=====================
Cryptographic signing
=====================

.. module:: django.core.signing
   :synopsis: Django's signing framework.

The golden rule of Web application security is to never trust data from
untrusted sources. Sometimes it can be useful to pass data through an
untrusted medium. Cryptographically signed values can be passed through an
untrusted channel safe in the knowledge that any tampering will be detected.

Django provides both a low-level API for signing values and a high-level API
for setting and reading signed cookies, one of the most common uses of
signing in Web applications.

You may also find signing useful for the following:

* Generating "recover my account" URLs for sending to users who have
  lost their password.

* Ensuring data stored in hidden form fields has not been tampered with.

* Generating one-time secret URLs for allowing temporary access to a
  protected resource, for example a downloadable file that a user has
  paid for.

Protecting the SECRET_KEY
=========================

When you create a new Django project using :djadmin:`startproject`, the
``settings.py`` file is generated automatically and gets a random
:setting:`SECRET_KEY` value. This value is the key to securing signed
data -- it is vital you keep this secure, or attackers could use it to
generate their own signed values.

Using the low-level API
=======================

Django's signing methods live in the ``django.core.signing`` module.
To sign a value, first instantiate a ``Signer`` instance::

    >>> from django.core.signing import Signer
    >>> signer = Signer()
    >>> value = signer.sign('My string')
    >>> value
    'My string:GdMGD6HNQ_qdgxYP8yBZAdAIV1w'

The signature is appended to the end of the string, following the colon.
You can retrieve the original value using the ``unsign`` method::

    >>> original = signer.unsign(value)
    >>> original
    u'My string'

If the signature or value have been altered in any way, a
``django.core.signing.BadSignature`` exception will be raised::

    >>> from django.core import signing
    >>> value += 'm'
    >>> try:
    ...    original = signer.unsign(value)
    ... except signing.BadSignature:
    ...    print("Tampering detected!")

By default, the ``Signer`` class uses the :setting:`SECRET_KEY` setting to
generate signatures. You can use a different secret by passing it to the
``Signer`` constructor::

    >>> signer = Signer('my-other-secret')
    >>> value = signer.sign('My string')
    >>> value
    'My string:EkfQJafvGyiofrdGnuthdxImIJw'

.. class:: Signer(key=None, sep=':', salt=None)

    Returns a signer which uses ``key`` to generate signatures and ``sep`` to
    separate values. ``sep`` cannot be in the `URL safe base64 alphabet
    <http://tools.ietf.org/html/rfc4648#section-5>`_.  This alphabet contains
    alphanumeric characters, hyphens, and underscores.

Using the salt argument
-----------------------

If you do not wish for every occurrence of a particular string to have the same
signature hash, you can use the optional ``salt`` argument to the ``Signer``
class. Using a salt will seed the signing hash function with both the salt and
your :setting:`SECRET_KEY`::

    >>> signer = Signer()
    >>> signer.sign('My string')
    'My string:GdMGD6HNQ_qdgxYP8yBZAdAIV1w'
    >>> signer = Signer(salt='extra')
    >>> signer.sign('My string')
    'My string:Ee7vGi-ING6n02gkcJ-QLHg6vFw'
    >>> signer.unsign('My string:Ee7vGi-ING6n02gkcJ-QLHg6vFw')
    u'My string'

Using salt in this way puts the different signatures into different
namespaces.  A signature that comes from one namespace (a particular salt
value) cannot be used to validate the same plaintext string in a different
namespace that is using a different salt setting. The result is to prevent an
attacker from using a signed string generated in one place in the code as input
to another piece of code that is generating (and verifying) signatures using a
different salt.

Unlike your :setting:`SECRET_KEY`, your salt argument does not need to stay
secret.

Verifying timestamped values
----------------------------

``TimestampSigner`` is a subclass of :class:`~Signer` that appends a signed
timestamp to the value. This allows you to confirm that a signed value was
created within a specified period of time::

    >>> from django.core.signing import TimestampSigner
    >>> signer = TimestampSigner()
    >>> value = signer.sign('hello')
    >>> value
    'hello:1NMg5H:oPVuCqlJWmChm1rA2lyTUtelC-c'
    >>> signer.unsign(value)
    u'hello'
    >>> signer.unsign(value, max_age=10)
    ...
    SignatureExpired: Signature age 15.5289158821 > 10 seconds
    >>> signer.unsign(value, max_age=20)
    u'hello'

.. class:: TimestampSigner(key=None, sep=':', salt=None)

    .. method:: sign(value)

        Sign ``value`` and append current timestamp to it.

    .. method:: unsign(value, max_age=None)

        Checks if ``value`` was signed less than ``max_age`` seconds ago,
        otherwise raises ``SignatureExpired``.

Protecting complex data structures
----------------------------------

If you wish to protect a list, tuple or dictionary you can do so using the
signing module's ``dumps`` and ``loads`` functions. These imitate Python's
pickle module, but use JSON serialization under the hood. JSON ensures that
even if your :setting:`SECRET_KEY` is stolen an attacker will not be able
to execute arbitrary commands by exploiting the pickle format.::

    >>> from django.core import signing
    >>> value = signing.dumps({"foo": "bar"})
    >>> value
    'eyJmb28iOiJiYXIifQ:1NMg1b:zGcDE4-TCkaeGzLeW9UQwZesciI'
    >>> signing.loads(value)
    {'foo': 'bar'}

.. function:: dumps(obj, key=None, salt='django.core.signing', compress=False)

    Returns URL-safe, sha1 signed base64 compressed JSON string. Serialized
    object is signed using :class:`~TimestampSigner`.

.. function:: loads(string, key=None, salt='django.core.signing', max_age=None)

    Reverse of ``dumps()``, raises ``BadSignature`` if signature fails.
    Checks ``max_age`` (in seconds) if given.
