:mod:`secrets` --- Generate secure random numbers for managing secrets ====================================================================== .. module:: secrets :synopsis: Generate secure random numbers for managing secrets. .. moduleauthor:: Steven D'Aprano .. sectionauthor:: Steven D'Aprano .. versionadded:: 3.6 .. testsetup:: from secrets import * __name__ = '' **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 `_, 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``, using a "constant-time compare" to reduce the risk of `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 `_, 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 `_: .. 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() .. # This modeline must appear within the last ten lines of the file. kate: indent-width 3; remove-trailing-space on; replace-tabs on; encoding utf-8;