
"crypt" --- Function to check Unix passwords
********************************************

This module implements an interface to the *crypt(3)* routine, which
is a one-way hash function based upon a modified DES algorithm; see
the Unix man page for further details.  Possible uses include storing
hashed passwords so you can check passwords without storing the actual
password, or attempting to crack Unix passwords with a dictionary.

Notice that the behavior of this module depends on the actual
implementation  of the *crypt(3)* routine in the running system.
Therefore, any extensions available on the current implementation will
also  be available on this module.


Hashing Methods
===============

New in version 3.3.

The "crypt" module defines the list of hashing methods (not all
methods are available on all platforms):

crypt.METHOD_SHA512

   A Modular Crypt Format method with 16 character salt and 86
   character hash.  This is the strongest method.

crypt.METHOD_SHA256

   Another Modular Crypt Format method with 16 character salt and 43
   character hash.

crypt.METHOD_MD5

   Another Modular Crypt Format method with 8 character salt and 22
   character hash.

crypt.METHOD_CRYPT

   The traditional method with a 2 character salt and 13 characters of
   hash.  This is the weakest method.


Module Attributes
=================

New in version 3.3.

crypt.methods

   A list of available password hashing algorithms, as
   "crypt.METHOD_*" objects.  This list is sorted from strongest to
   weakest, and is guaranteed to have at least "crypt.METHOD_CRYPT".


Module Functions
================

The "crypt" module defines the following functions:

crypt.crypt(word, salt=None)

   *word* will usually be a user's password as typed at a prompt or
   in a graphical interface.  The optional *salt* is either a string
   as returned from "mksalt()", one of the "crypt.METHOD_*" values
   (though not all may be available on all platforms), or a full
   encrypted password including salt, as returned by this function.
   If *salt* is not provided, the strongest method will be used (as
   returned by "methods()".

   Checking a password is usually done by passing the plain-text
   password as *word* and the full results of a previous "crypt()"
   call, which should be the same as the results of this call.

   *salt* (either a random 2 or 16 character string, possibly prefixed
   with "$digit$" to indicate the method) which will be used to
   perturb the encryption algorithm.  The characters in *salt* must be
   in the set "[./a-zA-Z0-9]", with the exception of Modular Crypt
   Format which prefixes a "$digit$".

   Returns the hashed password as a string, which will be composed of
   characters from the same alphabet as the salt.

   Since a few *crypt(3)* extensions allow different values, with
   different sizes in the *salt*, it is recommended to use  the full
   crypted password as salt when checking for a password.

   Changed in version 3.3: Accept "crypt.METHOD_*" values in addition
   to strings for *salt*.

crypt.mksalt(method=None)

   Return a randomly generated salt of the specified method.  If no
   *method* is given, the strongest method available as returned by
   "methods()" is used.

   The return value is a string either of 2 characters in length for
   "crypt.METHOD_CRYPT", or 19 characters starting with "$digit$" and
   16 random characters from the set "[./a-zA-Z0-9]", suitable for
   passing as the *salt* argument to "crypt()".

   New in version 3.3.


Examples
========

A simple example illustrating typical use (a constant-time comparison
operation is needed to limit exposure to timing attacks.
"hmac.compare_digest()" is suitable for this purpose):

   import pwd
   import crypt
   import getpass
   from hmac import compare_digest as compare_hash

   def login():
       username = input('Python login: ')
       cryptedpasswd = pwd.getpwnam(username)[1]
       if cryptedpasswd:
           if cryptedpasswd == 'x' or cryptedpasswd == '*':
               raise ValueError('no support for shadow passwords')
           cleartext = getpass.getpass()
           return compare_hash(crypt.crypt(cleartext, cryptedpasswd), cryptedpasswd)
       else:
           return True

To generate a hash of a password using the strongest available method
and check it against the original:

   import crypt
   from hmac import compare_digest as compare_hash

   hashed = crypt.crypt(plaintext)
   if not compare_hash(hashed, crypt.crypt(plaintext, hashed)):
      raise ValueError("hashed version doesn't validate against original")
