feat(tools): add ECDSA P-256 firmware signing tool

tools/test_sign_firmware.py

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+import os, sys, tempfile
+from pathlib import Path
+
+import pytest
+from cryptography.hazmat.primitives.asymmetric import ec
+from cryptography.hazmat.primitives import hashes, serialization
+from cryptography.hazmat.primitives.asymmetric.utils import decode_dss_signature
+
+REPO_ROOT = Path(__file__).parent.parent
+sys.path.insert(0, str(REPO_ROOT / "tools"))
+from sign_firmware import sign_firmware, load_private_key
+
+
+@pytest.fixture()
+def keypair(tmp_path):
+    key = ec.generate_private_key(ec.SECP256R1())
+    pem_path = tmp_path / "key.pem"
+    pem_path.write_bytes(key.private_bytes(
+        serialization.Encoding.PEM,
+        serialization.PrivateFormat.PKCS8,
+        serialization.NoEncryption(),
+    ))
+    return key, pem_path
+
+
+def test_signature_is_64_bytes(keypair, tmp_path):
+    key, key_path = keypair
+    firmware = tmp_path / "fw.bin"
+    firmware.write_bytes(b"fake firmware data" * 100)
+    sig = sign_firmware(firmware, key_path)
+    assert len(sig) == 64
+
+
+def test_signature_verifies(keypair, tmp_path):
+    key, key_path = keypair
+    data = b"test firmware payload"
+    firmware = tmp_path / "fw.bin"
+    firmware.write_bytes(data)
+    sig_raw = sign_firmware(firmware, key_path)
+
+    # Convert raw r||s back to DER for cryptography lib verify
+    r = int.from_bytes(sig_raw[:32], 'big')
+    s = int.from_bytes(sig_raw[32:], 'big')
+    from cryptography.hazmat.primitives.asymmetric.utils import encode_dss_signature
+    sig_der = encode_dss_signature(r, s)
+
+    key.public_key().verify(sig_der, data, ec.ECDSA(hashes.SHA256()))
+
+
+def test_wrong_key_fails_verification(keypair, tmp_path):
+    key, key_path = keypair
+    firmware = tmp_path / "fw.bin"
+    firmware.write_bytes(b"firmware")
+    sig_raw = sign_firmware(firmware, key_path)
+
+    other_key = ec.generate_private_key(ec.SECP256R1())
+    r = int.from_bytes(sig_raw[:32], 'big')
+    s = int.from_bytes(sig_raw[32:], 'big')
+    from cryptography.hazmat.primitives.asymmetric.utils import encode_dss_signature
+    sig_der = encode_dss_signature(r, s)
+
+    with pytest.raises(Exception):
+        other_key.public_key().verify(sig_der, b"firmware", ec.ECDSA(hashes.SHA256()))