feat(core): HMAC-sign client-carried state (A1, #21)

Component state round-trips to the client unsigned — the #1 security
gap (Epic A). This adds HMAC-SHA256 state signing at the
StateSerializer choke point so every adapter is covered.

- New stdlib-only core/signing.py: StateSigner + CorruptStateError.
  Versioned token format cfs1.<b64url(payload)>.<b64url(mac)>; the MAC
  binds the version prefix to prevent cross-format confusion; verify
  uses hmac.compare_digest.
- Key config: StateSigner.configure(secret) or STATE_SIGNING_KEY env
  var. Sequence / comma-separated values enable rotation: first key
  signs, all keys verify (A2 groundwork). Unconfigured = disabled
  (legacy pass-through) with a one-time prominent warning.
- Enabled mode: all outbound state is signed; inbound MUST be a valid
  token — plain JSON strings and raw dicts raise CorruptStateError
  (no unsigned fallback).
- Closed the dict bypass: FastAPI, Flask, Litestar, Django FBV/CBV,
  and the WebSocket manager now route inbound state through the new
  StateSerializer.load_untrusted() single entry point (a client could
  previously submit state as a JSON object and skip deserialization).
- Django ComponentView.handle_error() now maps client input errors
  (ValueError, incl. corrupt state) to 400 instead of 500, matching
  the other adapters.
- Docs: docs/STATE_SIGNING.md (per-adapter setup + rotation
  procedure, covers A2 doc scope); README + CHANGELOG updated.
- Tests: 36 new tests in tests/test_signing.py (round-trip, tamper /
  truncation / garbage rejection, rotation, env pickup, disabled-mode
  parity, FastAPI integration incl. tampered-state 400).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: fsecada01

CHANGELOG.md

@@ -5,6 +5,29 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- **HMAC-signed client state (Epic A, A1 — #21)** — new stdlib-only
+  `core/signing.py` with `StateSigner` (HMAC-SHA256, versioned
+  `cfs1.<payload>.<mac>` token format) and `CorruptStateError`. Enable via
+  `StateSigner.configure(secret)` or the `STATE_SIGNING_KEY` environment
+  variable; comma-separated / sequence values enable key rotation (first key
+  signs, all keys verify). When enabled, all outbound state is signed and
+  inbound state must be a valid token — tampered, unsigned, or raw-dict state
+  is rejected with HTTP 400. When disabled, legacy plain-JSON behavior is
+  preserved and a one-time warning is logged. See `docs/STATE_SIGNING.md`.
+
+### Security
+
+- Closed the **dict bypass**: all adapters (FastAPI, Flask, Litestar, Django
+  FBV/CBV, WebSocket) now route inbound state through
+  `StateSerializer.load_untrusted()`, so a client can no longer submit state
+  as a raw JSON object to skip deserialization/verification.
+- Django `ComponentView.handle_error()` now maps client input errors
+  (`ValueError`, including corrupt state) to `400` instead of `500`.
+
 ## [0.5.0b0] - 2026-06-24
 
 ### Added

README.md

@@ -238,8 +238,9 @@ Generated from docstrings by pdoc and deployed to GitHub Pages on every push to
 | [Architecture Overview](docs/server_component_spec.md) | Core design and component lifecycle |
 | [Django Implementation](docs/DJANGO_IMPLEMENTATION.md) | Django adapter setup and patterns |
 | [Class-Based Views](docs/CBV_GUIDE.md) | CBV auth/permission patterns |
+| [State Signing](docs/STATE_SIGNING.md) | HMAC-signed client state: setup per adapter + key rotation |
 | [E-Commerce Example](docs/examples/ecommerce.md) | Real-time cart + product demo |
-| [Multi-Step Wizard](docs/examples/wizard.md) | FastAPI wizard recipe (unsigned state — pending Epic A1) |
+| [Multi-Step Wizard](docs/examples/wizard.md) | FastAPI wizard recipe (enable [state signing](docs/STATE_SIGNING.md) in production) |
 | [CSRF & CSWSH Guide](docs/SECURITY_CSRF.md) | Per-adapter CSRF coverage audit + WebSocket hijacking guidance |
 
 ### AI / LLM Context
@@ -593,7 +594,7 @@ and [epic issues](https://github.com/fsecada01/component-framework/issues?q=is%3
 > optimistic UI once it lands).
 
 **0.6.0b — Hardening Foundation** *(Tier 0: security & rendering fidelity)*
-- [ ] Signed / tamper-proof state (HMAC) — *security gate*
+- [x] Signed / tamper-proof state (HMAC) — *security gate* — see [State Signing](docs/STATE_SIGNING.md)
 - [ ] DOM morphing — preserve focus / scroll / in-flight input; stable list keys
 - [ ] CSRF coverage for FastAPI / Litestar / Flask HTTP paths (today Django-only)
 - [ ] 422 re-render on form-validation failure (adapters currently return 200)

docs/STATE_SIGNING.md

@@ -0,0 +1,148 @@
+# State Signing (HMAC)
+
+Component state round-trips through the client on every dispatch: the server
+returns a serialized `state` field, and the client echoes it back with the
+next event. Without a signature, a client can tamper with any state field
+before echoing it back.
+
+State signing closes this gap. When enabled, every outbound state string is
+an HMAC-SHA256 signed token, and every inbound state **must** be a valid
+token — plain JSON strings and raw JSON objects are rejected with
+`CorruptStateError` (HTTP 400 on all adapters).
+
+## Token format
+
+```
+cfs1.<base64url(json_payload)>.<base64url(hmac_sha256_mac)>
+```
+
+The MAC covers `cfs1.<payload>` — the version prefix is bound into the
+signature, so tokens cannot be replayed across future format versions.
+
+## Enabling signing
+
+Signing is configured once per process, at application startup. There are two
+equivalent mechanisms:
+
+### 1. Environment variable (no code changes)
+
+```bash
+export STATE_SIGNING_KEY="$(openssl rand -hex 32)"
+```
+
+Comma-separated values enable key rotation (first key signs, all verify):
+
+```bash
+export STATE_SIGNING_KEY="new-key,old-key"
+```
+
+### 2. Explicit configuration
+
+```python
+from component_framework import StateSigner
+
+StateSigner.configure("your-secret-key")            # single key
+StateSigner.configure(["new-key", "old-key"])       # rotation
+StateSigner.configure(None)                          # explicitly disable (overrides env)
+```
+
+`StateSigner.configure()` takes precedence over the environment variable.
+
+If neither is set, signing is **disabled** and the framework falls back to
+legacy plain-JSON state — a one-time warning is logged. Do not run production
+deployments unsigned.
+
+## Per-adapter setup
+
+The signer is process-global and lives below the adapter layer, so setup is
+the same everywhere: configure it before the app starts serving.
+
+### FastAPI
+
+```python
+from fastapi import FastAPI
+from component_framework import StateSigner
+from component_framework.adapters.fastapi import create_component_routes
+
+StateSigner.configure(os.environ["MY_SECRET"])  # or just set STATE_SIGNING_KEY
+
+app = FastAPI()
+create_component_routes(app)
+```
+
+### Litestar
+
+```python
+from litestar import Litestar
+from component_framework import StateSigner
+from component_framework.adapters.litestar import component_endpoint
+
+StateSigner.configure(os.environ["MY_SECRET"])
+
+app = Litestar(route_handlers=[component_endpoint])
+```
+
+### Flask
+
+```python
+from flask import Flask
+from component_framework import StateSigner
+from component_framework.adapters.flask import register_component_routes
+
+app = Flask(__name__)
+StateSigner.configure(app.config["SECRET_KEY"])  # reusing SECRET_KEY is fine
+register_component_routes(app)
+```
+
+### Django
+
+Configure in `settings.py` (or an `AppConfig.ready()` hook):
+
+```python
+# settings.py
+from component_framework import StateSigner
+
+StateSigner.configure(SECRET_KEY)  # or a dedicated key
+```
+
+## Key rotation procedure
+
+1. **Prepend** the new key: `STATE_SIGNING_KEY="new-key,old-key"` (or
+   `StateSigner.configure(["new-key", "old-key"])`). New responses are signed
+   with `new-key`; states still in flight (signed with `old-key`) continue to
+   verify.
+2. **Wait** until in-flight client states have expired — i.e. long enough that
+   no open browser tab still holds a state signed with the old key. For most
+   apps a deploy cycle or a day is plenty.
+3. **Drop** the old key: `STATE_SIGNING_KEY="new-key"`. Old tokens now fail
+   verification (clients simply re-mount their components).
+
+## WebSocket pushes
+
+WebSocket traffic goes through the same `StateSerializer` choke point:
+inbound `component_event` messages are verified, and both dispatch responses
+and server-initiated `push_update()` broadcasts carry signed state.
+
+## Error handling
+
+Tampered, truncated, unsigned, or foreign-key state raises
+`component_framework.CorruptStateError` (a `ComponentError` subclass). The
+bundled adapters map it to:
+
+| Adapter | Response |
+|---------|----------|
+| FastAPI | `400` (`HTTPException`) |
+| Litestar | `400` (`HTTPException`) |
+| Flask | `400` JSON error |
+| Django (FBV + CBV) | `400` `JsonResponse` |
+| WebSocket | `{"type": "error", ...}` frame |
+
+## What signing does and does not protect
+
+- **Protects:** integrity — the client cannot alter state fields, forge
+  state, or splice state across format versions.
+- **Does not protect:** confidentiality — the payload is base64-encoded JSON,
+  readable by the client. Never put secrets in component state.
+- **Does not prevent replay of a stale-but-valid token** for the same
+  component. Treat state as untrusted input in handlers and enforce
+  authorization server-side (see Epic A3, locked fields, for the follow-up).

src/component_framework/__init__.py

@@ -0,0 +1,8 @@
+"""Component Framework — server-driven components for Python web frameworks."""
+
+from .core.signing import CorruptStateError, StateSigner
+
+__all__ = [
+    "CorruptStateError",
+    "StateSigner",
+]

src/component_framework/adapters/django_views.py

@@ -75,13 +75,11 @@ def component_view(request: HttpRequest, name: str) -> JsonResponse:
         except json.JSONDecodeError as e:
             return JsonResponse({"error": f"Invalid JSON in payload/params: {e}"}, status=400)
 
-        # Deserialize state
-        state = None
-        if state_str:
-            try:
-                state = StateSerializer.deserialize(state_str)
-            except Exception as e:
-                return JsonResponse({"error": f"Invalid state: {e}"}, status=400)
+        # Deserialize state (verified against the signing key when enabled)
+        try:
+            state = StateSerializer.load_untrusted(state_str)
+        except Exception as e:
+            return JsonResponse({"error": f"Invalid state: {e}"}, status=400)
 
         # Compute optimistic patch BEFORE dispatch (uses pre-dispatch state).
         # Hydrate state so get_optimistic_patch can access current state values.
@@ -254,12 +252,14 @@ def parse_payload(self, payload_str: str | dict) -> dict:
         except json.JSONDecodeError as e:
             raise ValueError(f"Invalid payload JSON: {e}")
 
-    def parse_state(self, state_str: str | None) -> dict | None:
-        """Parse state JSON string."""
-        if not state_str:
-            return None
+    def parse_state(self, state_raw: str | dict | None) -> dict | None:
+        """Parse client-supplied state (signed token, JSON string, or dict).
+
+        Routes through :meth:`StateSerializer.load_untrusted` so raw dicts
+        cannot bypass signature verification when signing is enabled.
+        """
         try:
-            return StateSerializer.deserialize(state_str)
+            return StateSerializer.load_untrusted(state_raw)
         except Exception as e:
             raise ValueError(f"Invalid state: {e}")
 
@@ -304,7 +304,14 @@ def component_not_found(self, name: str) -> JsonResponse:
         return JsonResponse({"error": f"Component '{name}' not found"}, status=404)
 
     def handle_error(self, error: Exception, name: str) -> JsonResponse:
-        """Handle errors."""
+        """Handle errors.
+
+        Client input errors (bad JSON, tampered/corrupt state) map to 400;
+        everything else maps to 500.
+        """
+        if isinstance(error, ValueError):
+            logger.warning("Bad request for component '%s': %s", name, error)
+            return JsonResponse({"error": str(error)}, status=400)
         logger.exception(f"Error processing component '{name}'")
         return JsonResponse({"error": "Internal server error"}, status=500)
 

src/component_framework/adapters/fastapi.py

@@ -62,16 +62,10 @@ def _extract_params(data: dict) -> tuple[dict, str | None, dict, dict | None]:
     params = _parse_json_str(data.get("params"), default={}) or {}
     event = data.get("event")
     payload = _parse_json_str(data.get("payload"), default={}) or {}
-    state_raw = data.get("state")
-
-    state = None
-    if state_raw:
-        try:
-            state = (
-                StateSerializer.deserialize(state_raw) if isinstance(state_raw, str) else state_raw
-            )
-        except Exception as e:
-            raise HTTPException(status_code=400, detail=f"Invalid state: {e}")
+    try:
+        state = StateSerializer.load_untrusted(data.get("state"))
+    except Exception as e:
+        raise HTTPException(status_code=400, detail=f"Invalid state: {e}")
 
     return params, event, payload, state
 

src/component_framework/adapters/flask.py

@@ -98,16 +98,10 @@ def _extract_params(data: dict) -> tuple[dict, str | None, dict, dict | None]:
     params = _parse_json_str(data.get("params"), default={}) or {}
     event = data.get("event")
     payload = _parse_json_str(data.get("payload"), default={}) or {}
-    state_raw = data.get("state")
-
-    state = None
-    if state_raw:
-        try:
-            state = (
-                StateSerializer.deserialize(state_raw) if isinstance(state_raw, str) else state_raw
-            )
-        except Exception as e:
-            raise ValueError(f"Invalid state: {e}")
+    try:
+        state = StateSerializer.load_untrusted(data.get("state"))
+    except Exception as e:
+        raise ValueError(f"Invalid state: {e}")
 
     return params, event, payload, state
 

src/component_framework/adapters/litestar.py

@@ -62,16 +62,10 @@ def _extract_params(data: dict) -> tuple[dict, str | None, dict, dict | None]:
     params = _parse_json_str(data.get("params"), default={}) or {}
     event = data.get("event")
     payload = _parse_json_str(data.get("payload"), default={}) or {}
-    state_raw = data.get("state")
-
-    state = None
-    if state_raw:
-        try:
-            state = (
-                StateSerializer.deserialize(state_raw) if isinstance(state_raw, str) else state_raw
-            )
-        except Exception as e:
-            raise HTTPException(status_code=400, detail=f"Invalid state: {e}")
+    try:
+        state = StateSerializer.load_untrusted(data.get("state"))
+    except Exception as e:
+        raise HTTPException(status_code=400, detail=f"Invalid state: {e}")
 
     return params, event, payload, state
 

src/component_framework/core/__init__.py

@@ -13,15 +13,18 @@
 )
 from .registry import ComponentRegistry, registry
 from .renderer import Renderer
+from .signing import CorruptStateError, StateSigner
 from .state import InMemoryStateStore, StateStore
 from .streaming import StreamingComponent, format_sse_frame
 from .websocket import ComponentWebSocketManager, WebSocketConnection, ws_manager
 
 __all__ = [
     "Component",
     "ComponentError",
+    "CorruptStateError",
     "EventNotFoundError",
     "StateSerializer",
+    "StateSigner",
     "compose",
     "SlotRenderer",
     "FormComponent",