feat(FN-004): merge fusion/fn-004

apps/api/app/auth/jwt.py

@@ -0,0 +1,53 @@
+from typing import Any
+
+import jwt
+
+from app.config import settings
+
+
+def decode_token(token: str) -> dict[str, Any]:
+    """Decode a JWT token.
+
+    When authentik_issuer_url is configured, validates the token
+    against the OIDC discovery document JWKS.
+    Otherwise, decodes without verification (local development only).
+    """
+    if settings.authentik_issuer_url:
+        import httpx
+
+        issuer = settings.authentik_issuer_url.rstrip("/")
+        discovery_url = f"{issuer}/.well-known/openid-configuration"
+        with httpx.Client() as client:
+            resp = client.get(discovery_url)
+            resp.raise_for_status()
+            discovery = resp.json()
+            jwks_uri = discovery["jwks_uri"]
+
+            jwks_resp = client.get(jwks_uri)
+            jwks_resp.raise_for_status()
+            jwks = jwks_resp.json()
+
+        signing_key = jwt.algorithms.RSAAlgorithm.from_jwk(
+            _find_matching_key(jwks, token)
+        )
+
+        return jwt.decode(
+            token,
+            signing_key,  # type: ignore[arg-type]
+            algorithms=["RS256"],
+            audience=settings.authentik_client_id,
+            issuer=settings.authentik_issuer_url,
+        )
+
+    return jwt.decode(token, options={"verify_signature": False})
+
+
+def _find_matching_key(jwks: dict[str, Any], token: str) -> dict[str, Any]:
+    """Find the key in JWKS that matches the token's kid header."""
+    unverified_header = jwt.get_unverified_header(token)
+    kid = unverified_header.get("kid")
+    for key in jwks.get("keys", []):
+        key_dict: dict[str, Any] = key
+        if key_dict.get("kid") == kid:
+            return key_dict
+    raise RuntimeError(f"No matching JWKS key found for kid={kid}")