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Functions for creating and restoring url-safe signed JSON objects.
The format used looks like this:
>>> signing.dumps("hello")
There are two components here, separated by a ':'. The first component is a
URLsafe base64 encoded JSON of the object passed to dumps(). The second
component is a base64 encoded hmac/SHA1 hash of "$first_component:$secret"
signing.loads(s) checks the signature and returns the deserialized object.
If the signature fails, a BadSignature exception is raised.
>>> signing.loads("ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv422nZA4sgmk")
>>> signing.loads("ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv422nZA4sgmk-modified")
BadSignature: Signature failed: ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv422nZA4sgmk-modified
You can optionally compress the JSON prior to base64 encoding it to save
space, using the compress=True argument. This checks if compression actually
helps and only applies compression if the result is a shorter string:
>>> signing.dumps(range(1, 20), compress=True)
The fact that the string is compressed is signalled by the prefixed '.' at the
start of the base64 JSON.
There are 65 url-safe characters: the 64 used by url-safe base64 and the ':'.
These functions make use of all of them.
from __future__ import unicode_literals
import base64
import datetime
import json
import re
import time
import zlib
from django.conf import settings
from django.utils import baseconv
from django.utils.crypto import constant_time_compare, salted_hmac
from django.utils.encoding import force_bytes, force_str, force_text
from django.utils.module_loading import import_string
_SEP_UNSAFE = re.compile(r'^[A-z0-9-_=]*$')
class BadSignature(Exception):
Signature does not match
class SignatureExpired(BadSignature):
Signature timestamp is older than required max_age
def b64_encode(s):
return base64.urlsafe_b64encode(s).strip(b'=')
def b64_decode(s):
pad = b'=' * (-len(s) % 4)
return base64.urlsafe_b64decode(s + pad)
def base64_hmac(salt, value, key):
return b64_encode(salted_hmac(salt, value, key).digest())
def get_cookie_signer(salt='django.core.signing.get_cookie_signer'):
Signer = import_string(settings.SIGNING_BACKEND)
key = force_bytes(settings.SECRET_KEY)
return Signer(b'django.http.cookies' + key, salt=salt)
class JSONSerializer(object):
Simple wrapper around json to be used in signing.dumps and
def dumps(self, obj):
return json.dumps(obj, separators=(',', ':')).encode('latin-1')
def loads(self, data):
return json.loads(data.decode('latin-1'))
def dumps(obj, key=None, salt='django.core.signing', serializer=JSONSerializer, compress=False):
Returns URL-safe, sha1 signed base64 compressed JSON string. If key is
None, settings.SECRET_KEY is used instead.
If compress is True (not the default) checks if compressing using zlib can
save some space. Prepends a '.' to signify compression. This is included
in the signature, to protect against zip bombs.
Salt can be used to namespace the hash, so that a signed string is
only valid for a given namespace. Leaving this at the default
value or re-using a salt value across different parts of your
application without good cause is a security risk.
The serializer is expected to return a bytestring.
data = serializer().dumps(obj)
# Flag for if it's been compressed or not
is_compressed = False
if compress:
# Avoid zlib dependency unless compress is being used
compressed = zlib.compress(data)
if len(compressed) < (len(data) - 1):
data = compressed
is_compressed = True
base64d = b64_encode(data)
if is_compressed:
base64d = b'.' + base64d
return TimestampSigner(key, salt=salt).sign(base64d)
def loads(s, key=None, salt='django.core.signing', serializer=JSONSerializer, max_age=None):
Reverse of dumps(), raises BadSignature if signature fails.
The serializer is expected to accept a bytestring.
# TimestampSigner.unsign always returns unicode but base64 and zlib
# compression operate on bytes.
base64d = force_bytes(TimestampSigner(key, salt=salt).unsign(s, max_age=max_age))
decompress = False
if base64d[:1] == b'.':
# It's compressed; uncompress it first
base64d = base64d[1:]
decompress = True
data = b64_decode(base64d)
if decompress:
data = zlib.decompress(data)
return serializer().loads(data)
class Signer(object):
def __init__(self, key=None, sep=':', salt=None):
# Use of native strings in all versions of Python
self.key = key or settings.SECRET_KEY
self.sep = force_str(sep)
if _SEP_UNSAFE.match(self.sep):
raise ValueError(
'Unsafe Signer separator: %r (cannot be empty or consist of '
'only A-z0-9-_=)' % sep,
self.salt = force_str(salt or
'%s.%s' % (self.__class__.__module__, self.__class__.__name__))
def signature(self, value):
signature = base64_hmac(self.salt + 'signer', value, self.key)
# Convert the signature from bytes to str only on Python 3
return force_str(signature)
def sign(self, value):
value = force_str(value)
return str('%s%s%s') % (value, self.sep, self.signature(value))
def unsign(self, signed_value):
signed_value = force_str(signed_value)
if self.sep not in signed_value:
raise BadSignature('No "%s" found in value' % self.sep)
value, sig = signed_value.rsplit(self.sep, 1)
if constant_time_compare(sig, self.signature(value)):
return force_text(value)
raise BadSignature('Signature "%s" does not match' % sig)
class TimestampSigner(Signer):
def timestamp(self):
return baseconv.base62.encode(int(time.time()))
def sign(self, value):
value = force_str(value)
value = str('%s%s%s') % (value, self.sep, self.timestamp())
return super(TimestampSigner, self).sign(value)
def unsign(self, value, max_age=None):
Retrieve original value and check it wasn't signed more
than max_age seconds ago.
result = super(TimestampSigner, self).unsign(value)
value, timestamp = result.rsplit(self.sep, 1)
timestamp = baseconv.base62.decode(timestamp)
if max_age is not None:
if isinstance(max_age, datetime.timedelta):
max_age = max_age.total_seconds()
# Check timestamp is not older than max_age
age = time.time() - timestamp
if age > max_age:
raise SignatureExpired(
'Signature age %s > %s seconds' % (age, max_age))
return value
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