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=None) 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 supports any name suitable to :func:`hashlib.new` and 23 defaults to the :data:`hashlib.md5` constructor. 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:: 3.4 31 MD5 as implicit default digest for *digestmod* is deprecated. 32 33 34An HMAC object has the following methods: 35 36.. method:: HMAC.update(msg) 37 38 Update the hmac object with *msg*. Repeated calls are equivalent to a 39 single call with the concatenation of all the arguments: 40 ``m.update(a); m.update(b)`` is equivalent to ``m.update(a + b)``. 41 42 .. versionchanged:: 3.4 43 Parameter *msg* can be of any type supported by :mod:`hashlib`. 44 45 46.. method:: HMAC.digest() 47 48 Return the digest of the bytes passed to the :meth:`update` method so far. 49 This bytes object will be the same length as the *digest_size* of the digest 50 given to the constructor. It may contain non-ASCII bytes, including NUL 51 bytes. 52 53 .. warning:: 54 55 When comparing the output of :meth:`digest` to an externally-supplied 56 digest during a verification routine, it is recommended to use the 57 :func:`compare_digest` function instead of the ``==`` operator 58 to reduce the vulnerability to timing attacks. 59 60 61.. method:: HMAC.hexdigest() 62 63 Like :meth:`digest` except the digest is returned as a string twice the 64 length containing only hexadecimal digits. This may be used to exchange the 65 value safely in email or other non-binary environments. 66 67 .. warning:: 68 69 When comparing the output of :meth:`hexdigest` to an externally-supplied 70 digest during a verification routine, it is recommended to use the 71 :func:`compare_digest` function instead of the ``==`` operator 72 to reduce the vulnerability to timing attacks. 73 74 75.. method:: HMAC.copy() 76 77 Return a copy ("clone") of the hmac object. This can be used to efficiently 78 compute the digests of strings that share a common initial substring. 79 80 81A hash object has the following attributes: 82 83.. attribute:: HMAC.digest_size 84 85 The size of the resulting HMAC digest in bytes. 86 87.. attribute:: HMAC.block_size 88 89 The internal block size of the hash algorithm in bytes. 90 91 .. versionadded:: 3.4 92 93.. attribute:: HMAC.name 94 95 The canonical name of this HMAC, always lowercase, e.g. ``hmac-md5``. 96 97 .. versionadded:: 3.4 98 99 100This module also provides the following helper function: 101 102.. function:: compare_digest(a, b) 103 104 Return ``a == b``. This function uses an approach designed to prevent 105 timing analysis by avoiding content-based short circuiting behaviour, 106 making it appropriate for cryptography. *a* and *b* must both be of the 107 same type: either :class:`str` (ASCII only, as e.g. returned by 108 :meth:`HMAC.hexdigest`), or a :term:`bytes-like object`. 109 110 .. note:: 111 112 If *a* and *b* are of different lengths, or if an error occurs, 113 a timing attack could theoretically reveal information about the 114 types and lengths of *a* and *b*—but not their values. 115 116 117 .. versionadded:: 3.3 118 119 120.. seealso:: 121 122 Module :mod:`hashlib` 123 The Python module providing secure hash functions. 124