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)

docs/kms-provisioning.md

@@ -0,0 +1,53 @@
+# KMS asymmetric key provisioning (C-1.1)
+
+This document records the C-1.1 verification for the Nova OIDC signing
+KMS key and the provisioning path used by `nova idp setup`.
+
+## C-1.1 verification (P4)
+
+C-1.1 requires verifying KMS asymmetric key support **before**
+implementation. The verification command is:
+
+```
+aws kms create-key \
+  --key-spec ECC_NIST_P256 \
+  --key-usage SIGN_VERIFY \
+  --description nova-oidc-signing
+```
+
+**Result on the P4 build host:** AWS credentials are not available
+(`Unable to locate credentials`), so the live verification could not
+run. This is recorded as a **P4 CI gate**: the `nova idp setup --check`
+command (Wave 8) performs this verification when AWS creds are present
+and reports it as a missing prerequisite when they are not. The code
+proceeds against the documented KMS API (REQ-337); tests use a test
+ECDSA P-256 keypair + mocked `boto3.client("kms")` (no real AWS calls).
+
+KMS asymmetric signing keys (`ECC_NIST_P256` + `SIGN_VERIFY`) are GA
+in all commercial regions (announced 2020-11). The
+`ECDSA_SHA_256` signing algorithm is supported. Confidence: high.
+
+## Key spec (REQ-337)
+
+* **Key spec:** `ECC_NIST_P256` (NIST P-256 / secp256r1)
+* **Key usage:** `SIGN_VERIFY`
+* **Signing algorithm:** `ECDSA_SHA_256` (JWS `ES256`)
+* **Alias:** `alias/nova-oidc-signing`
+* **Rotation:** manual, 90 days (matches D-069 CMK cadence). New key +
+  re-point alias + JWKS serves both `kid`s during overlap.
+
+## DER → raw ECDSA conversion (the #1 gotcha)
+
+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. The conversion (in
+`core/kms_signing.py:der_to_raw_ecdsa`):
+
+```python
+from cryptography.hazmat.primitives.asymmetric.utils import decode_dss_signature
+r, s = decode_dss_signature(der_sig)
+raw = r.to_bytes(32, "big") + s.to_bytes(32, "big")
+```
+
+This is verified by `tests/test_kms_signing.py` and the CAP-037
+round-trip test (`tests/test_kms_roundtrip.py`).

tests/test_kms_signing.py

@@ -0,0 +1,172 @@
+"""KMS signing tests (REQ-337, C-1.1).
+
+Tests :func:`core.kms_signing.der_to_raw_ecdsa` with a known DER
+signature and the full :func:`sign_jwt` round-trip with a mocked KMS
+client (no real AWS calls — C-1.1 documented as a CI gate in
+``docs/kms-provisioning.md``).
+"""
+
+from __future__ import annotations
+
+import base64
+import json
+import sys
+from pathlib import Path
+from unittest import mock
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+import core.kms_signing as kms_signing
+from core.kms_signing import der_to_raw_ecdsa, sign_jwt, get_jwk
+
+from cryptography.hazmat.primitives.asymmetric import ec, utils
+from cryptography.hazmat.primitives import hashes, serialization
+import jwt as pyjwt
+
+
+# ---------------------------------------------------------------------------
+# Test keypair — generated once per session (P-256).
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture(scope="module")
+def test_keypair():
+    priv = ec.generate_private_key(ec.SECP256R1())
+    pub = priv.public_key()
+    return priv, pub
+
+
+@pytest.fixture(scope="module")
+def test_pub_der(test_keypair):
+    _priv, pub = test_keypair
+    return pub.public_bytes(
+        encoding=serialization.Encoding.DER,
+        format=serialization.PublicFormat.SubjectPublicKeyInfo,
+    )
+
+
+class _MockKmsSignClient:
+    """Mock KMS client that signs with a test ECDSA private key (DER)."""
+
+    def __init__(self, priv, pub_der, key_id="alias/nova-oidc-signing"):
+        self._priv = priv
+        self._pub_der = pub_der
+        self._key_id = key_id
+
+    def sign(self, KeyId, Message, MessageType, SigningAlgorithm):
+        assert SigningAlgorithm == "ECDSA_SHA_256"
+        assert MessageType == "RAW"
+        der = self._priv.sign(Message, ec.ECDSA(hashes.SHA256()))
+        return {"Signature": der, "KeyId": KeyId}
+
+    def get_public_key(self, KeyId):
+        return {"PublicKey": self._pub_der, "KeyId": KeyId}
+
+
+# ---------------------------------------------------------------------------
+# der_to_raw_ecdsa — unit test with a known DER signature.
+# ---------------------------------------------------------------------------
+
+
+def test_der_to_raw_ecdsa_known_vector():
+    # Minimal DER: SEQUENCE { INTEGER r, INTEGER s }.
+    # r=5, s=7 → DER: 30 06 02 01 05 02 01 07
+    der = b"\x30\x06\x02\x01\x05\x02\x01\x07"
+    raw = der_to_raw_ecdsa(der)
+    assert len(raw) == 64  # 32 + 32
+    r = int.from_bytes(raw[:32], "big")
+    s = int.from_bytes(raw[32:], "big")
+    assert r == 5
+    assert s == 7
+
+
+def test_der_to_raw_ecdsa_real_signature(test_keypair):
+    priv, _ = test_keypair
+    msg = b"test message for der->raw"
+    der = priv.sign(msg, ec.ECDSA(hashes.SHA256()))
+    raw = der_to_raw_ecdsa(der)
+    assert len(raw) == 64
+    # Round-trip: raw → (r, s) should verify against the message.
+    r = int.from_bytes(raw[:32], "big")
+    s = int.from_bytes(raw[32:], "big")
+    from cryptography.hazmat.primitives.asymmetric.utils import encode_dss_signature
+    der2 = encode_dss_signature(r, s)
+    # Verifying with the re-encoded DER proves the raw split is correct.
+    priv.public_key().verify(der2, msg, ec.ECDSA(hashes.SHA256()))
+
+
+def test_der_to_raw_ecdsa_rejects_oversized_coord():
+    # r needs 33 bytes (2**256+1) → should raise.
+    from cryptography.hazmat.primitives.asymmetric.utils import encode_dss_signature
+    der = encode_dss_signature(2**256 + 1, 1)
+    with pytest.raises(ValueError):
+        der_to_raw_ecdsa(der, coord_len=32)
+
+
+# ---------------------------------------------------------------------------
+# sign_jwt — full round-trip with mock KMS + pyjwt verification.
+# ---------------------------------------------------------------------------
+
+
+def test_sign_jwt_roundtrip_verifies_with_pyjwt(test_keypair, test_pub_der):
+    priv, pub = test_keypair
+    client = _MockKmsSignClient(priv, test_pub_der)
+    kms_signing.set_kms_client_for_testing(client)
+    try:
+        claims = {
+            "sub": "user-1",
+            "aud": "nova-cli",
+            "iss": "nova-idp",
+            "exp": 9999999999,
+            "iat": 1700000000,
+            "jti": "test-jti",
+            "roles": ["developer"],
+        }
+        token = sign_jwt(claims, key_id="alias/nova-oidc-signing")
+        parts = token.split(".")
+        assert len(parts) == 3  # header.payload.signature
+
+        # Verify the header.
+        header = json.loads(base64.urlsafe_b64decode(parts[0] + "=="))
+        assert header["alg"] == "ES256"
+        assert header["typ"] == "JWT"
+        assert header["kid"] == "alias/nova-oidc-signing"
+
+        # Verify the signature with pyjwt using the test public key.
+        pem = pub.public_bytes(
+            encoding=serialization.Encoding.PEM,
+            format=serialization.PublicFormat.SubjectPublicKeyInfo,
+        ).decode("ascii")
+        decoded = pyjwt.decode(token, pem, algorithms=["ES256"], options={"verify_aud": False})
+        assert decoded["sub"] == "user-1"
+        assert decoded["jti"] == "test-jti"
+        assert decoded["roles"] == ["developer"]
+    finally:
+        kms_signing.set_kms_client_for_testing(None)
+
+
+def test_get_jwk_returns_valid_ec_jwk(test_keypair, test_pub_der):
+    priv, pub = test_keypair
+    client = _MockKmsSignClient(priv, test_pub_der)
+    kms_signing.set_kms_client_for_testing(client)
+    try:
+        jwk = get_jwk(key_id="alias/nova-oidc-signing")
+        assert jwk["kty"] == "EC"
+        assert jwk["crv"] == "P-256"
+        assert jwk["kid"] == "alias/nova-oidc-signing"
+        assert jwk["alg"] == "ES256"
+        # x and y are 32 bytes → 43 base64url chars (no padding).
+        assert len(jwk["x"]) == 43
+        assert len(jwk["y"]) == 43
+        # Cross-verify: a JWT signed with the test private key verifies
+        # with pyjwt using this JWK as the key.
+        client2 = _MockKmsSignClient(priv, test_pub_der)
+        kms_signing.set_kms_client_for_testing(client2)
+        token = sign_jwt({"sub": "x", "exp": 9999999999, "iat": 1, "jti": "j"})
+        key = pyjwt.PyJWK(jwk).key
+        decoded = pyjwt.decode(token, key, algorithms=["ES256"], options={"verify_aud": False})
+        assert decoded["sub"] == "x"
+    finally:
+        kms_signing.set_kms_client_for_testing(None)