"hmac" — Keyed-Hashing for Message Authentication
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**Source code:** Lib/hmac.py

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This module implements the HMAC algorithm as described by **RFC
2104**. The interface allows to use any hash function with a *fixed*
digest size. In particular, extendable output functions such as
SHAKE-128 or SHAKE-256 cannot be used with HMAC.

hmac.new(key, msg=None, digestmod)

   Return a new hmac object.  *key* is a bytes or bytearray object
   giving the secret key.  If *msg* is present, the method call
   "update(msg)" is made. *digestmod* is the digest name, digest
   constructor or module for the HMAC object to use.  It may be any
   name suitable to "hashlib.new()". Despite its argument position, it
   is required.

   Changed in version 3.4: Parameter *key* can be a bytes or bytearray
   object. Parameter *msg* can be of any type supported by "hashlib".
   Parameter *digestmod* can be the name of a hash algorithm.

   Changed in version 3.8: The *digestmod* argument is now required.
   Pass it as a keyword argument to avoid awkwardness when you do not
   have an initial *msg*.

hmac.digest(key, msg, digest)

   Return digest of *msg* for given secret *key* and *digest*. The
   function is equivalent to "HMAC(key, msg, digest).digest()", but
   uses an optimized C or inline implementation, which is faster for
   messages that fit into memory. The parameters *key*, *msg*, and
   *digest* have the same meaning as in "new()".

   CPython implementation detail, the optimized C implementation is
   only used when *digest* is a string and name of a digest algorithm,
   which is supported by OpenSSL.

   Added in version 3.7.

An HMAC object has the following methods:

HMAC.update(msg)

   Update the hmac object with *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)".

   Changed in version 3.4: Parameter *msg* can be of any type
   supported by "hashlib".

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.

A hash object has the following attributes:

HMAC.digest_size

   The size of the resulting HMAC digest in bytes.

HMAC.block_size

   The internal block size of the hash algorithm in bytes.

   Added in version 3.4.

HMAC.name

   The canonical name of this HMAC, always lowercase, e.g. "hmac-md5".

   Added in version 3.4.

Changed in version 3.10: Removed the undocumented attributes
"HMAC.digest_cons", "HMAC.inner", and "HMAC.outer".

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.

   Added in version 3.3.

   Changed in version 3.10: The function uses OpenSSL’s
   "CRYPTO_memcmp()" internally when available.

See also:

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