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"""
Django's standard crypto functions and utilities.
"""
import hashlib
import hmac
import random
import time
from django.conf import settings
from django.utils.encoding import force_bytes
# Use the system PRNG if possible
try:
random = random.SystemRandom()
using_sysrandom = True
except NotImplementedError:
import warnings
warnings.warn('A secure pseudo-random number generator is not available '
'on your system. Falling back to Mersenne Twister.')
using_sysrandom = False
def salted_hmac(key_salt, value, secret=None):
"""
Return the HMAC-SHA1 of 'value', using a key generated from key_salt and a
secret (which defaults to settings.SECRET_KEY).
A different key_salt should be passed in for every application of HMAC.
"""
if secret is None:
secret = settings.SECRET_KEY
key_salt = force_bytes(key_salt)
secret = force_bytes(secret)
# We need to generate a derived key from our base key. We can do this by
# passing the key_salt and our base key through a pseudo-random function and
# SHA1 works nicely.
key = hashlib.sha1(key_salt + secret).digest()
# If len(key_salt + secret) > sha_constructor().block_size, the above
# line is redundant and could be replaced by key = key_salt + secret, since
# the hmac module does the same thing for keys longer than the block size.
# However, we need to ensure that we *always* do this.
return hmac.new(key, msg=force_bytes(value), digestmod=hashlib.sha1)
def get_random_string(length=12,
allowed_chars='abcdefghijklmnopqrstuvwxyz'
'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'):
"""
Return a securely generated random string.
The default length of 12 with the a-z, A-Z, 0-9 character set returns
a 71-bit value. log_2((26+26+10)^12) =~ 71 bits
"""
if not using_sysrandom:
# This is ugly, and a hack, but it makes things better than
# the alternative of predictability. This re-seeds the PRNG
# using a value that is hard for an attacker to predict, every
# time a random string is required. This may change the
# properties of the chosen random sequence slightly, but this
# is better than absolute predictability.
random.seed(
hashlib.sha256(
('%s%s%s' % (random.getstate(), time.time(), settings.SECRET_KEY)).encode()
).digest()
)
return ''.join(random.choice(allowed_chars) for i in range(length))
def constant_time_compare(val1, val2):
"""Return True if the two strings are equal, False otherwise."""
return hmac.compare_digest(force_bytes(val1), force_bytes(val2))
def pbkdf2(password, salt, iterations, dklen=0, digest=None):
"""Return the hash of password using pbkdf2."""
if digest is None:
digest = hashlib.sha256
if not dklen:
dklen = None
password = force_bytes(password)
salt = force_bytes(salt)
return hashlib.pbkdf2_hmac(digest().name, password, salt, iterations, dklen)