diff --git a/fastapi_third_party_auth/auth.py b/fastapi_third_party_auth/auth.py
index 850211d..5dd792d 100644
--- a/fastapi_third_party_auth/auth.py
+++ b/fastapi_third_party_auth/auth.py
@@ -15,6 +15,7 @@ def test_auth(authenticated_user: IDToken = Security(auth.required)):
         return f"Hello {authenticated_user.preferred_username}"
 """
 
+from logging import getLogger
 from typing import List
 from typing import Optional
 from typing import Type
@@ -33,14 +34,16 @@ def test_auth(authenticated_user: IDToken = Security(auth.required)):
 from fastapi.security import OAuth2
 from fastapi.security import SecurityScopes
 from jose import ExpiredSignatureError
-from jose import JWTError
 from jose import jwt
-from jose.exceptions import JWTClaimsError
+from jose.exceptions import JWTClaimsError, JWKError, JWTError, JWSError
+from requests.exceptions import ConnectionError
 
 from fastapi_third_party_auth import discovery
 from fastapi_third_party_auth.grant_types import GrantType
 from fastapi_third_party_auth.idtoken_types import IDToken
 
+logger = getLogger(__name__)
+
 
 class Auth(OAuth2):
     def __init__(
@@ -81,8 +84,19 @@ def __init__(
         self.client_id = client_id
         self.idtoken_model = idtoken_model
         self.scopes = scopes
-
+        
         self.discover = discovery.configure(cache_ttl=signature_cache_ttl)
+        self.grant_types = grant_types
+
+        try:
+            flows = self.get_flows()
+        except ConnectionError as e:
+            logger.warning("Could not discover OIDC flows %s", e)
+            flows = OAuthFlows()
+
+        super().__init__(scheme_name="OIDC", flows=flows, auto_error=False)
+
+    def get_flows(self) -> OAuthFlows:
         oidc_discoveries = self.discover.auth_server(
             openid_connect_url=self.openid_connect_url
         )
@@ -91,36 +105,32 @@ def __init__(
         # }
 
         flows = OAuthFlows()
-        if GrantType.AUTHORIZATION_CODE in grant_types:
+        if GrantType.AUTHORIZATION_CODE in self.grant_types:
             flows.authorizationCode = OAuthFlowAuthorizationCode(
                 authorizationUrl=self.discover.authorization_url(oidc_discoveries),
                 tokenUrl=self.discover.token_url(oidc_discoveries),
                 # scopes=scopes_dict,
             )
 
-        if GrantType.CLIENT_CREDENTIALS in grant_types:
+        if GrantType.CLIENT_CREDENTIALS in self.grant_types:
             flows.clientCredentials = OAuthFlowClientCredentials(
                 tokenUrl=self.discover.token_url(oidc_discoveries),
                 # scopes=scopes_dict,
             )
 
-        if GrantType.PASSWORD in grant_types:
+        if GrantType.PASSWORD in self.grant_types:
             flows.password = OAuthFlowPassword(
                 tokenUrl=self.discover.token_url(oidc_discoveries),
                 # scopes=scopes_dict,
             )
 
-        if GrantType.IMPLICIT in grant_types:
+        if GrantType.IMPLICIT in self.grant_types:
             flows.implicit = OAuthFlowImplicit(
                 authorizationUrl=self.discover.authorization_url(oidc_discoveries),
                 # scopes=scopes_dict,
             )
-
-        super().__init__(
-            scheme_name="OIDC",
-            flows=flows,
-            auto_error=False,
-        )
+        
+        return flows
 
     async def __call__(self, request: Request) -> None:
         return None
@@ -189,6 +199,33 @@ def optional(
             auto_error=False,
         )
 
+
+    def _find_key(self, token: str) -> dict:
+        oidc_discoveries = self.discover.auth_server(
+            openid_connect_url=self.openid_connect_url
+        )
+        try:
+            keys = self.discover.public_keys(oidc_discoveries)["keys"]
+        except KeyError as e:
+            raise JWKError("Badly formed JWKs_uri") from e
+
+        header = jwt.get_unverified_header(token)
+        try:
+            kid = header['kid']
+        except KeyError as e:
+            raise JWTError("field 'kid' is missing from JWT headers") from e
+
+        for key in keys:
+            try:
+                key_kid = key['kid']
+            except KeyError as e:
+                raise JWKError("field 'kid' is missing from JWK") from e
+            if key_kid == kid:
+                return key
+        
+        raise JWKError(f"Could not find JWK 'kid'={kid}")
+
+
     def authenticate_user(
         self,
         security_scopes: SecurityScopes,
@@ -222,12 +259,16 @@ def authenticate_user(
                 )
             else:
                 return None
-
-        oidc_discoveries = self.discover.auth_server(
-            openid_connect_url=self.openid_connect_url
-        )
-        key = self.discover.public_keys(oidc_discoveries)
+        
+        try:
+            oidc_discoveries = self.discover.auth_server(
+                openid_connect_url=self.openid_connect_url
+            )
+        except ConnectionError as e:
+            logger.warning("Could not reach auth server %e", e)
+            raise HTTPException(503, detail="Could not reach auth server") from e
         algorithms = self.discover.signing_algos(oidc_discoveries)
+        key = self._find_key(authorization_credentials.credentials)
 
         try:
             id_token = jwt.decode(
@@ -245,7 +286,8 @@ def authenticate_user(
             )
 
             if (
-                type(id_token["aud"]) == list
+                "aud" in id_token
+                and type(id_token["aud"]) == list
                 and len(id_token["aud"]) >= 1
                 and "azp" not in id_token
             ):