==================================================================
:class:`passlib.hash.sha256_crypt` - SHA-256 Crypt
==================================================================

.. currentmodule:: passlib.hash

SHA-256 Crypt and SHA-512 Crypt were developed in 2008 by Ulrich Drepper [#f1]_,
designed as the successor to :class:`~passlib.hash.md5_crypt`.
They include fixes and advancements such as variable rounds, and use of NIST-approved cryptographic primitives.
The design involves repeated composition of the underlying digest algorithm,
using various arbitrary permutations of inputs.
SHA-512 / SHA-256 Crypt are currently the default password hash for many systems
(notably Linux), and have no known weaknesses.
SHA-256 Crypt is one of the four hashes Passlib :ref:`recommends <recommended-hashes>`
for new applications.
This class can be used directly as follows::

    >>> from passlib.hash import sha256_crypt

    >>> # generate new salt, hash password
    >>> hash = sha256_crypt.hash("password")
    >>> hash
    '$5$rounds=80000$wnsT7Yr92oJoP28r$cKhJImk5mfuSKV9b3mumNzlbstFUplKtQXXMo4G6Ep5'

    >>> # same, but with explict number of rounds
    >>> sha256_crypt.using(rounds=12345).hash("password")
    '$5$rounds=12345$q3hvJE5mn5jKRsW.$BbbYTFiaImz9rTy03GGi.Jf9YY5bmxN0LU3p3uI1iUB'

    >>> # verify password
    >>> sha256_crypt.verify("password", hash)
    True
    >>> sha256_crypt.verify("letmein", hash)
    False

.. seealso::

    * :ref:`password hash usage <password-hash-examples>` -- for more usage examples

    * :doc:`sha512_crypt <passlib.hash.sha512_crypt>` -- the companion 512-bit version of this hash.

Interface
=========
.. autoclass:: sha256_crypt()

.. note::

    This class will use the first available of two possible backends:

    * stdlib :func:`crypt()`, if the host OS supports SHA256-Crypt (most Linux systems).
    * a pure python implementation of SHA256-Crypt built into Passlib.

    You can see which backend is in use by calling the :meth:`get_backend()` method.

Format & Algorithm
==================
An example sha256-crypt hash (of the string ``password``) is:

    ``$5$rounds=80000$wnsT7Yr92oJoP28r$cKhJImk5mfuSKV9b3mumNzlbstFUplKtQXXMo4G6Ep5``

An sha256-crypt hash string has the format :samp:`$5$rounds={rounds}${salt}${checksum}`, where:

* ``$5$`` is the prefix used to identify sha256-crypt hashes,
  following the :ref:`modular-crypt-format`

* :samp:`{rounds}` is the decimal number of rounds to use (80000 in the example).

* :samp:`{salt}` is 0-16 characters drawn from ``[./0-9A-Za-z]``, providing a
  96-bit salt (``wnsT7Yr92oJoP28r`` in the example).

* :samp:`{checksum}` is 43 characters drawn from the same set, encoding a 256-bit
  checksum (``cKhJImk5mfuSKV9b3mumNzlbstFUplKtQXXMo4G6Ep5`` in the example).

The official implementation allows omitting the ``rounds`` section when it's set to 5000,
resulting in an alternate hash format: :samp:`$5${salt}${checksum}`.
(Passlib supports this via the ``implicit_rounds`` constructor parameter).

The algorithm used by SHA256-Crypt is laid out in detail
in the specification document linked to below [#f1]_.

Security Issues
===============
* The algorithm's initialization stage contains a loop which varies linearly with the
  square of the password size; and further loops which vary linearly with the
  password size * rounds.

    - This means an attacker could provide a maliciously large password at the login screen
      to attempt a DOS on a publically visible login.  For example, a 32kib password
      would require hashing 1gib of data.
      Passlib mitigates this by limiting the maximum password size to 4k by default.

    - An attacker could also theoretically determine a password's size by observing
      the time taken on a successful login, and then attempting verification themselves
      to find the size password which has an equivalent delay.  This has not been applied
      in practice, probably due to the fact that (for normal passwords < 64 bytes),
      the contribution of the password size to the overall time taken is below
      the observable noise level when evesdropping on the timings of successful logins
      for a single user.

Deviations
==========
This implementation of sha256-crypt differs from the specification,
and other implementations, in a few ways:

* Zero-Padded Rounds:

  The specification does not specify how to deal with zero-padding
  within the rounds portion of the hash. No existing examples
  or test vectors have zero padding, and allowing it would
  result in multiple encodings for the same configuration / hash.
  To prevent this situation, Passlib will throw an error if the rounds
  parameter in a hash has leading zeros.

* Restricted salt string character set:

  The underlying algorithm can unambiguously handle salt strings
  which contain any possible byte value besides ``\x00`` and ``$``.
  However, Passlib strictly limits salts to the
  :data:`hash64 <passlib.utils.binary.HASH64_CHARS>` character set,
  as nearly all implementations of sha256-crypt generate
  and expect salts containing those characters,
  but may have unexpected behaviors for other character values.

* Unicode Policy:

  The underlying algorithm takes in a password specified
  as a series of non-null bytes, and does not specify what encoding
  should be used; though a ``us-ascii`` compatible encoding
  is implied by nearly all implementations of sha256-crypt
  as well as all known reference hashes.

  In order to provide support for unicode strings,
  Passlib will encode unicode passwords using ``utf-8``
  before running them through sha256-crypt. If a different
  encoding is desired by an application, the password should be encoded
  before handing it to Passlib.

.. rubric:: Footnotes

.. [#f1] Ulrich Drepper's SHA-256/512-Crypt specification, reference
   implementation, and test vectors -
   `sha-crypt specification <http://www.akkadia.org/drepper/sha-crypt.html>`_
