238 lines
7.9 KiB
Python
238 lines
7.9 KiB
Python
"""
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Functions for creating and restoring url-safe signed JSON objects.
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The format used looks like this:
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>>> signing.dumps("hello")
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'ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv422nZA4sgmk'
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There are two components here, separated by a ':'. The first component is a
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URLsafe base64 encoded JSON of the object passed to dumps(). The second
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component is a base64 encoded hmac/SHA1 hash of "$first_component:$secret"
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signing.loads(s) checks the signature and returns the deserialized object.
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If the signature fails, a BadSignature exception is raised.
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>>> signing.loads("ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv422nZA4sgmk")
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'hello'
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>>> signing.loads("ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv422nZA4sgmk-modified")
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...
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BadSignature: Signature failed: ImhlbGxvIg:1QaUZC:YIye-ze3TTx7gtSv422nZA4sgmk-modified
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You can optionally compress the JSON prior to base64 encoding it to save
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space, using the compress=True argument. This checks if compression actually
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helps and only applies compression if the result is a shorter string:
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>>> signing.dumps(list(range(1, 20)), compress=True)
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'.eJwFwcERACAIwLCF-rCiILN47r-GyZVJsNgkxaFxoDgxcOHGxMKD_T7vhAml:1QaUaL:BA0thEZrp4FQVXIXuOvYJtLJSrQ'
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The fact that the string is compressed is signalled by the prefixed '.' at the
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start of the base64 JSON.
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There are 65 url-safe characters: the 64 used by url-safe base64 and the ':'.
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These functions make use of all of them.
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"""
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import base64
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import datetime
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import json
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import time
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import zlib
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from django.conf import settings
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from django.utils.crypto import constant_time_compare, salted_hmac
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from django.utils.encoding import force_bytes
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from django.utils.module_loading import import_string
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from django.utils.regex_helper import _lazy_re_compile
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_SEP_UNSAFE = _lazy_re_compile(r'^[A-z0-9-_=]*$')
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BASE62_ALPHABET = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
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class BadSignature(Exception):
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"""Signature does not match."""
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pass
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class SignatureExpired(BadSignature):
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"""Signature timestamp is older than required max_age."""
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pass
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def b62_encode(s):
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if s == 0:
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return '0'
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sign = '-' if s < 0 else ''
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s = abs(s)
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encoded = ''
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while s > 0:
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s, remainder = divmod(s, 62)
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encoded = BASE62_ALPHABET[remainder] + encoded
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return sign + encoded
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def b62_decode(s):
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if s == '0':
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return 0
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sign = 1
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if s[0] == '-':
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s = s[1:]
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sign = -1
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decoded = 0
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for digit in s:
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decoded = decoded * 62 + BASE62_ALPHABET.index(digit)
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return sign * decoded
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def b64_encode(s):
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return base64.urlsafe_b64encode(s).strip(b'=')
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def b64_decode(s):
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pad = b'=' * (-len(s) % 4)
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return base64.urlsafe_b64decode(s + pad)
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def base64_hmac(salt, value, key, algorithm='sha1'):
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return b64_encode(salted_hmac(salt, value, key, algorithm=algorithm).digest()).decode()
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def get_cookie_signer(salt='django.core.signing.get_cookie_signer'):
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Signer = import_string(settings.SIGNING_BACKEND)
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key = force_bytes(settings.SECRET_KEY) # SECRET_KEY may be str or bytes.
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return Signer(b'django.http.cookies' + key, salt=salt)
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class JSONSerializer:
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"""
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Simple wrapper around json to be used in signing.dumps and
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signing.loads.
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"""
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def dumps(self, obj):
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return json.dumps(obj, separators=(',', ':')).encode('latin-1')
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def loads(self, data):
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return json.loads(data.decode('latin-1'))
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def dumps(obj, key=None, salt='django.core.signing', serializer=JSONSerializer, compress=False):
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"""
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Return URL-safe, hmac signed base64 compressed JSON string. If key is
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None, use settings.SECRET_KEY instead. The hmac algorithm is the default
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Signer algorithm.
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If compress is True (not the default), check if compressing using zlib can
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save some space. Prepend a '.' to signify compression. This is included
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in the signature, to protect against zip bombs.
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Salt can be used to namespace the hash, so that a signed string is
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only valid for a given namespace. Leaving this at the default
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value or re-using a salt value across different parts of your
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application without good cause is a security risk.
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The serializer is expected to return a bytestring.
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"""
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return TimestampSigner(key, salt=salt).sign_object(obj, serializer=serializer, compress=compress)
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def loads(s, key=None, salt='django.core.signing', serializer=JSONSerializer, max_age=None):
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"""
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Reverse of dumps(), raise BadSignature if signature fails.
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The serializer is expected to accept a bytestring.
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"""
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return TimestampSigner(key, salt=salt).unsign_object(s, serializer=serializer, max_age=max_age)
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class Signer:
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def __init__(self, key=None, sep=':', salt=None, algorithm=None):
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self.key = key or settings.SECRET_KEY
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self.sep = sep
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if _SEP_UNSAFE.match(self.sep):
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raise ValueError(
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'Unsafe Signer separator: %r (cannot be empty or consist of '
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'only A-z0-9-_=)' % sep,
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)
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self.salt = salt or '%s.%s' % (self.__class__.__module__, self.__class__.__name__)
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self.algorithm = algorithm or 'sha256'
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def signature(self, value):
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return base64_hmac(self.salt + 'signer', value, self.key, algorithm=self.algorithm)
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def sign(self, value):
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return '%s%s%s' % (value, self.sep, self.signature(value))
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def unsign(self, signed_value):
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if self.sep not in signed_value:
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raise BadSignature('No "%s" found in value' % self.sep)
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value, sig = signed_value.rsplit(self.sep, 1)
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if constant_time_compare(sig, self.signature(value)):
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return value
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raise BadSignature('Signature "%s" does not match' % sig)
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def sign_object(self, obj, serializer=JSONSerializer, compress=False):
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"""
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Return URL-safe, hmac signed base64 compressed JSON string.
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If compress is True (not the default), check if compressing using zlib
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can save some space. Prepend a '.' to signify compression. This is
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included in the signature, to protect against zip bombs.
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The serializer is expected to return a bytestring.
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"""
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data = serializer().dumps(obj)
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# Flag for if it's been compressed or not.
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is_compressed = False
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if compress:
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# Avoid zlib dependency unless compress is being used.
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compressed = zlib.compress(data)
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if len(compressed) < (len(data) - 1):
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data = compressed
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is_compressed = True
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base64d = b64_encode(data).decode()
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if is_compressed:
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base64d = '.' + base64d
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return self.sign(base64d)
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def unsign_object(self, signed_obj, serializer=JSONSerializer, **kwargs):
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# Signer.unsign() returns str but base64 and zlib compression operate
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# on bytes.
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base64d = self.unsign(signed_obj, **kwargs).encode()
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decompress = base64d[:1] == b'.'
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if decompress:
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# It's compressed; uncompress it first.
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base64d = base64d[1:]
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data = b64_decode(base64d)
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if decompress:
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data = zlib.decompress(data)
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return serializer().loads(data)
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class TimestampSigner(Signer):
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def timestamp(self):
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return b62_encode(int(time.time()))
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def sign(self, value):
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value = '%s%s%s' % (value, self.sep, self.timestamp())
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return super().sign(value)
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def unsign(self, value, max_age=None):
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"""
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Retrieve original value and check it wasn't signed more
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than max_age seconds ago.
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"""
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result = super().unsign(value)
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value, timestamp = result.rsplit(self.sep, 1)
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timestamp = b62_decode(timestamp)
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if max_age is not None:
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if isinstance(max_age, datetime.timedelta):
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max_age = max_age.total_seconds()
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# Check timestamp is not older than max_age
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age = time.time() - timestamp
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if age > max_age:
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raise SignatureExpired(
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'Signature age %s > %s seconds' % (age, max_age))
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return value
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