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
---

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)