1:mod:`hmac` --- Keyed-Hashing for Message Authentication
2========================================================
3
4.. module:: hmac
5   :synopsis: Keyed-Hashing for Message Authentication (HMAC) implementation
6
7.. moduleauthor:: Gerhard Häring <ghaering@users.sourceforge.net>
8.. sectionauthor:: Gerhard Häring <ghaering@users.sourceforge.net>
9
10**Source code:** :source:`Lib/hmac.py`
11
12--------------
13
14This module implements the HMAC algorithm as described by :rfc:`2104`.
15
16
17.. function:: new(key, msg=None, digestmod='')
18
19   Return a new hmac object.  *key* is a bytes or bytearray object giving the
20   secret key.  If *msg* is present, the method call ``update(msg)`` is made.
21   *digestmod* is the digest name, digest constructor or module for the HMAC
22   object to use.  It may be any name suitable to :func:`hashlib.new`.
23   Despite its argument position, it is required.
24
25   .. versionchanged:: 3.4
26      Parameter *key* can be a bytes or bytearray object.
27      Parameter *msg* can be of any type supported by :mod:`hashlib`.
28      Parameter *digestmod* can be the name of a hash algorithm.
29
30   .. deprecated-removed:: 3.4 3.8
31      MD5 as implicit default digest for *digestmod* is deprecated.
32      The digestmod parameter is now required.  Pass it as a keyword
33      argument to avoid awkwardness when you do not have an initial msg.
34
35
36.. function:: digest(key, msg, digest)
37
38   Return digest of *msg* for given secret *key* and *digest*. The
39   function is equivalent to ``HMAC(key, msg, digest).digest()``, but
40   uses an optimized C or inline implementation, which is faster for messages
41   that fit into memory. The parameters *key*, *msg*, and *digest* have
42   the same meaning as in :func:`~hmac.new`.
43
44   CPython implementation detail, the optimized C implementation is only used
45   when *digest* is a string and name of a digest algorithm, which is
46   supported by OpenSSL.
47
48   .. versionadded:: 3.7
49
50
51An HMAC object has the following methods:
52
53.. method:: HMAC.update(msg)
54
55   Update the hmac object with *msg*.  Repeated calls are equivalent to a
56   single call with the concatenation of all the arguments:
57   ``m.update(a); m.update(b)`` is equivalent to ``m.update(a + b)``.
58
59   .. versionchanged:: 3.4
60      Parameter *msg* can be of any type supported by :mod:`hashlib`.
61
62
63.. method:: HMAC.digest()
64
65   Return the digest of the bytes passed to the :meth:`update` method so far.
66   This bytes object will be the same length as the *digest_size* of the digest
67   given to the constructor.  It may contain non-ASCII bytes, including NUL
68   bytes.
69
70   .. warning::
71
72      When comparing the output of :meth:`digest` to an externally-supplied
73      digest during a verification routine, it is recommended to use the
74      :func:`compare_digest` function instead of the ``==`` operator
75      to reduce the vulnerability to timing attacks.
76
77
78.. method:: HMAC.hexdigest()
79
80   Like :meth:`digest` except the digest is returned as a string twice the
81   length containing only hexadecimal digits.  This may be used to exchange the
82   value safely in email or other non-binary environments.
83
84   .. warning::
85
86      When comparing the output of :meth:`hexdigest` to an externally-supplied
87      digest during a verification routine, it is recommended to use the
88      :func:`compare_digest` function instead of the ``==`` operator
89      to reduce the vulnerability to timing attacks.
90
91
92.. method:: HMAC.copy()
93
94   Return a copy ("clone") of the hmac object.  This can be used to efficiently
95   compute the digests of strings that share a common initial substring.
96
97
98A hash object has the following attributes:
99
100.. attribute:: HMAC.digest_size
101
102   The size of the resulting HMAC digest in bytes.
103
104.. attribute:: HMAC.block_size
105
106   The internal block size of the hash algorithm in bytes.
107
108   .. versionadded:: 3.4
109
110.. attribute:: HMAC.name
111
112   The canonical name of this HMAC, always lowercase, e.g. ``hmac-md5``.
113
114   .. versionadded:: 3.4
115
116
117This module also provides the following helper function:
118
119.. function:: compare_digest(a, b)
120
121   Return ``a == b``.  This function uses an approach designed to prevent
122   timing analysis by avoiding content-based short circuiting behaviour,
123   making it appropriate for cryptography.  *a* and *b* must both be of the
124   same type: either :class:`str` (ASCII only, as e.g. returned by
125   :meth:`HMAC.hexdigest`), or a :term:`bytes-like object`.
126
127   .. note::
128
129      If *a* and *b* are of different lengths, or if an error occurs,
130      a timing attack could theoretically reveal information about the
131      types and lengths of *a* and *b*—but not their values.
132
133   .. versionadded:: 3.3
134
135
136.. seealso::
137
138   Module :mod:`hashlib`
139      The Python module providing secure hash functions.
140