93 lines
3.0 KiB
ReStructuredText
93 lines
3.0 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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.. versionadded:: 2.2
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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[, digestmod]])
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Return a new hmac object. If *msg* is present, the method call ``update(msg)``
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is made. *digestmod* is the digest constructor or module for the HMAC object to
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use. It defaults to the :data:`hashlib.md5` constructor.
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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 the string *msg*. Repeated calls are equivalent to
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a single call with the concatenation of all the arguments: ``m.update(a);
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m.update(b)`` is equivalent to ``m.update(a + b)``.
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.. method:: HMAC.digest()
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Return the digest of the strings passed to the :meth:`update` method so far.
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This string will be the same length as the *digest_size* of the digest given to
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the constructor. It may contain non-ASCII characters, including NUL 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 length
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containing only hexadecimal digits. This may be used to exchange the value
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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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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:`unicode` 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:: 2.7.7
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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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