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File Content: `constant-time.rst`

.. 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)

    Compares ``a`` and ``b`` with one another. If ``a`` and ``b`` have
    different lengths, this returns ``False`` immediately. Otherwise it
    compares them in a way that takes the same amount of time, regardless of
    how many characters are the same between the two.

    .. 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 bytes a: The left-hand side.
    :param bytes b: The right-hand side.
    :returns bool: ``True`` if ``a`` has the same bytes as ``b``, otherwise
                   ``False``.
    :raises TypeError: This exception is raised if ``a`` or ``b`` is not
                       ``bytes``.


.. _`Coda Hale's blog post`: https://codahale.com/a-lesson-in-timing-attacks/

Name	Size	Permission
asymmetric	---	0755
mac	---	0755
aead.rst	10322 bytes	0644
constant-time.rst	1592 bytes	0644
cryptographic-hashes.rst	9153 bytes	0644
index.rst	246 bytes	0644
key-derivation-functions.rst	39929 bytes	0644
keywrap.rst	3116 bytes	0644
padding.rst	4670 bytes	0644
symmetric-encryption.rst	31500 bytes	0644
twofactor.rst	10562 bytes	0644

File Content: constant-time.rst

File Content: `constant-time.rst`