:mod:secrets --- Generate secure random numbers for managing secrets

.. module:: secrets :synopsis: Generate secure random numbers for managing secrets.

.. moduleauthor:: Steven D'Aprano [email protected] .. sectionauthor:: Steven D'Aprano [email protected] .. 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 particularly, :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 a and b are equal, otherwise False, in such a way as 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 alphabet = string.ascii_letters + string.digits password = ''.join(choice(alphabet) for i in range(8))

.. note::

Applications should not store passwords in a recoverable format <http://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 alphabet = string.ascii_letters + string.digits while True: password = ''.join(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::

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(choice(words) for i in range(4))

Generate a hard-to-guess temporary URL containing a security token suitable for password recovery applications:

.. testcode::

url = 'https://mydomain.com/reset=' + token_urlsafe()

..

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