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93 lines
3.2 KiB
ReStructuredText
93 lines
3.2 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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for Python.
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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=None)
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Return a new hmac object. *key* is a bytes object giving the secret key. If
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*msg* is present, the method call ``update(msg)`` is made. *digestmod* is
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the digest constructor or module for the HMAC object to use. It defaults to
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the :func:`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 bytes object *msg*. Repeated calls are
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equivalent to a 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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.. 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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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 any type that supports the buffer protocol
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(e.g. :class:`bytes`).
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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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.. seealso::
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Module :mod:`hashlib`
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The Python module providing secure hash functions.
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