# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το 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")
    u'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.
    """

    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
        """
        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())

    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
        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):
        """
        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