Add JWTPayload(dict) for extended verification

The JWTPayload class allows PyJWT.decode() to expose header, signature,
signing_input, and compute_hash_digest() (based on header) without
changing the pyjwt API in a breaking way.
Merely making this info accessible to the client (without specify an
additional verification callback scheme) is simpler for everyone.

Include doc on why JWTPayload is a good idea (in a docstring), since
it's a little unusual to subclass `dict`. The intent is to make the JWT
payload change as little as possible while still making it easy to add
more verification after the fact.

Add a simple test for `JWTPayload.compute_hash_digest()`

Closes #314, #295

jwt/api_jws.py

@@ -99,11 +99,17 @@ def encode(self, payload, key, algorithm='HS256', headers=None,
 
         # Segments
         signing_input = b'.'.join(segments)
-        try:
-            alg_obj = self._algorithms[algorithm]
-            key = alg_obj.prepare_key(key)
-            signature = alg_obj.sign(signing_input, key)
+        alg_obj = self.get_algo_by_name(algorithm)
+        key = alg_obj.prepare_key(key)
+        signature = alg_obj.sign(signing_input, key)
+
+        segments.append(base64url_encode(signature))
+
+        return b'.'.join(segments)
 
+    def get_algo_by_name(self, algorithm):
+        try:
+            return self._algorithms[algorithm]
         except KeyError:
             if not has_crypto and algorithm in requires_cryptography:
                 raise NotImplementedError(
@@ -113,10 +119,6 @@ def encode(self, payload, key, algorithm='HS256', headers=None,
             else:
                 raise NotImplementedError('Algorithm not supported')
 
-        segments.append(base64url_encode(signature))
-
-        return b'.'.join(segments)
-
     def decode(self, jws, key='', verify=True, algorithms=None, options=None,
                **kwargs):
 
@@ -138,8 +140,7 @@ def decode(self, jws, key='', verify=True, algorithms=None, options=None,
                           'Please use verify_signature in options instead.',
                           DeprecationWarning, stacklevel=2)
         elif verify_signature:
-            self._verify_signature(payload, signing_input, header, signature,
-                                   key, algorithms)
+            self._verify_signature(payload, signing_input, header, signature, key, algorithms)
 
         return payload
 
@@ -191,7 +192,7 @@ def _load(self, jwt):
         except (TypeError, binascii.Error):
             raise DecodeError('Invalid crypto padding')
 
-        return (payload, signing_input, header, signature)
+        return payload, signing_input, header, signature
 
     def _verify_signature(self, payload, signing_input, header, signature,
                           key='', algorithms=None):

jwt/api_jwt.py

@@ -5,6 +5,7 @@
 from datetime import datetime, timedelta
 
 from .api_jws import PyJWS
+from .jwt_payload import JWTPayload
 from .algorithms import Algorithm, get_default_algorithms  # NOQA
 from .compat import string_types
 from .exceptions import (
@@ -88,7 +89,7 @@ def decode(self, jwt, key='', verify=True, algorithms=None, options=None,
             merged_options = merge_dict(self.options, options)
             self._validate_claims(payload, merged_options, **kwargs)
 
-        return payload
+        return JWTPayload(self, payload, signing_input, header, signature)
 
     def _validate_claims(self, payload, options, audience=None, issuer=None,
                          leeway=0, **kwargs):

jwt/jwt_payload.py

@@ -0,0 +1,78 @@
+"""
+= JWTPayload
+
+A JWTPayload is the result of PyJWT.decode()
+
+It
+- is a dict (namely, the decoded payload)
+- has signing_input, header, and signature as attributes
+- exposes JWTPayload.compute_hash_digest(<string>)
+  which selects a hash algo (and implementation) based on the header and uses
+  it to compute a message digest
+
+
+== Design Decision: Why JWTPayload?
+
+This implementation path was chosen to handle a desire to support additional
+verification of JWTs without changing the API of pyjwt to v2.0
+
+Because JWTPayload inherits from dict, it behaves the same as the raw dict
+objects that PyJWT.decode() used to return (prior to this addition). Unless you
+check `type(PyJWT.decode()) is dict`, you likely won't see any change.
+
+It exposes the information previously hidden by PyJWT.decode to allow complex
+verification methods to be added to pyjwt client code (rather than baked into
+pyjwt itself).
+
+It also allows carefully selected methods (like compute_hash_digest) to be
+exposed which are derived from these data.
+"""
+try:
+    from cryptography.hazmat.primitives import hashes
+    from cryptography.hazmat.backends import default_backend
+    has_crypto = True
+except ImportError:
+    has_crypto = False
+
+
+class JWTPayload(dict):
+    """
+    A decoded JWT payload.
+    When treated directly as a dict, represents the JWT Payload (which is
+    typically what clients want).
+
+    :ivar signing_input: The signing input as a bytestring
+    :ivar header: The JWT header as a **dict**
+    :ivar signature: The JWT signature as a string
+    """
+    def __init__(self, jwt_api, payload, signing_input, header, signature,
+                 *args, **kwargs):
+        super(JWTPayload, self).__init__(payload, *args, **kwargs)
+        self.signing_input = signing_input
+        self.header = header
+        self.signature = signature
+
+        self._jwt_api = jwt_api
+
+    def compute_hash_digest(self, bytestr):
+        """
+        Given a bytestring, compute a hash digest of the bytestring and
+        return it, using the algorithm specified by the JWT header.
+
+        When `cryptography` is present, it will be used.
+
+        This method is necessary in order to support computation of the OIDC
+        at_hash claim.
+        """
+        algorithm = self.header.get('alg')
+        alg_obj = self._jwt_api.get_algo_by_name(algorithm)
+        hash_alg = alg_obj.hash_alg
+
+        if has_crypto and (
+                isinstance(hash_alg, type) and
+                issubclass(hash_alg, hashes.HashAlgorithm)):
+            digest = hashes.Hash(hash_alg(), backend=default_backend())
+            digest.update(bytestr)
+            return digest.finalize()
+        else:
+            return hash_alg(bytestr).digest()

tests/test_api_jwt.py

@@ -1,4 +1,4 @@
-
+import hashlib
 import json
 import time
 from calendar import timegm
@@ -511,3 +511,11 @@ def test_decode_no_algorithms_verify_false(self, jwt, payload):
             pass
         else:
             assert False, "Unexpected DeprecationWarning raised."
+
+    def test_decoded_payload_can_compute_hash(self, jwt, payload):
+        secret = 'secret'
+        jwt_message = jwt.encode(payload, secret, algorithm='HS256')
+        decoded_payload = jwt.decode(jwt_message, secret)
+
+        assert (decoded_payload.compute_hash_digest(b'abc') ==
+                hashlib.sha256(b'abc').digest())