Checking for missing keys earlier

This will help catch errors where public keys are not retrievable. This is a partial solution to #283.

fusillade/utils/security.py

@@ -26,7 +26,7 @@
 
 
 @functools.lru_cache(maxsize=32)
-def get_openid_config(openid_provider=None):
+def get_openid_config(openid_provider=None) -> dict:
     """
 
     :param openid_provider: the openid provider's domain.
@@ -43,28 +43,36 @@ def get_openid_config(openid_provider=None):
     return res.json()
 
 
-def get_jwks_uri(openid_provider):
+def get_jwks_uri(openid_provider) -> str:
     if openid_provider.endswith(gserviceaccount_domain):
         return f"https://www.googleapis.com/service_accounts/v1/jwk/{openid_provider}"
     else:
         return get_openid_config(openid_provider)["jwks_uri"]
 
 
 @functools.lru_cache(maxsize=32)
-def get_public_keys(openid_provider):
+def get_public_key(issuer, kid: str) -> bytearray:
     """
     Fetches the public key from an OIDC Identity provider to verify the JWT.
-    :param openid_provider: the openid provider's domain.
-    :return: Public Keys
+    :param issuer: the openid provider's domain.
+    :param kid: the key identifier for verifying the JWT
+    :return: A Public Key
     """
-    keys = session.get(get_jwks_uri(openid_provider)).json()["keys"]
-    return {
+    public_keys = {
         key["kid"]: rsa.RSAPublicNumbers(
             e=int.from_bytes(base64.urlsafe_b64decode(key["e"] + "==="), byteorder="big"),
             n=int.from_bytes(base64.urlsafe_b64decode(key["n"] + "==="), byteorder="big")
         ).public_key(backend=default_backend())
-        for key in keys
+        for key in session.get(get_jwks_uri(issuer)).json()["keys"]
     }
+    try:
+        return public_keys[kid]
+    except KeyError:
+        logger.error({"message": "Failed to fetched public key from openid provider.",
+                      "public_keys": public_keys,
+                      "issuer": issuer,
+                      "kid": kid})
+        raise FusilladeHTTPException(503, 'Service Unavailable', "Failed to fetched public key from openid provider.")
 
 
 def verify_jwt(token: str) -> typing.Optional[typing.Mapping]:
@@ -84,15 +92,7 @@ def verify_jwt(token: str) -> typing.Optional[typing.Mapping]:
         raise FusilladeHTTPException(401, 'Unauthorized', 'Failed to decode token.')
 
     issuer = unverified_token['iss']
-    public_keys = get_public_keys(issuer)
-    try:
-        public_key = public_keys[token_header["kid"]]
-    except KeyError:
-        logger.error({"message": "Failed to fetched public key from openid provider.",
-                      "public_keys": public_keys,
-                      "issuer": issuer,
-                      "kid": token_header["kid"]})
-        raise FusilladeHTTPException(503, 'Service Unavailable', "Failed to fetched public key from openid provider.")
+    public_key = get_public_key(issuer, token_header["kid"])
     try:
         verified_tok = jwt.decode(token,
                                   key=public_key,