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

**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.


15.1.1. Hash algorithms
=======================

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 *bytes-like objects* (normally "bytes") 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 data larger than 2047 bytes at object creation or on
  update.

Note: Feeding string objects into "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'

hashlib.new(name[, data])

   Is a generic constructor that takes the string name of the desired
   algorithm as its first parameter.  It 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'

Hashlib provides the following constant attributes:

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 3.2.

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 3.2.

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 attributes:

hash.name

   The canonical name of this hash, always lowercase and always
   suitable as a parameter to "new()" to create another hash of this
   type.

   Changed in version 3.4: The name attribute has been present in
   CPython since its inception, but until Python 3.4 was not formally
   specified, so may not exist on some platforms.

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)".

   Changed in version 3.1: The Python GIL is released to allow other
   threads to run while hash updates on data larger than 2047 bytes is
   taking place when using hash algorithms supplied by OpenSSL.

hash.digest()

   Return the digest of the data passed to the "update()" method so
   far. This is a bytes object 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.


15.1.2. 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(hash_name, password, salt, iterations, dklen=None)

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

   The string *hash_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 length (e.g. 1024). *salt*
   should be about 16 or more bytes from a proper source, e.g.
   "os.urandom()".

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

   *dklen* is the length of the derived key. If *dklen* is "None" then
   the digest size of the hash algorithm *hash_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 3.4.

   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.

  https://www.ietf.org/rfc/rfc2898.txt
     PKCS #5: Password-Based Cryptography Specification Version 2.0
