4 files changed, 119 insertions, 0 deletions
diff --git a/cryptography/hazmat/primitives/constant_time.py b/cryptography/hazmat/primitives/constant_time.py
new file mode 100644
index 00000000..a8351504
--- /dev/null
+++ b/cryptography/hazmat/primitives/constant_time.py
@@ -0,0 +1,53 @@
+# 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.
+
+from __future__ import absolute_import, division, print_function
+
+import cffi
+
+import six
+
+
+_ffi = cffi.FFI()
+_ffi.cdef("""
+bool Cryptography_constant_time_bytes_eq(uint8_t *, size_t, uint8_t *, size_t);
+""")
+_lib = _ffi.verify("""
+#include <stdbool.h>
+
+bool Cryptography_constant_time_bytes_eq(uint8_t *a, size_t len_a, uint8_t *b,
+                                         size_t len_b) {
+    size_t i = 0;
+    uint8_t mismatch = 0;
+    if (len_a != len_b) {
+        return false;
+    }
+    for (i = 0; i < len_a; i++) {
+        mismatch |= a[i] ^ b[i];
+    }
+
+    /* Make sure any bits set are copied to the lowest bit */
+    mismatch |= mismatch >> 4;
+    mismatch |= mismatch >> 2;
+    mismatch |= mismatch >> 1;
+    /* Now check the low bit to see if it's set */
+    return (mismatch & 1) == 0;
+}
+""")
+
+
+def bytes_eq(a, b):
+    if isinstance(a, six.text_type) or isinstance(b, six.text_type):
+        raise TypeError("Unicode-objects must be encoded before comparing")
+
+    return _lib.Cryptography_constant_time_bytes_eq(a, len(a), b, len(b)) == 1
diff --git a/docs/hazmat/primitives/constant-time.rst b/docs/hazmat/primitives/constant-time.rst
new file mode 100644
index 00000000..2e8e26d7
--- /dev/null
+++ b/docs/hazmat/primitives/constant-time.rst
@@ -0,0 +1,24 @@
+.. hazmat::
+
+Constant time functions
+=======================
+
+.. currentmodule:: cryptography.hazmat.primitives.constant_time
+
+In order for cryptographic operations to not leak information through timing
+side channels, constant time operations need to be made available.
+
+.. function:: bytes_eq(a, b)
+
+    Compare ``a`` and ``b`` to one another in constant time.
+
+    .. doctest::
+
+        >>> from cryptography.hazmat.primitives import constant_time
+        >>> constant_time.bytes_eq(b"foo", b"foo")
+        True
+        >>> constant_time.bytes_eq(b"foo", b"bar")
+        False
+
+    :param a: ``bytes``.
+    :param b: ``bytes``.
diff --git a/docs/hazmat/primitives/index.rst b/docs/hazmat/primitives/index.rst
index 614c414a..b115fdbc 100644
--- a/docs/hazmat/primitives/index.rst
+++ b/docs/hazmat/primitives/index.rst
@@ -10,4 +10,5 @@ Primitives
     hmac
     symmetric-encryption
     padding
+    constant-time
     interfaces
diff --git a/tests/hazmat/primitives/test_constant_time.py b/tests/hazmat/primitives/test_constant_time.py
new file mode 100644
index 00000000..dd910dee
--- /dev/null
+++ b/tests/hazmat/primitives/test_constant_time.py
@@ -0,0 +1,41 @@
+# 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.
+
+from __future__ import absolute_import, division, print_function
+
+import pytest
+
+import six
+
+from cryptography.hazmat.primitives import constant_time
+
+
+class TestConstantTimeBytesEq(object):
+    def test_reject_unicode(self):
+        with pytest.raises(TypeError):
+            constant_time.bytes_eq(b"foo", six.u("foo"))
+
+        with pytest.raises(TypeError):
+            constant_time.bytes_eq(six.u("foo"), b"foo")
+
+        with pytest.raises(TypeError):
+            constant_time.bytes_eq(six.u("foo"), six.u("foo"))
+
+    def test_compares(self):
+        assert constant_time.bytes_eq(b"foo", b"foo") is True
+
+        assert constant_time.bytes_eq(b"foo", b"bar") is False
+
+        assert constant_time.bytes_eq(b"foobar", b"foo") is False
+
+        assert constant_time.bytes_eq(b"foo", b"foobar") is False