Add support for user based secret key

In order to address this issue :
#17

I propose the following solution:
- Add a JWT_SECRET_KEY setting that should be used for hmac algorithm
- The SIGNING_KEY and VERIFYING_KEY should be used only for asymmetric
algorithm
- Add a GET_USER_SECRET_KEY setting which expect a function that will
be called with the user id as defined by the USER_ID_FIELD and return a
key that can change for instance when the user changed his password

README.rst

@@ -145,8 +145,10 @@ Some of Simple JWT's behavior can be customized through settings variables in
       'BLACKLIST_AFTER_ROTATION': True,
 
       'ALGORITHM': 'HS256',
-      'SIGNING_KEY': settings.SECRET_KEY,
+      'JWT_SECRET_KEY': settings.SECRET_KEY,
+      'SIGNING_KEY': None,
       'VERIFYING_KEY': None,
+      'GET_USER_SECRET_KEY': None
 
       'AUTH_HEADER_TYPES': ('Bearer',),
       'USER_ID_FIELD': 'id',
@@ -202,24 +204,34 @@ ALGORITHM
   key.  Likewise, the ``VERIFYING_KEY`` setting must be set to a string which
   contains an RSA public key.
 
-SIGNING_KEY
-  The signing key which is used to sign the content of generated tokens.  For
-  HMAC signing, this should be a random string with at least as many bits of
-  data as is required by the signing protocol.  For RSA signing, this
-  should be a string which contains an RSA private key which is 2048 bits or
-  longer.  Since Simple JWT defaults to using 256-bit HMAC signing, the
-  ``SIGNING_KEY`` setting defaults to the value of the ``SECRET_KEY`` setting
+JWT_SECRET_KEY
+  The signing key which is used to sign the content of generated tokens when HMAC algorithm
+  is chosen, this should be a random string with at least as many bits of
+  data as is required by the signing protocol.
+  Since Simple JWT defaults to using 256-bit HMAC signing, the
+  ``JWT_SECRET_KEY`` setting defaults to the value of the ``SECRET_KEY`` setting
   for your django project.  Although this is the most reasonable default that
   Simple JWT can provide, it is recommended that developers change this setting
-  to a value which is independent from the django project secret key.  This
-  will make changing the signing key used for tokens easier in the event that
+  to a value which is independent from the django project secret key.
+  This will make changing the signing key used for tokens easier in the event that
   it is compromised.
 
+GET_USER_SECRET_KEY
+  A function that will be called with the value of ``USER_ID_FIELD`` when decoding a token.
+  It should return a secret key that is user dependant. This lets you specify a secret key for
+  a user and for instance make the key change once the user changes his password in order to
+  invalidate token created before the password change.
+
+SIGNING_KEY
+  The signing key which is used to sign the content of generated tokens when RSA algorithm
+  is chosen, this should be a string which contains an RSA private key which is 2048 bits or
+  longer.
+
 VERIFYING_KEY
   The verifying key which is used to verify the content of generated tokens.
   If an HMAC algorithm has been specified by the ``ALGORITHM`` setting, the
   ``VERIFYING_KEY`` setting will be ignored and the value of the
-  ``SIGNING_KEY`` setting will be used.  If an RSA algorithm has been specified
+  ``JWT_SECRET_KEY`` setting will be used.  If an RSA algorithm has been specified
   by the ``ALGORITHM`` setting, the ``VERIFYING_KEY`` setting must be set to a
   string which contains an RSA public key.
 

rest_framework_simplejwt/backends.py

@@ -6,6 +6,7 @@
 
 from .exceptions import TokenBackendError
 from .utils import format_lazy
+from .settings import api_settings
 
 ALLOWED_ALGORITHMS = (
     'HS256',
@@ -18,22 +19,33 @@
 
 
 class TokenBackend(object):
-    def __init__(self, algorithm, signing_key=None, verifying_key=None):
+    def __init__(self, algorithm, signing_key=None, verifying_key=None, secret_key=None, get_user_secret_key=None):
         if algorithm not in ALLOWED_ALGORITHMS:
             raise TokenBackendError(format_lazy(_("Unrecognized algorithm type '{}'"), algorithm))
 
         self.algorithm = algorithm
         self.signing_key = signing_key
-        if algorithm.startswith('HS'):
-            self.verifying_key = signing_key
-        else:
-            self.verifying_key = verifying_key
+        self.verifying_key = verifying_key
+        self.secret_key = secret_key
+        self.get_user_secret_key = get_user_secret_key
+
+    def get_secret_key(self, payload):
+        if self.get_user_secret_key:
+            return self.get_user_secret_key(payload[api_settings.USER_ID_FIELD])
+        return self.secret_key
+
+    def get_signing_key(self, payload):
+        return self.signing_key or self.get_secret_key(payload)
+
+    def get_verifying_key(self, payload):
+        return self.verifying_key or self.get_secret_key(payload)
 
     def encode(self, payload):
         """
         Returns an encoded token for the given payload dictionary.
         """
-        token = jwt.encode(payload, self.signing_key, algorithm=self.algorithm)
+        signing_key = self.get_signing_key(payload)
+        token = jwt.encode(payload, signing_key, algorithm=self.algorithm)
         return token.decode('utf-8')
 
     def decode(self, token, verify=True):
@@ -45,6 +57,8 @@ def decode(self, token, verify=True):
         signature check fails, or if its 'exp' claim indicates it has expired.
         """
         try:
-            return jwt.decode(token, self.verifying_key, algorithms=[self.algorithm], verify=verify)
+            unverified_payload = jwt.decode(token, None, False)
+            verifying_key = self.get_verifying_key(unverified_payload)
+            return jwt.decode(token, verifying_key, algorithms=[self.algorithm], verify=verify)
         except InvalidTokenError:
             raise TokenBackendError(_('Token is invalid or expired'))

rest_framework_simplejwt/settings.py

@@ -18,7 +18,9 @@
     'BLACKLIST_AFTER_ROTATION': True,
 
     'ALGORITHM': 'HS256',
-    'SIGNING_KEY': settings.SECRET_KEY,
+    'JWT_SECRET_KEY': settings.SECRET_KEY,
+    'GET_USER_SECRET_KEY': None,
+    'SIGNING_KEY': None,
     'VERIFYING_KEY': None,
 
     'AUTH_HEADER_TYPES': ('Bearer',),

rest_framework_simplejwt/state.py

@@ -6,5 +6,10 @@
 from .settings import api_settings
 
 User = get_user_model()
-token_backend = TokenBackend(api_settings.ALGORITHM, api_settings.SIGNING_KEY,
-                             api_settings.VERIFYING_KEY)
+token_backend = TokenBackend(
+    api_settings.ALGORITHM,
+    signing_key=api_settings.SIGNING_KEY,
+    verifying_key=api_settings.VERIFYING_KEY,
+    secret_key=api_settings.JWT_SECRET_KEY,
+    get_user_secret_key=api_settings.GET_USER_SECRET_KEY
+)

tests/test_backends.py

@@ -7,6 +7,7 @@
 from rest_framework_simplejwt.backends import TokenBackend
 from rest_framework_simplejwt.exceptions import TokenBackendError
 from rest_framework_simplejwt.utils import aware_utcnow, make_utc
+from random import randint
 
 SECRET = 'not_secret'
 
@@ -55,8 +56,9 @@
 
 class TestTokenBackend(TestCase):
     def setUp(self):
-        self.hmac_token_backend = TokenBackend('HS256', SECRET)
-        self.rsa_token_backend = TokenBackend('RS256', PRIVATE_KEY, PUBLIC_KEY)
+        self.hmac_token_backend = TokenBackend('HS256', secret_key=SECRET)
+        self.hmac_custom_user_key_token_backend = TokenBackend('HS256', get_user_secret_key= lambda a: a)
+        self.rsa_token_backend = TokenBackend('RS256', signing_key=PRIVATE_KEY, verifying_key=PUBLIC_KEY)
         self.payload = {'foo': 'bar'}
 
     def test_init(self):
@@ -96,6 +98,31 @@ def test_encode_rsa(self):
             ),
         )
 
+    def test_encode_hmac_custom(self):
+        payload = {
+            'id': '1234'
+        }
+        hmac_token = self.hmac_custom_user_key_token_backend.encode(payload)
+        self.assertIn(
+            hmac_token,
+            (
+                'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6IjEyMzQifQ.RMZuO9SRBYS0pLh8DVhvknBSs80OfFvzxbl-y9b5pnc'
+                'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpZCI6IjEyMzQifQ.1weakZ8seqK0UKjpLrpfJaCEr9B7rJiYLGa1TR64QF4'
+            )
+        )
+
+    def test_decode_hmac_custom_changed_user(self):
+        token_backend =  TokenBackend('HS256', get_user_secret_key= lambda a: str(a) + str(randint(0, 1000000000)))
+        payload = {
+            'exp': make_utc(datetime(year=2000, month=1, day=1)),
+            'id': '1234'
+        }
+
+        token = token_backend.encode(payload)
+
+        with self.assertRaises(TokenBackendError):
+            self.hmac_token_backend.decode(token)
+
     def test_decode_hmac_with_no_expiry(self):
         no_exp_token = jwt.encode(self.payload, SECRET, algorithm='HS256')
 
@@ -162,7 +189,7 @@ def test_decode_rsa_with_no_expiry_no_verify(self):
         no_exp_token = jwt.encode(self.payload, PRIVATE_KEY, algorithm='RS256')
 
         self.assertEqual(
-            self.hmac_token_backend.decode(no_exp_token, verify=False),
+            self.rsa_token_backend.decode(no_exp_token, verify=False),
             self.payload,
         )
 
@@ -198,7 +225,7 @@ def test_decode_rsa_with_invalid_sig_no_verify(self):
         invalid_token = token_2_payload + '.' + token_1_sig
 
         self.assertEqual(
-            self.hmac_token_backend.decode(invalid_token, verify=False),
+            self.rsa_token_backend.decode(invalid_token, verify=False),
             self.payload,
         )
 

tests/test_tokens.py

@@ -97,9 +97,9 @@ def test_init_bad_sig_token_given(self):
         # Test backend rejects encoded token (expired or bad signature)
         payload = {'foo': 'bar'}
         payload['exp'] = aware_utcnow() + timedelta(days=1)
-        token_1 = jwt.encode(payload, api_settings.SIGNING_KEY, algorithm='HS256')
+        token_1 = jwt.encode(payload, api_settings.JWT_SECRET_KEY, algorithm='HS256')
         payload['foo'] = 'baz'
-        token_2 = jwt.encode(payload, api_settings.SIGNING_KEY, algorithm='HS256')
+        token_2 = jwt.encode(payload, api_settings.JWT_SECRET_KEY, algorithm='HS256')
 
         token_2_payload = token_2.rsplit('.', 1)[0]
         token_1_sig = token_1.rsplit('.', 1)[-1]
@@ -112,9 +112,9 @@ def test_init_bad_sig_token_given_no_verify(self):
         # Test backend rejects encoded token (expired or bad signature)
         payload = {'foo': 'bar'}
         payload['exp'] = aware_utcnow() + timedelta(days=1)
-        token_1 = jwt.encode(payload, api_settings.SIGNING_KEY, algorithm='HS256')
+        token_1 = jwt.encode(payload, api_settings.JWT_SECRET_KEY, algorithm='HS256')
         payload['foo'] = 'baz'
-        token_2 = jwt.encode(payload, api_settings.SIGNING_KEY, algorithm='HS256')
+        token_2 = jwt.encode(payload, api_settings.JWT_SECRET_KEY, algorithm='HS256')
 
         token_2_payload = token_2.rsplit('.', 1)[0]
         token_1_sig = token_1.rsplit('.', 1)[-1]