
``hmac`` --- Keyed-Hashing for Message Authentication
*****************************************************

**Source code:** Lib/hmac.py

======================================================================

This module implements the HMAC algorithm as described by **RFC
2104**.

hmac.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 ``hashlib.md5()``
   constructor.

An HMAC object has the following methods:

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)``.

HMAC.digest()

   Return the digest of the bytes passed to the ``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 ``digest()`` to an externally-
     supplied digest during a verification routine, it is recommended
     to use the ``compare_digest()`` function instead of the ``==``
     operator to reduce the vulnerability to timing attacks.

HMAC.hexdigest()

   Like ``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 ``hexdigest()`` to an externally-
     supplied digest during a verification routine, it is recommended
     to use the ``compare_digest()`` function instead of the ``==``
     operator to reduce the vulnerability to timing attacks.

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:

hmac.compare_digest(a, b)

   Return ``a == b``.  This function uses an approach designed to
   prevent timing analysis by avoiding content-based short circuiting
   behaviour, making it appropriate for cryptography.  *a* and *b*
   must both be of the same type: either ``str`` (ASCII only, as e.g.
   returned by ``HMAC.hexdigest()``), or a *bytes-like object*.

   Note: If *a* and *b* are of different lengths, or if an error occurs, a
     timing attack could theoretically reveal information about the
     types and lengths of *a* and *b*--but not their values.

   New in version 3.3.

See also:

   Module ``hashlib``
      The Python module providing secure hash functions.
