• Language: en

Source code for django.core.signing

"""
Functions for creating and restoring url-safe signed JSON objects.

The format used looks like this:

>>> signing.dumps("hello")
'ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv422nZA4sgmk'

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/SHA-256 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")
'hello'
>>> signing.loads("ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv42-modified")
...
BadSignature: Signature "ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv42-modified" does not match

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(list(range(1, 20)), compress=True)
'.eJwFwcERACAIwLCF-rCiILN47r-GyZVJsNgkxaFxoDgxcOHGxMKD_T7vhAml:1QaUaL:BA0thEZrp4FQVXIXuOvYJtLJSrQ'

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.
"""

import base64
import datetime
import json
import time
import zlib

from django.conf import settings
from django.utils.crypto import constant_time_compare, salted_hmac
from django.utils.encoding import force_bytes
from django.utils.module_loading import import_string
from django.utils.regex_helper import _lazy_re_compile

_SEP_UNSAFE = _lazy_re_compile(r"^[A-z0-9-_=]*$")
BASE62_ALPHABET = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"


class BadSignature(Exception):
    """Signature does not match."""

    pass


class SignatureExpired(BadSignature):
    """Signature timestamp is older than required max_age."""

    pass


def b62_encode(s):
    if s == 0:
        return "0"
    sign = "-" if s < 0 else ""
    s = abs(s)
    encoded = ""
    while s > 0:
        s, remainder = divmod(s, 62)
        encoded = BASE62_ALPHABET[remainder] + encoded
    return sign + encoded


def b62_decode(s):
    if s == "0":
        return 0
    sign = 1
    if s[0] == "-":
        s = s[1:]
        sign = -1
    decoded = 0
    for digit in s:
        decoded = decoded * 62 + BASE62_ALPHABET.index(digit)
    return sign * decoded


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, algorithm="sha1"):
    return b64_encode(
        salted_hmac(salt, value, key, algorithm=algorithm).digest()
    ).decode()


def _cookie_signer_key(key):
    # SECRET_KEYS items may be str or bytes.
    return b"django.http.cookies" + force_bytes(key)


def get_cookie_signer(salt="django.core.signing.get_cookie_signer"):
    Signer = import_string(settings.SIGNING_BACKEND)
    return Signer(
        key=_cookie_signer_key(settings.SECRET_KEY),
        fallback_keys=map(_cookie_signer_key, settings.SECRET_KEY_FALLBACKS),
        salt=salt,
    )


class JSONSerializer:
    """
    Simple wrapper around json to be used in signing.dumps and
    signing.loads.
    """

    def dumps(self, obj):
        return json.dumps(obj, separators=(",", ":")).encode("latin-1")

    def loads(self, data):
        return json.loads(data.decode("latin-1"))


[docs]def dumps( obj, key=None, salt="django.core.signing", serializer=JSONSerializer, compress=False ): """ Return URL-safe, hmac signed base64 compressed JSON string. If key is None, use settings.SECRET_KEY instead. The hmac algorithm is the default Signer algorithm. If compress is True (not the default), check if compressing using zlib can save some space. Prepend 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. """ return TimestampSigner(key, salt=salt).sign_object( obj, serializer=serializer, compress=compress )
[docs]def loads( s, key=None, salt="django.core.signing", serializer=JSONSerializer, max_age=None, fallback_keys=None, ): """ Reverse of dumps(), raise BadSignature if signature fails. The serializer is expected to accept a bytestring. """ return TimestampSigner(key, salt=salt, fallback_keys=fallback_keys).unsign_object( s, serializer=serializer, max_age=max_age, )
[docs]class Signer: def __init__( self, key=None, sep=":", salt=None, algorithm=None, fallback_keys=None, ): self.key = key or settings.SECRET_KEY self.fallback_keys = ( fallback_keys if fallback_keys is not None else settings.SECRET_KEY_FALLBACKS ) self.sep = 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 = salt or "%s.%s" % ( self.__class__.__module__, self.__class__.__name__, ) self.algorithm = algorithm or "sha256" def signature(self, value, key=None): key = key or self.key return base64_hmac(self.salt + "signer", value, key, algorithm=self.algorithm) def sign(self, value): return "%s%s%s" % (value, self.sep, self.signature(value)) def unsign(self, 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) for key in [self.key, *self.fallback_keys]: if constant_time_compare(sig, self.signature(value, key)): return value raise BadSignature('Signature "%s" does not match' % sig) def sign_object(self, obj, serializer=JSONSerializer, compress=False): """ Return URL-safe, hmac signed base64 compressed JSON string. If compress is True (not the default), check if compressing using zlib can save some space. Prepend a '.' to signify compression. This is included in the signature, to protect against zip bombs. 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).decode() if is_compressed: base64d = "." + base64d return self.sign(base64d) def unsign_object(self, signed_obj, serializer=JSONSerializer, **kwargs): # Signer.unsign() returns str but base64 and zlib compression operate # on bytes. base64d = self.unsign(signed_obj, **kwargs).encode() decompress = base64d[:1] == b"." if decompress: # It's compressed; uncompress it first. base64d = base64d[1:] data = b64_decode(base64d) if decompress: data = zlib.decompress(data) return serializer().loads(data)
[docs]class TimestampSigner(Signer): def timestamp(self): return b62_encode(int(time.time()))
[docs] def sign(self, value): value = "%s%s%s" % (value, self.sep, self.timestamp()) return super().sign(value)
[docs] def unsign(self, value, max_age=None): """ Retrieve original value and check it wasn't signed more than max_age seconds ago. """ result = super().unsign(value) value, timestamp = result.rsplit(self.sep, 1) timestamp = b62_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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