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.

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