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db5aed931f
hashlib.compare_digest uses OpenSSL's CRYPTO_memcmp() function when OpenSSL is available. Note: The _operator module is a builtin module. I don't want to add libcrypto dependency to libpython. Therefore I duplicated the wrapper function and added a copy to _hashopenssl.c.
151 lines
4.9 KiB
ReStructuredText
151 lines
4.9 KiB
ReStructuredText
:mod:`hmac` --- Keyed-Hashing for Message Authentication
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========================================================
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.. module:: hmac
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:synopsis: Keyed-Hashing for Message Authentication (HMAC) implementation
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.. moduleauthor:: Gerhard Häring <ghaering@users.sourceforge.net>
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.. sectionauthor:: Gerhard Häring <ghaering@users.sourceforge.net>
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**Source code:** :source:`Lib/hmac.py`
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--------------
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This module implements the HMAC algorithm as described by :rfc:`2104`.
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.. function:: new(key, msg=None, digestmod='')
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Return a new hmac object. *key* is a bytes or bytearray object giving the
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secret key. If *msg* is present, the method call ``update(msg)`` is made.
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*digestmod* is the digest name, digest constructor or module for the HMAC
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object to use. It may be any name suitable to :func:`hashlib.new`.
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Despite its argument position, it is required.
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.. versionchanged:: 3.4
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Parameter *key* can be a bytes or bytearray object.
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Parameter *msg* can be of any type supported by :mod:`hashlib`.
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Parameter *digestmod* can be the name of a hash algorithm.
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.. deprecated-removed:: 3.4 3.8
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MD5 as implicit default digest for *digestmod* is deprecated.
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The digestmod parameter is now required. Pass it as a keyword
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argument to avoid awkwardness when you do not have an initial msg.
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.. function:: digest(key, msg, digest)
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Return digest of *msg* for given secret *key* and *digest*. The
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function is equivalent to ``HMAC(key, msg, digest).digest()``, but
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uses an optimized C or inline implementation, which is faster for messages
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that fit into memory. The parameters *key*, *msg*, and *digest* have
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the same meaning as in :func:`~hmac.new`.
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CPython implementation detail, the optimized C implementation is only used
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when *digest* is a string and name of a digest algorithm, which is
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supported by OpenSSL.
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.. versionadded:: 3.7
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An HMAC object has the following methods:
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.. method:: HMAC.update(msg)
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Update the hmac object with *msg*. Repeated calls are equivalent to a
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single call with the concatenation of all the arguments:
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``m.update(a); m.update(b)`` is equivalent to ``m.update(a + b)``.
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.. versionchanged:: 3.4
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Parameter *msg* can be of any type supported by :mod:`hashlib`.
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.. method:: HMAC.digest()
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Return the digest of the bytes passed to the :meth:`update` method so far.
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This bytes object will be the same length as the *digest_size* of the digest
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given to the constructor. It may contain non-ASCII bytes, including NUL
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bytes.
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.. warning::
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When comparing the output of :meth:`digest` to an externally-supplied
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digest during a verification routine, it is recommended to use the
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:func:`compare_digest` function instead of the ``==`` operator
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to reduce the vulnerability to timing attacks.
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.. method:: HMAC.hexdigest()
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Like :meth:`digest` except the digest is returned as a string twice the
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length containing only hexadecimal digits. This may be used to exchange the
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value safely in email or other non-binary environments.
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.. warning::
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When comparing the output of :meth:`hexdigest` to an externally-supplied
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digest during a verification routine, it is recommended to use the
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:func:`compare_digest` function instead of the ``==`` operator
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to reduce the vulnerability to timing attacks.
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.. method:: HMAC.copy()
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Return a copy ("clone") of the hmac object. This can be used to efficiently
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compute the digests of strings that share a common initial substring.
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A hash object has the following attributes:
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.. attribute:: HMAC.digest_size
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The size of the resulting HMAC digest in bytes.
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.. attribute:: HMAC.block_size
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The internal block size of the hash algorithm in bytes.
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.. versionadded:: 3.4
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.. attribute:: HMAC.name
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The canonical name of this HMAC, always lowercase, e.g. ``hmac-md5``.
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.. versionadded:: 3.4
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.. deprecated:: 3.9
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The undocumented attributes ``HMAC.digest_cons``, ``HMAC.inner``, and
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``HMAC.outer`` are internal implementation details and will be removed in
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Python 3.10.
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This module also provides the following helper function:
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.. function:: compare_digest(a, b)
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Return ``a == b``. This function uses an approach designed to prevent
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timing analysis by avoiding content-based short circuiting behaviour,
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making it appropriate for cryptography. *a* and *b* must both be of the
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same type: either :class:`str` (ASCII only, as e.g. returned by
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:meth:`HMAC.hexdigest`), or a :term:`bytes-like object`.
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.. note::
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If *a* and *b* are of different lengths, or if an error occurs,
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a timing attack could theoretically reveal information about the
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types and lengths of *a* and *b*—but not their values.
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.. versionadded:: 3.3
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.. versionchanged:: 3.10
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The function uses OpenSSL's ``CRYPTO_memcmp()`` internally when
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available.
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.. seealso::
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Module :mod:`hashlib`
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The Python module providing secure hash functions.
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