paskia/authcode.py

@@ -23,15 +23,15 @@ AUTH_CODE_LIFETIME = timedelta(seconds=60)
 class OIDCCode(msgspec.Struct):
     """An OIDC authorization code pending token exchange.
 
-    PKCE uses S256 only (verified at auth time).
+    PKCE uses S256 only when provided (verified at token exchange).
     """
 
     session_key: str
     created: datetime
     redirect_uri: str
     scope: str
-    nonce: str
-    code_challenge: str
+    nonce: str | None = None
+    code_challenge: str | None = None
 
 
 class CookieCode(msgspec.Struct):

paskia/fastapi/oid.py

@@ -196,23 +196,24 @@ async def _handle_authorization_code(
             status_code=400,
         )
 
-    # Verify PKCE (S256 only, enforced at auth time)
-    if not code_verifier:
-        return JSONResponse(
-            {
-                "error": "invalid_grant",
-                "error_description": "Missing code_verifier",
-            },
-            status_code=400,
-        )
-    if not _verify_pkce(code_verifier, oidc_code.code_challenge):
-        return JSONResponse(
-            {
-                "error": "invalid_grant",
-                "error_description": "Invalid code_verifier",
-            },
-            status_code=400,
-        )
+    # Verify PKCE if code_challenge was provided at authorization time
+    if oidc_code.code_challenge:
+        if not code_verifier:
+            return JSONResponse(
+                {
+                    "error": "invalid_grant",
+                    "error_description": "Missing code_verifier",
+                },
+                status_code=400,
+            )
+        if not _verify_pkce(code_verifier, oidc_code.code_challenge):
+            return JSONResponse(
+                {
+                    "error": "invalid_grant",
+                    "error_description": "Invalid code_verifier",
+                },
+                status_code=400,
+            )
 
     # Get user from session
     user = db.data().users.get(session.user_uuid)

paskia/fastapi/ws.py

@@ -158,25 +158,19 @@ async def websocket_authenticate(
             await ws.send_json({"status": 400, "detail": "Scope must include openid"})
             return
 
-        # PKCE is required with S256
-        if not code_challenge:
-            await ws.send_json(
-                {"status": 400, "detail": "PKCE code_challenge is required"}
-            )
-            return
-        if code_challenge_method and code_challenge_method != "S256":
-            await ws.send_json(
-                {
-                    "status": 400,
-                    "detail": "Only S256 code_challenge_method is supported",
-                }
-            )
-            return
-
-        # Nonce is required for OIDC
-        if not nonce:
-            await ws.send_json({"status": 400, "detail": "nonce is required for OIDC"})
-            return
+        # PKCE: when code_challenge is present, only S256 is supported
+        # If method is omitted, default to S256 per best practice (not "plain" per RFC 7636)
+        # When code_challenge is absent, ignore code_challenge_method entirely
+        if code_challenge:
+            if code_challenge_method and code_challenge_method != "S256":
+                await ws.send_json(
+                    {
+                        "status": 400,
+                        "detail": "Only S256 code_challenge_method is supported",
+                    }
+                )
+                return
+            # Default to S256 when method not specified (implicit)
 
         # Validate state parameter if provided (defensive against injection)
         if state: