feat(P04): KMS ECDSA P-256 signing + DER->raw conversion (REQ-337, C-1.1, security-engineer)

---ci---
project: acdl
phase: 4
milestone: v1.28
status: execute
persona: security-engineer
---

core/kms_signing.py

@@ -0,0 +1,151 @@
+"""KMS-signed JWT issuance for the Nova IdP (REQ-337, REQ-336).
+
+Signs OIDC tokens with an AWS KMS asymmetric key (``ECC_NIST_P256``,
+``ECDSA_SHA_256`` → JWS ``ES256``) and exposes the public key as a JWK
+for the JWKS endpoint (REQ-338).
+
+## DER → raw ECDSA conversion (the #1 gotcha, RESEARCH §5)
+
+KMS ``sign()`` returns a **DER-encoded** ASN.1 ECDSA signature. JWS
+(RFC 7515 §3.1.3) requires the **raw** ``r‖s`` concatenation, each
+coordinate 32 bytes big-endian. :func:`der_to_raw_ecdsa` performs the
+conversion via ``cryptography``'s ``decode_dss_signature``. This is the
+core of REQ-337 and is verified by the CAP-037 round-trip test.
+
+## Lazy boto3
+
+``boto3.client("kms")`` is constructed lazily so the module imports
+without AWS creds (mirrors ``nova_idp_auth.py``). Tests inject a mock
+client via :func:`set_kms_client_for_testing`.
+"""
+
+from __future__ import annotations
+
+import base64
+import json
+import os
+from typing import Any
+
+import boto3
+
+from cryptography.hazmat.primitives.asymmetric.utils import decode_dss_signature
+from cryptography.hazmat.primitives.asymmetric.ec import (
+    EllipticCurvePublicKey,
+)
+from cryptography.hazmat.primitives.serialization import load_der_public_key
+from cryptography.hazmat.primitives.asymmetric import ec
+
+
+# Default KMS key alias for Nova OIDC signing (REQ-337).
+DEFAULT_KEY_ID = os.environ.get("NOVA_OIDC_KMS_KEY_ID", "alias/nova-oidc-signing")
+
+_kms_client = None
+
+
+def _get_kms_client():
+    """Lazy boto3 KMS client singleton (mirrors nova_idp_auth.py)."""
+    global _kms_client
+    if _kms_client is None:
+        _kms_client = boto3.client("kms")
+    return _kms_client
+
+
+def set_kms_client_for_testing(client: Any) -> None:
+    """Inject a mock KMS client for tests (no real AWS calls)."""
+    global _kms_client
+    _kms_client = client
+
+
+def _b64url(data: bytes) -> str:
+    """Base64url encode without padding (RFC 7515 §2)."""
+    return base64.urlsafe_b64encode(data).rstrip(b"=").decode("ascii")
+
+
+def der_to_raw_ecdsa(der_sig: bytes, coord_len: int = 32) -> bytes:
+    """Convert a DER-encoded ECDSA signature to raw ``r‖s`` (JWS format).
+
+    KMS returns DER; JWS requires raw ``r‖s`` concatenation, each
+    coordinate ``coord_len`` bytes big-endian (32 for P-256, 48 for
+    P-384). Uses ``cryptography``'s ``decode_dss_signature`` to parse
+    the DER, then zero-pads each integer to ``coord_len``.
+
+    Raises:
+        ValueError: if a coordinate does not fit in ``coord_len`` bytes
+            (the integer is larger than the curve allows — indicates a
+            malformed signature or wrong ``coord_len``).
+    """
+    r, s = decode_dss_signature(der_sig)
+    if r.bit_length() > coord_len * 8 or s.bit_length() > coord_len * 8:
+        raise ValueError(
+            f"ECDSA coordinate does not fit in {coord_len} bytes "
+            f"(r={r.bit_length()} bits, s={s.bit_length()} bits)"
+        )
+    return r.to_bytes(coord_len, "big") + s.to_bytes(coord_len, "big")
+
+
+def sign_jwt(claims: dict, key_id: str = DEFAULT_KEY_ID) -> str:
+    """Build + sign a JWT with KMS (REQ-337, REQ-336).
+
+    Args:
+        claims: the JWT claims payload (``sub, aud, iss, exp, iat, jti,
+            roles`` per REQ-336, plus ``typ`` for PATs).
+        key_id: the KMS key ID or alias (default
+            ``alias/nova-oidc-signing``).
+
+    Returns:
+        The compact JWS (``header.payload.signature``), ``ES256``,
+        with the signature in raw ``r‖s`` form (DER→raw converted).
+    """
+    header = {"alg": "ES256", "typ": "JWT", "kid": key_id}
+    signing_input = (
+        _b64url(json.dumps(header, separators=(",", ":"), sort_keys=True).encode())
+        + "."
+        + _b64url(json.dumps(claims, separators=(",", ":"), sort_keys=True).encode())
+    )
+    resp = _get_kms_client().sign(
+        KeyId=key_id,
+        Message=signing_input.encode("ascii"),
+        MessageType="RAW",
+        SigningAlgorithm="ECDSA_SHA_256",
+    )
+    der_sig = resp["Signature"]
+    raw_sig = der_to_raw_ecdsa(der_sig)
+    return signing_input + "." + _b64url(raw_sig)
+
+
+def get_jwk(key_id: str = DEFAULT_KEY_ID) -> dict:
+    """Fetch the KMS public key and return it as a JWK (REQ-338).
+
+    Calls ``kms.get_public_key`` → DER SPKI → ``cryptography``'s
+    ``load_der_public_key`` → JWK ``{"kty":"EC","crv":"P-256","kid":...,
+    "x":...,"y":...}``. The ``x``/``y`` are base64url-encoded
+    big-endian 32-byte coordinates.
+    """
+    resp = _get_kms_client().get_public_key(KeyId=key_id)
+    pub = load_der_public_key(resp["PublicKey"])
+    if not isinstance(pub, EllipticCurvePublicKey):
+        raise ValueError(
+            f"KMS public key is not an EC key (got {type(pub).__name__})"
+        )
+    nums = pub.public_numbers()
+    # P-256 coordinates are 32 bytes big-endian.
+    x = nums.x.to_bytes(32, "big")
+    y = nums.y.to_bytes(32, "big")
+    return {
+        "kty": "EC",
+        "crv": "P-256",
+        "kid": key_id,
+        "x": _b64url(x),
+        "y": _b64url(y),
+        "alg": "ES256",
+        "use": "sig",
+    }
+
+
+if __name__ == "__main__":  # pragma: no cover - CLI inspection helper
+    import sys
+
+    if "--print-jwk" in sys.argv:
+        print(json.dumps(get_jwk(), indent=2))
+    else:
+        print("usage: python3 -m core.kms_signing --print-jwks", file=sys.stderr)