1 files changed, 108 insertions, 15 deletions
diff --git a/Doc/library/crypt.rst b/Doc/library/crypt.rst
index 0be571e..b4c90cd 100644
--- a/Doc/library/crypt.rst
+++ b/Doc/library/crypt.rst
@@ -15,9 +15,9 @@
 
 This module implements an interface to the :manpage:`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 allowing Python scripts to
-accept typed passwords from the user, or attempting to crack Unix passwords with
-a dictionary.
+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.
 
 .. index:: single: crypt(3)
 
@@ -26,15 +26,74 @@ the :manpage:`crypt(3)` routine in the running system.  Therefore, any
 extensions available on the current implementation will also  be available on
 this module.
 
+Hashing Methods
+---------------
 
-.. function:: crypt(word, salt)
+.. versionadded:: 3.3
+
+The :mod:`crypt` module defines the list of hashing methods (not all methods
+are available on all platforms):
+
+.. data:: METHOD_SHA512
+
+   A Modular Crypt Format method with 16 character salt and 86 character
+   hash.  This is the strongest method.
+
+.. data:: METHOD_SHA256
+
+   Another Modular Crypt Format method with 16 character salt and 43
+   character hash.
+
+.. data:: METHOD_MD5
+
+   Another Modular Crypt Format method with 8 character salt and 22
+   character hash.
+
+.. data:: METHOD_CRYPT
+
+   The traditional method with a 2 character salt and 13 characters of
+   hash.  This is the weakest method.
+
+
+Module Attributes
+-----------------
+
+.. versionadded:: 3.3
+
+.. attribute:: 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 :mod:`crypt` module defines the following functions:
+
+.. function:: crypt(word, salt=None)
 
    *word* will usually be a user's password as typed at a prompt or  in a graphical
-   interface.  *salt* is usually a random two-character string which will be used
-   to perturb the DES algorithm in one of 4096 ways.  The characters in *salt* must
-   be in the set ``[./a-zA-Z0-9]``.  Returns the hashed password as a string, which
-   will be composed of characters from the same alphabet as the salt (the first two
-   characters represent the salt itself).
+   interface.  The optional *salt* is either a string as returned from
+   :func:`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
+   :func:`methods`.
+
+   Checking a password is usually done by passing the plain-text password
+   as *word* and the full results of a previous :func:`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.
 
    .. index:: single: crypt(3)
 
@@ -42,18 +101,52 @@ this module.
    different sizes in the *salt*, it is recommended to use  the full crypted
    password as salt when checking for a password.
 
-A simple example illustrating typical use::
+   .. versionchanged:: 3.3
+      Accept ``crypt.METHOD_*`` values in addition to strings for *salt*.
+
+
+.. function:: mksalt(method=None)
 
-   import crypt, getpass, pwd
+   Return a randomly generated salt of the specified method.  If no
+   *method* is given, the strongest method available as returned by
+   :func:`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 :func:`crypt`.
+
+   .. versionadded:: 3.3
+
+Examples
+--------
+
+A simple example illustrating typical use (a constant-time comparison
+operation is needed to limit exposure to timing attacks.
+:func:`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:')
+       username = input('Python login: ')
        cryptedpasswd = pwd.getpwnam(username)[1]
        if cryptedpasswd:
            if cryptedpasswd == 'x' or cryptedpasswd == '*':
-               raise "Sorry, currently no support for shadow passwords"
+               raise ValueError('no support for shadow passwords')
            cleartext = getpass.getpass()
-           return crypt.crypt(cleartext, cryptedpasswd) == cryptedpasswd
+           return compare_hash(crypt.crypt(cleartext, cryptedpasswd), cryptedpasswd)
        else:
-           return 1
+           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")