Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 1 | :mod:`hmac` --- Keyed-Hashing for Message Authentication |
| 2 | ======================================================== |
| 3 | |
| 4 | .. module:: hmac |
Georg Brandl | 80b75fd | 2010-10-17 09:43:35 +0000 | [diff] [blame] | 5 | :synopsis: Keyed-Hashing for Message Authentication (HMAC) implementation |
| 6 | for Python. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 7 | .. moduleauthor:: Gerhard Häring <ghaering@users.sourceforge.net> |
| 8 | .. sectionauthor:: Gerhard Häring <ghaering@users.sourceforge.net> |
| 9 | |
Raymond Hettinger | 469271d | 2011-01-27 20:38:46 +0000 | [diff] [blame] | 10 | **Source code:** :source:`Lib/hmac.py` |
| 11 | |
| 12 | -------------- |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 13 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 14 | This module implements the HMAC algorithm as described by :rfc:`2104`. |
| 15 | |
| 16 | |
Georg Brandl | 036490d | 2009-05-17 13:00:36 +0000 | [diff] [blame] | 17 | .. function:: new(key, msg=None, digestmod=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 18 | |
Georg Brandl | 80b75fd | 2010-10-17 09:43:35 +0000 | [diff] [blame] | 19 | Return a new hmac object. *key* is a bytes object giving the secret key. If |
| 20 | *msg* is present, the method call ``update(msg)`` is made. *digestmod* is |
| 21 | the digest constructor or module for the HMAC object to use. It defaults to |
| 22 | the :func:`hashlib.md5` constructor. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 23 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 24 | |
| 25 | An HMAC object has the following methods: |
| 26 | |
Georg Brandl | 9701eb6 | 2012-02-05 09:25:22 +0100 | [diff] [blame] | 27 | .. method:: HMAC.update(msg) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 28 | |
Georg Brandl | 80b75fd | 2010-10-17 09:43:35 +0000 | [diff] [blame] | 29 | Update the hmac object with the bytes object *msg*. Repeated calls are |
| 30 | equivalent to a single call with the concatenation of all the arguments: |
| 31 | ``m.update(a); m.update(b)`` is equivalent to ``m.update(a + b)``. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 32 | |
| 33 | |
Georg Brandl | 9701eb6 | 2012-02-05 09:25:22 +0100 | [diff] [blame] | 34 | .. method:: HMAC.digest() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 35 | |
Georg Brandl | 80b75fd | 2010-10-17 09:43:35 +0000 | [diff] [blame] | 36 | Return the digest of the bytes passed to the :meth:`update` method so far. |
| 37 | This bytes object will be the same length as the *digest_size* of the digest |
| 38 | given to the constructor. It may contain non-ASCII bytes, including NUL |
| 39 | bytes. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 40 | |
Charles-François Natali | 7feb9f4 | 2012-05-13 19:53:07 +0200 | [diff] [blame] | 41 | .. warning:: |
| 42 | |
| 43 | When comparing the output of :meth:`digest` to an externally-supplied |
| 44 | digest during a verification routine, it is recommended to use the |
Nick Coghlan | 807770e | 2012-06-15 21:14:08 +1000 | [diff] [blame] | 45 | :func:`compare_digest` function instead of the ``==`` operator |
| 46 | to reduce the vulnerability to timing attacks. |
Charles-François Natali | 7feb9f4 | 2012-05-13 19:53:07 +0200 | [diff] [blame] | 47 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 48 | |
Georg Brandl | 9701eb6 | 2012-02-05 09:25:22 +0100 | [diff] [blame] | 49 | .. method:: HMAC.hexdigest() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 50 | |
Georg Brandl | 80b75fd | 2010-10-17 09:43:35 +0000 | [diff] [blame] | 51 | Like :meth:`digest` except the digest is returned as a string twice the |
| 52 | length containing only hexadecimal digits. This may be used to exchange the |
| 53 | value safely in email or other non-binary environments. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 54 | |
Charles-François Natali | 7feb9f4 | 2012-05-13 19:53:07 +0200 | [diff] [blame] | 55 | .. warning:: |
| 56 | |
Antoine Pitrou | 9df73da | 2012-06-24 16:03:50 +0200 | [diff] [blame] | 57 | When comparing the output of :meth:`hexdigest` to an externally-supplied |
| 58 | digest during a verification routine, it is recommended to use the |
| 59 | :func:`compare_digest` function instead of the ``==`` operator |
| 60 | to reduce the vulnerability to timing attacks. |
Charles-François Natali | 7feb9f4 | 2012-05-13 19:53:07 +0200 | [diff] [blame] | 61 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 62 | |
Georg Brandl | 9701eb6 | 2012-02-05 09:25:22 +0100 | [diff] [blame] | 63 | .. method:: HMAC.copy() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 64 | |
| 65 | Return a copy ("clone") of the hmac object. This can be used to efficiently |
| 66 | compute the digests of strings that share a common initial substring. |
| 67 | |
| 68 | |
Charles-François Natali | 7feb9f4 | 2012-05-13 19:53:07 +0200 | [diff] [blame] | 69 | This module also provides the following helper function: |
| 70 | |
Nick Coghlan | 807770e | 2012-06-15 21:14:08 +1000 | [diff] [blame] | 71 | .. function:: compare_digest(a, b) |
Charles-François Natali | 7feb9f4 | 2012-05-13 19:53:07 +0200 | [diff] [blame] | 72 | |
Antoine Pitrou | 5f762af | 2012-06-24 16:23:54 +0200 | [diff] [blame] | 73 | Return ``a == b``. This function uses an approach designed to prevent |
| 74 | timing analysis by avoiding content-based short circuiting behaviour, |
| 75 | making it appropriate for cryptography. *a* and *b* must both be of the |
| 76 | same type: either :class:`str` (ASCII only, as e.g. returned by |
Antoine Pitrou | 1524d75 | 2012-06-24 16:18:48 +0200 | [diff] [blame] | 77 | :meth:`HMAC.hexdigest`), or any type that supports the buffer protocol |
| 78 | (e.g. :class:`bytes`). |
Nick Coghlan | 807770e | 2012-06-15 21:14:08 +1000 | [diff] [blame] | 79 | |
Charles-François Natali | 7feb9f4 | 2012-05-13 19:53:07 +0200 | [diff] [blame] | 80 | .. note:: |
| 81 | |
Antoine Pitrou | f61e791 | 2012-06-24 16:21:10 +0200 | [diff] [blame] | 82 | If *a* and *b* are of different lengths, or if an error occurs, |
Larry Hastings | 48986d6 | 2012-06-25 00:59:34 -0700 | [diff] [blame] | 83 | a timing attack could theoretically reveal information about the |
| 84 | types and lengths of *a* and *b*--but not their values. |
Antoine Pitrou | f61e791 | 2012-06-24 16:21:10 +0200 | [diff] [blame] | 85 | |
Charles-François Natali | 7feb9f4 | 2012-05-13 19:53:07 +0200 | [diff] [blame] | 86 | |
Charles-François Natali | 9b704ec | 2012-05-15 21:00:32 +0200 | [diff] [blame] | 87 | .. versionadded:: 3.3 |
Charles-François Natali | 7feb9f4 | 2012-05-13 19:53:07 +0200 | [diff] [blame] | 88 | |
Georg Brandl | a1bc35f | 2012-06-24 16:07:33 +0200 | [diff] [blame] | 89 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 90 | .. seealso:: |
| 91 | |
| 92 | Module :mod:`hashlib` |
Ezio Melotti | 0639d5a | 2009-12-19 23:26:38 +0000 | [diff] [blame] | 93 | The Python module providing secure hash functions. |