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103 lines
3.9 KiB
Python
103 lines
3.9 KiB
Python
"""Inbound cron-fire token verification for Chronos (Phase 4E.1).
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When NAS relays an external scheduler fire to the agent, it POSTs
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``/api/cron/fire`` with a short-lived NAS-minted JWT. This module verifies that
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JWT before any job runs — the security boundary for remotely-triggered job
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execution.
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We verify a NAS-minted JWT (the trust path the agent already has) rather than
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let an external scheduler call the agent directly: the scheduler signs with
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NAS's keys, which the agent doesn't (and shouldn't) hold. See the plan's DQ-4.
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The verifier is pluggable (``get_fire_verifier``) so the escape-hatch mode
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(direct per-job cron-key) can swap in later with no handler change.
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Crypto is delegated to PyJWT (already a declared dependency) — we do NOT
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hand-roll JWT verification.
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"""
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from __future__ import annotations
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import logging
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from typing import Any, Callable, Dict, Optional
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logger = logging.getLogger("cron.chronos.verify")
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# The purpose claim that scopes a token to the fire endpoint. A general agent
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# JWT (without this claim) must NOT be replayable against /api/cron/fire.
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_FIRE_PURPOSE = "cron_fire"
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def verify_nas_fire_token(
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*,
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token: str,
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expected_audience: str,
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jwks_or_key: Optional[str] = None,
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issuer: Optional[str] = None,
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leeway_seconds: int = 30,
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) -> Optional[Dict[str, Any]]:
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"""Verify a NAS-minted cron-fire JWT. Return decoded claims, or None.
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Checks (all must pass):
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- signature against the NAS JWKS (``jwks_or_key`` is a JWKS URL) — RS256
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family; symmetric secrets are rejected (NAS signs asymmetrically).
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- ``aud`` == ``expected_audience`` (this agent: ``agent:{instance_id}``).
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- ``exp`` / ``nbf`` within ``leeway_seconds``.
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- ``iss`` == ``issuer`` when an issuer is configured.
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- ``purpose`` == ``"cron_fire"`` — so a general agent JWT can't be
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replayed against the fire endpoint.
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Returns None (never raises) on any failure, so the handler can answer 401
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without leaking which check failed.
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"""
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if not token or not expected_audience:
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return None
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if not jwks_or_key:
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# No verification key configured → cannot verify → refuse. We never
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# fall back to unsigned decode for a security boundary.
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logger.warning("cron fire: no JWKS/key configured; refusing token")
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return None
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try:
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import jwt
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from jwt import PyJWKClient
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# Resolve the signing key from the JWKS endpoint by the token's kid.
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signing_key = None
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if jwks_or_key.startswith("http://") or jwks_or_key.startswith("https://"):
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jwk_client = PyJWKClient(jwks_or_key)
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signing_key = jwk_client.get_signing_key_from_jwt(token).key
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else:
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# A PEM public key passed inline (test / pinned-key deployments).
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signing_key = jwks_or_key
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options = {"require": ["exp", "aud"]}
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decode_kwargs: Dict[str, Any] = dict(
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algorithms=["RS256", "RS384", "RS512", "ES256", "ES384"],
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audience=expected_audience,
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leeway=leeway_seconds,
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options=options,
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)
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if issuer:
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decode_kwargs["issuer"] = issuer
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claims = jwt.decode(token, signing_key, **decode_kwargs)
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except Exception as e:
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logger.warning("cron fire: token verification failed: %s", e)
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return None
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if claims.get("purpose") != _FIRE_PURPOSE:
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logger.warning("cron fire: token missing/!=%s purpose claim", _FIRE_PURPOSE)
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return None
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return claims
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def get_fire_verifier() -> Callable[..., Optional[Dict[str, Any]]]:
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"""Return the active inbound-fire verifier.
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Default = the NAS-JWT verifier. The DQ-4 escape hatch (direct per-job
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cron-key) would return a cron-key verifier here instead, selected by config
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— so the webhook handler never changes when the auth mode is swapped.
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"""
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return verify_nas_fire_token
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