Python 3.6.5 Documentation >  "crypt" — Function to check Unix passwords

"crypt" — Function to check Unix passwords
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**Source code:** Lib/crypt.py

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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.

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")