# documentação Django 2.0.8

# Código fonte para 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/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")
    'hello'
    >>> 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)
    '.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 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
    from django.utils.module_loading import import_string

    _SEP_UNSAFE = re.compile(r'^[A-z0-9-_=]*$')

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

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

    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()).decode()

    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:
        """
        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'))

    def dumps(obj, key=None, salt='django.core.signing', serializer=JSONSerializer, compress=False):
        """
        Return URL-safe, hmac/SHA1 signed base64 compressed JSON string. If key is
        None, use settings.SECRET_KEY instead.

        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.
        """
        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 TimestampSigner(key, salt=salt).sign(base64d)

    def loads(s, key=None, salt='django.core.signing', serializer=JSONSerializer, max_age=None):
        """
        Reverse of dumps(), raise BadSignature if signature fails.

        The serializer is expected to accept a bytestring.
        """
        # TimestampSigner.unsign() returns str 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:

        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 = 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__)

        def signature(self, value):
            return base64_hmac(self.salt + 'signer', value, self.key)

        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)
            if constant_time_compare(sig, self.signature(value)):
                return 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 = '%s%s%s' % (value, self.sep, self.timestamp())
            return super().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().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