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.. 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 used.
One should use these functions whenever you are comparing a secret to
something received. This includes things like HMAC signatures as described by
a `timing attack on KeyCzar`_.
.. 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``.
.. _`timing attack on KeyCzar`: http://rdist.root.org/2009/05/28/timing-attack-in-google-keyczar-library/
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