
``hashlib`` --- Secure hashes and message digests
*************************************************

This module implements a common interface to many different secure
hash and message digest algorithms.  Included are the FIPS secure hash
algorithms SHA1, SHA224, SHA256, SHA384, and SHA512 (defined in FIPS
180-2) as well as RSA's MD5 algorithm (defined in Internet **RFC
1321**).  The terms "secure hash" and "message digest" are
interchangeable.  Older algorithms were called message digests.  The
modern term is secure hash.

Note: If you want the adler32 or crc32 hash functions they are available
  in the ``zlib`` module.

Warning: Some algorithms have known hash collision weaknesses, see the FAQ at
  the end.

There is one constructor method named for each type of *hash*.  All
return a hash object with the same simple interface. For example: use
``sha1()`` to create a SHA1 hash object. You can now feed this object
with objects conforming to the buffer interface (normally ``bytes``
objects) using the ``update()`` method.  At any point you can ask it
for the *digest* of the concatenation of the data fed to it so far
using the ``digest()`` or ``hexdigest()`` methods.

Note: For better multithreading performance, the Python GIL is released
  for strings of more than 2047 bytes at object creation or on update.

Note: Feeding string objects is to ``update()`` is not supported, as
  hashes work on bytes, not on characters.

Constructors for hash algorithms that are always present in this
module are ``md5()``, ``sha1()``, ``sha224()``, ``sha256()``,
``sha384()``, and ``sha512()``.  Additional algorithms may also be
available depending upon the OpenSSL library that Python uses on your
platform.

For example, to obtain the digest of the byte string ``b'Nobody
inspects the spammish repetition'``:

   >>> import hashlib
   >>> m = hashlib.md5()
   >>> m.update(b"Nobody inspects")
   >>> m.update(b" the spammish repetition")
   >>> m.digest()
   b'\xbbd\x9c\x83\xdd\x1e\xa5\xc9\xd9\xde\xc9\xa1\x8d\xf0\xff\xe9'
   >>> m.digest_size
   16
   >>> m.block_size
   64

More condensed:

>>> hashlib.sha224(b"Nobody inspects the spammish repetition").hexdigest()
'a4337bc45a8fc544c03f52dc550cd6e1e87021bc896588bd79e901e2'

A generic ``new()`` constructor that takes the string name of the
desired algorithm as its first parameter also exists to allow access
to the above listed hashes as well as any other algorithms that your
OpenSSL library may offer.  The named constructors are much faster
than ``new()`` and should be preferred.

Using ``new()`` with an algorithm provided by OpenSSL:

>>> h = hashlib.new('ripemd160')
>>> h.update(b"Nobody inspects the spammish repetition")
>>> h.hexdigest()
'cc4a5ce1b3df48aec5d22d1f16b894a0b894eccc'

The following values are provided as constant attributes of the hash
objects returned by the constructors:

hashlib.digest_size

   The size of the resulting hash in bytes.

hashlib.block_size

   The internal block size of the hash algorithm in bytes.

A hash object has the following methods:

hash.update(arg)

   Update the hash object with the object *arg*, which must be
   interpretable as a buffer of bytes.  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)``.

hash.digest()

   Return the digest of the data passed to the ``update()`` method so
   far. This is a bytes array of size ``digest_size`` which may
   contain bytes in the whole range from 0 to 255.

hash.hexdigest()

   Like ``digest()`` except the digest is returned as a string object
   of double length, containing only hexadecimal digits.  This may be
   used to exchange the value safely in email or other non-binary
   environments.

hash.copy()

   Return a copy ("clone") of the hash object.  This can be used to
   efficiently compute the digests of data sharing a common initial
   substring.

See also:

   Module ``hmac``
      A module to generate message authentication codes using hashes.

   Module ``base64``
      Another way to encode binary hashes for non-binary environments.

   http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf
      The FIPS 180-2 publication on Secure Hash Algorithms.

   http://www.cryptography.com/cnews/hash.html
      Hash Collision FAQ with information on which algorithms have
      known issues and what that means regarding their use.
