherBackend)
+    def test_xts_wrong_key_size(self, backend):
+        with pytest.raises(ValueError):
+            ciphers.Cipher(AES(b"0" * 16), modes.XTS(b"0" * 16), backend)
+
+
 class TestCamellia(object):
     @pytest.mark.parametrize(("key", "keysize"), [
         (b"0" * 32, 128),