Add PyJWT._{de,en}code_payload hooks (#829)

* Add PyJWT._decode_payload hook

* Add PyJWT._encode_payload hook

jwt/api_jwt.py

@@ -60,12 +60,32 @@ def encode(
             if isinstance(payload.get(time_claim), datetime):
                 payload[time_claim] = timegm(payload[time_claim].utctimetuple())
 
-        json_payload = json.dumps(
-            payload, separators=(",", ":"), cls=json_encoder
-        ).encode("utf-8")
+        json_payload = self._encode_payload(
+            payload,
+            headers=headers,
+            json_encoder=json_encoder,
+        )
 
         return api_jws.encode(json_payload, key, algorithm, headers, json_encoder)
 
+    def _encode_payload(
+        self,
+        payload: Dict[str, Any],
+        headers: Optional[Dict[str, Any]] = None,
+        json_encoder: Optional[Type[json.JSONEncoder]] = None,
+    ) -> bytes:
+        """
+        Encode a given payload to the bytes to be signed.
+
+        This method is intended to be overridden by subclasses that need to
+        encode the payload in a different way, e.g. compress the payload.
+        """
+        return json.dumps(
+            payload,
+            separators=(",", ":"),
+            cls=json_encoder,
+        ).encode("utf-8")
+
     def decode_complete(
         self,
         jwt: str,
@@ -125,12 +145,7 @@ def decode_complete(
             detached_payload=detached_payload,
         )
 
-        try:
-            payload = json.loads(decoded["payload"])
-        except ValueError as e:
-            raise DecodeError(f"Invalid payload string: {e}")
-        if not isinstance(payload, dict):
-            raise DecodeError("Invalid payload string: must be a json object")
+        payload = self._decode_payload(decoded)
 
         merged_options = {**self.options, **options}
         self._validate_claims(
@@ -140,6 +155,22 @@ def decode_complete(
         decoded["payload"] = payload
         return decoded
 
+    def _decode_payload(self, decoded: Dict[str, Any]) -> Any:
+        """
+        Decode the payload from a JWS dictionary (payload, signature, header).
+
+        This method is intended to be overridden by subclasses that need to
+        decode the payload in a different way, e.g. decompress compressed
+        payloads.
+        """
+        try:
+            payload = json.loads(decoded["payload"])
+        except ValueError as e:
+            raise DecodeError(f"Invalid payload string: {e}")
+        if not isinstance(payload, dict):
+            raise DecodeError("Invalid payload string: must be a json object")
+        return payload
+
     def decode(
         self,
         jwt: str,

tests/test_compressed_jwt.py

@@ -0,0 +1,37 @@
+import json
+import zlib
+
+from jwt import PyJWT
+
+
+class CompressedPyJWT(PyJWT):
+    def _decode_payload(self, decoded):
+        return json.loads(
+            # wbits=-15 has zlib not worry about headers of crc's
+            zlib.decompress(decoded["payload"], wbits=-15).decode("utf-8")
+        )
+
+
+def test_decodes_complete_valid_jwt_with_compressed_payload():
+    # Test case from https://github.com/jpadilla/pyjwt/pull/753/files
+    example_payload = {"hello": "world"}
+    example_secret = "secret"
+    # payload made with the pako (https://nodeca.github.io/pako/) library in Javascript:
+    # Buffer.from(pako.deflateRaw('{"hello": "world"}')).toString('base64')
+    example_jwt = (
+        b"eyJhbGciOiAiSFMyNTYiLCAidHlwIjogIkpXVCJ9"
+        b".q1bKSM3JyVeyUlAqzy/KSVGqBQA="
+        b".08wHYeuh1rJXmcBcMrz6NxmbxAnCQp2rGTKfRNIkxiw="
+    )
+    decoded = CompressedPyJWT().decode_complete(
+        example_jwt, example_secret, algorithms=["HS256"]
+    )
+
+    assert decoded == {
+        "header": {"alg": "HS256", "typ": "JWT"},
+        "payload": example_payload,
+        "signature": (
+            b"\xd3\xcc\x07a\xeb\xa1\xd6\xb2W\x99\xc0\\2\xbc\xfa7"
+            b"\x19\x9b\xc4\t\xc2B\x9d\xab\x192\x9fD\xd2$\xc6,"
+        ),
+    }