| :mod:`hmac` --- Keyed-Hashing for Message Authentication |
| ======================================================== |
| |
| .. module:: hmac |
| :synopsis: Keyed-Hashing for Message Authentication (HMAC) implementation |
| for Python. |
| .. moduleauthor:: Gerhard Häring <ghaering@users.sourceforge.net> |
| .. sectionauthor:: Gerhard Häring <ghaering@users.sourceforge.net> |
| |
| **Source code:** :source:`Lib/hmac.py` |
| |
| -------------- |
| |
| This module implements the HMAC algorithm as described by :rfc:`2104`. |
| |
| |
| .. function:: new(key, msg=None, digestmod=None) |
| |
| Return a new hmac object. *key* is a bytes object giving the secret key. If |
| *msg* is present, the method call ``update(msg)`` is made. *digestmod* is |
| the digest constructor or module for the HMAC object to use. It defaults to |
| the :func:`hashlib.md5` constructor. |
| |
| |
| An HMAC object has the following methods: |
| |
| .. method:: HMAC.update(msg) |
| |
| Update the hmac object with the bytes object *msg*. Repeated calls are |
| equivalent to a single call with the concatenation of all the arguments: |
| ``m.update(a); m.update(b)`` is equivalent to ``m.update(a + b)``. |
| |
| |
| .. method:: HMAC.digest() |
| |
| Return the digest of the bytes passed to the :meth:`update` method so far. |
| This bytes object will be the same length as the *digest_size* of the digest |
| given to the constructor. It may contain non-ASCII bytes, including NUL |
| bytes. |
| |
| .. warning:: |
| |
| When comparing the output of :meth:`digest` to an externally-supplied |
| digest during a verification routine, it is recommended to use the |
| :func:`hmac.secure_compare` function instead of the ``==`` operator |
| to avoid potential timing attacks. |
| |
| |
| .. method:: HMAC.hexdigest() |
| |
| Like :meth:`digest` except the digest is returned as a string twice the |
| length containing only hexadecimal digits. This may be used to exchange the |
| value safely in email or other non-binary environments. |
| |
| .. warning:: |
| |
| When comparing the output of :meth:`hexdigest` to an externally-supplied |
| digest during a verification routine, it is recommended to use the |
| :func:`hmac.secure_compare` function instead of the ``==`` operator |
| to avoid potential timing attacks. |
| |
| |
| .. method:: HMAC.copy() |
| |
| Return a copy ("clone") of the hmac object. This can be used to efficiently |
| compute the digests of strings that share a common initial substring. |
| |
| |
| This module also provides the following helper function: |
| |
| .. function:: secure_compare(a, b) |
| |
| Returns the equivalent of ``a == b``, but using a time-independent |
| comparison method. Comparing the full lengths of the inputs *a* and *b*, |
| instead of short-circuiting the comparison upon the first unequal byte, |
| prevents leaking information about the inputs being compared and mitigates |
| potential timing attacks. The inputs must be either :class:`str` or |
| :class:`bytes` instances. |
| |
| .. note:: |
| |
| While the :func:`hmac.secure_compare` function prevents leaking the |
| contents of the inputs via a timing attack, it does leak the length |
| of the inputs. However, this generally is not a security risk. |
| |
| .. versionadded:: 3.3 |
| |
| .. seealso:: |
| |
| Module :mod:`hashlib` |
| The Python module providing secure hash functions. |