:mod:`secrets` --- Generate secure random numbers for managing secrets
======================================================================
.. module:: secrets
:synopsis: Generate secure random numbers for managing secrets.
.. moduleauthor:: Steven D'Aprano <steve+python@pearwood.info>
.. sectionauthor:: Steven D'Aprano <steve+python@pearwood.info>
.. versionadded:: 3.6
.. testsetup::
from secrets import *
__name__ = '<doctest>'
**Source code:** :source:`Lib/secrets.py`
-------------
The :mod:`secrets` module is used for generating cryptographically strong
random numbers suitable for managing data such as passwords, account
authentication, security tokens, and related secrets.
In particular, :mod:`secrets` should be used in preference to the
default pseudo-random number generator in the :mod:`random` module, which
is designed for modelling and simulation, not security or cryptography.
.. seealso::
:pep:`506`
Random numbers
--------------
The :mod:`secrets` module provides access to the most secure source of
randomness that your operating system provides.
.. class:: SystemRandom
A class for generating random numbers using the highest-quality
sources provided by the operating system. See
:class:`random.SystemRandom` for additional details.
.. function:: choice(sequence)
Return a randomly chosen element from a non-empty sequence.
.. function:: randbelow(n)
Return a random int in the range [0, *n*).
.. function:: randbits(k)
Return an int with *k* random bits.
Generating tokens
-----------------
The :mod:`secrets` module provides functions for generating secure
tokens, suitable for applications such as password resets,
hard-to-guess URLs, and similar.
.. function:: token_bytes([nbytes=None])
Return a random byte string containing *nbytes* number of bytes.
If *nbytes* is ``None`` or not supplied, a reasonable default is
used.
.. doctest::
>>> token_bytes(16) #doctest:+SKIP
b'\xebr\x17D*t\xae\xd4\xe3S\xb6\xe2\xebP1\x8b'
.. function:: token_hex([nbytes=None])
Return a random text string, in hexadecimal. The string has *nbytes*
random bytes, each byte converted to two hex digits. If *nbytes* is
``None`` or not supplied, a reasonable default is used.
.. doctest::
>>> token_hex(16) #doctest:+SKIP
'f9bf78b9a18ce6d46a0cd2b0b86df9da'
.. function:: token_urlsafe([nbytes=None])
Return a random URL-safe text string, containing *nbytes* random
bytes. The text is Base64 encoded, so on average each byte results
in approximately 1.3 characters. If *nbytes* is ``None`` or not
supplied, a reasonable default is used.
.. doctest::
>>> token_urlsafe(16) #doctest:+SKIP
'Drmhze6EPcv0fN_81Bj-nA'
How many bytes should tokens use?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
To be secure against
`brute-force attacks <https://en.wikipedia.org/wiki/Brute-force_attack>`_,
tokens need to have sufficient randomness. Unfortunately, what is
considered sufficient will necessarily increase as computers get more
powerful and able to make more guesses in a shorter period. As of 2015,
it is believed that 32 bytes (256 bits) of randomness is sufficient for
the typical use-case expected for the :mod:`secrets` module.
For those who want to manage their own token length, you can explicitly
specify how much randomness is used for tokens by giving an :class:`int`
argument to the various ``token_*`` functions. That argument is taken
as the number of bytes of randomness to use.
Otherwise, if no argument is provided, or if the argument is ``None``,
the ``token_*`` functions will use a reasonable default instead.
.. note::
That default is subject to change at any time, including during
maintenance releases.
Other functions
---------------
.. function:: compare_digest(a, b)
Return ``True`` if strings or
:term:`bytes-like objects <bytes-like object>`
*a* and *b* are equal, otherwise ``False``,
using a "constant-time compare" to reduce the risk of
`timing attacks <https://codahale.com/a-lesson-in-timing-attacks/>`_.
See :func:`hmac.compare_digest` for additional details.
Recipes and best practices
--------------------------
This section shows recipes and best practices for using :mod:`secrets`
to manage a basic level of security.
Generate an eight-character alphanumeric password:
.. testcode::
import string
import secrets
alphabet = string.ascii_letters + string.digits
password = ''.join(secrets.choice(alphabet) for i in range(8))
.. note::
Applications should not
`store passwords in a recoverable format <https://cwe.mitre.org/data/definitions/257.html>`_,
whether plain text or encrypted. They should be salted and hashed
using a cryptographically strong one-way (irreversible) hash function.
Generate a ten-character alphanumeric password with at least one
lowercase character, at least one uppercase character, and at least
three digits:
.. testcode::
import string
import secrets
alphabet = string.ascii_letters + string.digits
while True:
password = ''.join(secrets.choice(alphabet) for i in range(10))
if (any(c.islower() for c in password)
and any(c.isupper() for c in password)
and sum(c.isdigit() for c in password) >= 3):
break
Generate an `XKCD-style passphrase <https://xkcd.com/936/>`_:
.. testcode::
import secrets
# On standard Linux systems, use a convenient dictionary file.
# Other platforms may need to provide their own word-list.
with open('/usr/share/dict/words') as f:
words = [word.strip() for word in f]
password = ' '.join(secrets.choice(words) for i in range(4))
Generate a hard-to-guess temporary URL containing a security token
suitable for password recovery applications:
.. testcode::
import secrets
url = 'https://example.com/reset=' + secrets.token_urlsafe()
..
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