added setting JWT_PUBLIC_KEY_USING_HEADER_HANDLER

[devsnd]

added setting JWT_PUBLIC_KEY_USING_HEADER_HANDLER to dynamically fetch public keys based on the header

when setting an import path to a callable into JWT_PUBLIC_KEY_USING_HEADER_HANDLER, it will be called with the JWT header as parameter. This allows to e.g. fetch dynamically changing certificates, like
google does it for example.

[devsnd]

Dear DRF-JWT developers,

I made this small change that allows to get a public key based on the header of the JWT token. This allows for easy integration with e.g. firebase, as the public keys and certificates are rotated hourly.

Here is some example code of how you could use this:

import requests
from OpenSSL import crypto

def get_public_key_for_payload(unverified_payload):
    kid = unverified_payload['kid']
    certificates = requests.get('https://www.googleapis.com/robot/v1/metadata/x509/securetoken@system.gserviceaccount.com').json()
    certificate = certificates[kid]
    pubkey = certificate.get_pubkey()
    pem_pubkey = crypto.dump_publickey(crypto.FILETYPE_PEM, pubkey)
    return pem_pubkey

Then you could use this callback to get the right pubkey like this:

JWT_PUBLIC_KEY_USING_HEADER_HANDLER = 'my_package.get_public_key_for_payload'

Please note that you should cache the request based using the kid as cache key in production!

[codecov]

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@@            Coverage Diff            @@
##             master     #471   +/-   ##
=========================================
  Coverage          ?   90.34%           
=========================================
  Files             ?       12           
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  Branches          ?       29           
=========================================
  Hits              ?      739           
  Misses            ?       66           
  Partials          ?       13

Flag	Coverage Δ
#codecov	90.34% <ø> (?)
#dj110	87.04% <ø> (?)
#dj111	87.04% <ø> (?)
#dj18	89.48% <ø> (?)
#dj19	89.48% <ø> (?)
#drf31	89.48% <ø> (?)
#drf32	89.48% <ø> (?)
#drf33	89.48% <ø> (?)
#drf34	90.34% <ø> (?)
#drf35	89.97% <ø> (?)
#drf36	89.97% <ø> (?)
#py27	90.34% <ø> (?)
#py34	89.97% <ø> (?)
#py35	87.04% <ø> (?)
#py36	87.04% <ø> (?)

Impacted Files	Coverage Δ
rest_framework_jwt/settings.py	100% <ø> (ø)

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[devsnd]

I fixed the JWT_AUTH settings being clobbered by DRF when calling reload on the APISettings object. This means that @override_settings decorators now work properly.

By fixing the settings being clobbered by the original APISettings implementation of
restframework, we can now listen to settings changes using the setting_changed signal.

This signal is used by the @override_settings decorator. This means that you can now perform
tests that would for example override the JWT_VERIFY_EXPIRATION setting, allowing for testing
without generating new tokens on the fly.

E.g. this works now:

@override_settings(JWT_AUTH=dict(settings.JWT_AUTH, JWT_VERIFY_EXPIRATION=False))
def test(self):
    assert can_auth_with_expired_token()