secrets — Generate secure random numbers for managing secrets

Source code: Lib/secrets.py

The 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, secrets should be used in preference to the default pseudo-random number generator in the random module, which is designed for modelling and simulation, not security or cryptography.

15.3.1. Random numbers

The secrets module provides access to the most secure source of randomness that your operating system provides.

class secrets.SystemRandom

A class for generating random numbers using the highest-quality sources provided by the operating system. See random.SystemRandom for additional details.

secrets.choice(sequence)

Return a randomly-chosen element from a non-empty sequence.

secrets.randbelow(n)

Return a random int in the range [0, n).

secrets.randbits(k)

Return an int with k random bits.

15.3.2. Generating tokens

The secrets module provides functions for generating secure tokens, suitable for applications such as password resets, hard-to-guess URLs, and similar.

secrets.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.

>>> token_bytes(16)  
b'\xebr\x17D*t\xae\xd4\xe3S\xb6\xe2\xebP1\x8b'

secrets.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.

>>> token_hex(16)  
'f9bf78b9a18ce6d46a0cd2b0b86df9da'

secrets.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.

>>> token_urlsafe(16)  
'Drmhze6EPcv0fN_81Bj-nA'

15.3.2.1. 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 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 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.

15.3.3. Other functions

secrets.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. See hmac.compare_digest() for additional details.

15.3.4. Recipes and best practices

This section shows recipes and best practices for using secrets to manage a basic level of security.

Generate an eight-character alphanumeric password:

import string
alphabet = string.ascii_letters + string.digits
password = ''.join(choice(alphabet) for i in range(8))

Generate a ten-character alphanumeric password with at least one lowercase character, at least one uppercase character, and at least three digits:

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:

# 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:

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