14.1. "hashlib" — Secure hashes and message digests
***************************************************

New in version 2.5.

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

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

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, refer
  to the “See also” section 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 arbitrary strings using the "update()" method.  At any point you
can ask it for the *digest* of the concatenation of the strings fed to
it so far using the "digest()" or "hexdigest()" methods.

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 string "'Nobody inspects the
spammish repetition'":

>>> import hashlib
>>> m = hashlib.md5()
>>> m.update("Nobody inspects")
>>> m.update(" the spammish repetition")
>>> m.digest()
'\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("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("Nobody inspects the spammish repetition")
>>> h.hexdigest()
'cc4a5ce1b3df48aec5d22d1f16b894a0b894eccc'

This module provides the following constant attribute:

hashlib.algorithms

   A tuple providing the names of the hash algorithms guaranteed to be
   supported by this module.

   New in version 2.7.

hashlib.algorithms_guaranteed

   A set containing the names of the hash algorithms guaranteed to be
   supported by this module on all platforms.

   New in version 2.7.9.

hashlib.algorithms_available

   A set containing the names of the hash algorithms that are
   available in the running Python interpreter.  These names will be
   recognized when passed to "new()".  "algorithms_guaranteed" will
   always be a subset.  The same algorithm may appear multiple times
   in this set under different names (thanks to OpenSSL).

   New in version 2.7.9.

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

hash.digest_size

   The size of the resulting hash in bytes.

hash.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 string *arg*.  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 2.7: The Python GIL is released to allow other
   threads to run while hash updates on data larger than 2048 bytes is
   taking place when using hash algorithms supplied by OpenSSL.

hash.digest()

   Return the digest of the strings passed to the "update()" method so
   far. This is a string of "digest_size" bytes which may contain non-
   ASCII characters, including null bytes.

hash.hexdigest()

   Like "digest()" except the digest is returned as a string 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 strings that share a common
   initial substring.


14.1.1. Key derivation
======================

Key derivation and key stretching algorithms are designed for secure
password hashing. Naive algorithms such as "sha1(password)" are not
resistant against brute-force attacks. A good password hashing
function must be tunable, slow, and include a salt.

hashlib.pbkdf2_hmac(name, password, salt, rounds, dklen=None)

   The function provides PKCS#5 password-based key derivation function
   2. It uses HMAC as pseudorandom function.

   The string *name* is the desired name of the hash digest algorithm
   for HMAC, e.g. ‘sha1’ or ‘sha256’. *password* and *salt* are
   interpreted as buffers of bytes. Applications and libraries should
   limit *password* to a sensible value (e.g. 1024). *salt* should be
   about 16 or more bytes from a proper source, e.g. "os.urandom()".

   The number of *rounds* should be chosen based on the hash algorithm
   and computing power. As of 2013, at least 100,000 rounds of SHA-256
   is suggested.

   *dklen* is the length of the derived key. If *dklen* is "None" then
   the digest size of the hash algorithm *name* is used, e.g. 64 for
   SHA-512.

   >>> import hashlib, binascii
   >>> dk = hashlib.pbkdf2_hmac('sha256', b'password', b'salt', 100000)
   >>> binascii.hexlify(dk)
   b'0394a2ede332c9a13eb82e9b24631604c31df978b4e2f0fbd2c549944f9d79a5'

   New in version 2.7.8.

   Note: A fast implementation of *pbkdf2_hmac* is available with
     OpenSSL. The Python implementation uses an inline version of
     "hmac". It is about three times slower and doesn’t release the
     GIL.

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.

  https://en.wikipedia.org/wiki/Cryptographic_hash_function#Cryptogra
  phic_hash_algorithms
     Wikipedia article with information on which algorithms have known
     issues and what that means regarding their use.