diff options
-rw-r--r-- | cryptography/hazmat/primitives/constant_time.py | 53 | ||||
-rw-r--r-- | docs/conf.py | 1 | ||||
-rw-r--r-- | docs/hazmat/primitives/constant-time.rst | 38 | ||||
-rw-r--r-- | docs/hazmat/primitives/index.rst | 1 | ||||
-rw-r--r-- | docs/hazmat/primitives/symmetric-encryption.rst | 6 | ||||
-rw-r--r-- | tests/hazmat/primitives/test_constant_time.py | 41 | ||||
-rw-r--r-- | tox.ini | 1 |
7 files changed, 138 insertions, 3 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/conf.py b/docs/conf.py index 77050e72..5092e4d3 100644 --- a/docs/conf.py +++ b/docs/conf.py @@ -257,6 +257,5 @@ texinfo_documents = [ # How to display URL addresses: 'footnote', 'no', or 'inline'. #texinfo_show_urls = 'footnote' - # Example configuration for intersphinx: refer to the Python standard library. intersphinx_mapping = {'http://docs.python.org/': None} diff --git a/docs/hazmat/primitives/constant-time.rst b/docs/hazmat/primitives/constant-time.rst new file mode 100644 index 00000000..632e7c68 --- /dev/null +++ b/docs/hazmat/primitives/constant-time.rst @@ -0,0 +1,38 @@ +.. hazmat:: + +Constant time functions +======================= + +.. currentmodule:: cryptography.hazmat.primitives.constant_time + +This module contains functions for operating with secret data in a way that +does not leak information about that data through how long it takes to perform +the operation. These functions should be used whenever operating on secret data +along with data that is user supplied. + +An example would be comparing a HMAC signature received from a client to the +one generated by the server code for authentication purposes. + +For more information about this sort of issue, see `Coda Hale's blog post`_ +about the timing attacks on KeyCzar and Java's ``MessageDigest.isEqual()``. + + +.. function:: bytes_eq(a, b) + + Compare ``a`` and ``b`` to one another in constant time if they are of the + same length. + + .. 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: The left-hand side. + :param b bytes: The right-hand side. + :returns boolean: True if ``a`` has the same bytes as ``b``. + + +.. _`Coda Hale's blog post`: http://codahale.com/a-lesson-in-timing-attacks/ 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/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst index 9fccf552..f4d0457a 100644 --- a/docs/hazmat/primitives/symmetric-encryption.rst +++ b/docs/hazmat/primitives/symmetric-encryption.rst @@ -266,16 +266,18 @@ Modes A good construction looks like: - .. code-block:: pycon + .. doctest:: >>> import os + >>> from cryptography.hazmat.primitives.ciphers.modes import CBC >>> iv = os.urandom(16) >>> mode = CBC(iv) While the following is bad and will leak information: - .. code-block:: pycon + .. doctest:: + >>> from cryptography.hazmat.primitives.ciphers.modes import CBC >>> iv = "a" * 16 >>> mode = CBC(iv) 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 @@ -17,6 +17,7 @@ deps = basepython = python2.7 commands = sphinx-build -W -b html -d {envtmpdir}/doctrees docs docs/_build/html + sphinx-build -W -b latex -d {envtmpdir}/doctrees docs docs/_build/latex sphinx-build -W -b doctest -d {envtmpdir}/doctrees docs docs/_build/html sphinx-build -W -b linkcheck docs docs/_build/html |