Django2.0手册:Cryptographic signing



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 startproject, the
settings.py file is generated automatically and gets a random
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
'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 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)[source]

Returns a signer which uses key to generate signatures and sep to
separate values. sep cannot be in the URL safe base64 alphabet. 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 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')
'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 SECRET_KEY, your salt argument does not need to stay
secret.

Verifying timestamped values¶

TimestampSigner is a subclass of 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 datetime import timedelta
>>> from django.core.signing import TimestampSigner
>>> signer = TimestampSigner()
>>> value = signer.sign('hello')
>>> value
'hello:1NMg5H:oPVuCqlJWmChm1rA2lyTUtelC-c'
>>> signer.unsign(value)
'hello'
>>> signer.unsign(value, max_age=10)
...
SignatureExpired: Signature age 15.5289158821 > 10 seconds
>>> signer.unsign(value, max_age=20)
'hello'
>>> signer.unsign(value, max_age=timedelta(seconds=20))
'hello'
class TimestampSigner(key=None, sep=’:’, salt=None)[source]
sign(value)[source]

Sign value and append current timestamp to it.

unsign(value, max_age=None)[source]

Checks if value was signed less than max_age seconds ago,
otherwise raises SignatureExpired. The max_age parameter can
accept an integer or a datetime.timedelta object.

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 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'}

Because of the nature of JSON (there is no native distinction between lists
and tuples) if you pass in a tuple, you will get a list from
signing.loads(object):

>>> from django.core import signing
>>> value = signing.dumps(('a','b','c'))
>>> signing.loads(value)
['a', 'b', 'c']
dumps(obj, key=None, salt=’django.core.signing’, compress=False)[source]

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

loads(string, key=None, salt=’django.core.signing’, max_age=None)[source]

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