Add local Microsoft InvisMark payload disruption

tests/test_microsoft_invismark_local.py

@@ -0,0 +1,128 @@
+"""Clean-room tests for Paint's local ``Watermarker.dll`` pixel format."""
+
+from __future__ import annotations
+
+import uuid
+from pathlib import Path
+
+import numpy as np
+from PIL import Image
+
+from remove_ai_watermarks.microsoft_invismark import (
+    detect_local_invismark,
+    disrupt_local_invismark,
+    invismark_id_from_c2pa_info,
+)
+
+_WATERMARK_ID = uuid.UUID("83424621-03cb-40e3-9808-a9fae837156d")
+
+
+def _dct_matrix() -> np.ndarray:
+    matrix = np.empty((4, 4), dtype=np.float64)
+    for frequency in range(4):
+        scale = np.sqrt(1 / 4) if frequency == 0 else np.sqrt(2 / 4)
+        for sample in range(4):
+            matrix[frequency, sample] = scale * np.cos(np.pi * (2 * sample + 1) * frequency / 8)
+    return matrix
+
+
+def _synthetic_writer(image: np.ndarray, watermark_id: uuid.UUID, *, valid_checksum: bool = True) -> np.ndarray:
+    """Independent format producer; it deliberately does not call production helpers."""
+    guid = watermark_id.bytes_le
+    checksum = sum(guid) & 0xFF
+    if not valid_checksum:
+        checksum ^= 1
+    payload = bytes((0x4C,)) + guid + bytes((checksum,))
+    bits = np.unpackbits(np.frombuffer(payload, dtype=np.uint8))
+
+    rgb = image.astype(np.float64)
+    luminance = rgb @ np.array((0.299, 0.587, 0.114), dtype=np.float64)
+    ll = (luminance[0::2, 0::2] + luminance[0::2, 1::2] + luminance[1::2, 0::2] + luminance[1::2, 1::2]) * 0.5
+    original_ll = ll.copy()
+    dct_matrix = _dct_matrix()
+
+    for block_y in range(ll.shape[0] // 4):
+        for block_x in range(ll.shape[1] // 4):
+            top, left = block_y * 4, block_x * 4
+            block = ll[top : top + 4, left : left + 4]
+            coefficients = dct_matrix @ block @ dct_matrix.T
+            ac = coefficients.reshape(-1)[1:].reshape(3, 5)
+            u, singular, vh = np.linalg.svd(ac, full_matrices=False)
+            bit = int(bits[(block_y % 12) * 12 + (block_x % 12)])
+            singular[0] = (np.floor(singular[0] / 24) + 0.25 + 0.5 * bit) * 24
+            rewritten = coefficients.reshape(-1).copy()
+            rewritten[1:] = (u @ np.diag(singular) @ vh).reshape(-1)
+            ll[top : top + 4, left : left + 4] = dct_matrix.T @ rewritten.reshape(4, 4) @ dct_matrix
+
+    delta = (ll - original_ll) * 0.5
+    pixel_delta = np.repeat(np.repeat(delta, 2, axis=0), 2, axis=1)
+    return np.clip(np.rint(rgb + pixel_delta[..., None]), 0, 255).astype(np.uint8)
+
+
+def _write_carrier(path: Path, *, valid_checksum: bool = True) -> np.ndarray:
+    original = np.random.default_rng(20260913).integers(24, 232, size=(192, 192, 3), dtype=np.uint8)
+    encoded = _synthetic_writer(original, _WATERMARK_ID, valid_checksum=valid_checksum)
+    Image.fromarray(encoded, "RGB").save(path)
+    return original
+
+
+def test_detects_repeated_payload_and_windows_guid_order(tmp_path: Path) -> None:
+    source = tmp_path / "paint.png"
+    _write_carrier(source)
+
+    detection = detect_local_invismark(source)
+
+    assert detection is not None
+    assert detection.watermark_id == _WATERMARK_ID
+    assert detection.payload == b"\x4c" + _WATERMARK_ID.bytes_le + bytes((sum(_WATERMARK_ID.bytes_le) & 0xFF,))
+    assert detection.minimum_support >= 3
+
+
+def test_rejects_clean_and_malformed_payloads(tmp_path: Path) -> None:
+    clean = tmp_path / "clean.png"
+    malformed = tmp_path / "malformed.png"
+    Image.fromarray(
+        np.random.default_rng(20260913).integers(24, 232, size=(192, 192, 3), dtype=np.uint8),
+        "RGB",
+    ).save(clean)
+    _write_carrier(malformed, valid_checksum=False)
+
+    assert detect_local_invismark(clean) is None
+    assert detect_local_invismark(malformed) is None
+
+
+def test_disruption_requires_expected_guid_and_invalidates_payload(tmp_path: Path) -> None:
+    source = tmp_path / "paint.png"
+    wrong_output = tmp_path / "wrong.png"
+    output = tmp_path / "clean.png"
+    original = _write_carrier(source)
+
+    assert disrupt_local_invismark(source, wrong_output, expected_watermark_id=uuid.uuid4()) is None
+    assert not wrong_output.exists()
+
+    result = disrupt_local_invismark(source, output, expected_watermark_id=_WATERMARK_ID)
+
+    assert result is not None
+    assert result.watermark_id == _WATERMARK_ID
+    assert result.changed_blocks >= 432
+    assert output.exists()
+    assert detect_local_invismark(output) is None
+    cleaned = np.asarray(Image.open(output).convert("RGB"))
+    assert np.mean(np.abs(cleaned.astype(np.int16) - original.astype(np.int16))) < 2.0
+
+
+def test_cloud_image_creator_fixture_is_not_the_local_dll_format() -> None:
+    fixture = Path(__file__).parents[1] / "data/fixtures/provenance/microsoft-paint-invismark.png"
+
+    assert detect_local_invismark(fixture) is None
+
+
+def test_c2pa_uuid_requires_exact_algorithm_and_one_valid_value() -> None:
+    info = {
+        "soft_binding_algorithm": "com.microsoft.invismark.1",
+        "soft_binding_value": str(_WATERMARK_ID),
+    }
+
+    assert invismark_id_from_c2pa_info(info) == _WATERMARK_ID
+    assert invismark_id_from_c2pa_info({**info, "soft_binding_algorithm": "io.iscc.v0"}) is None
+    assert invismark_id_from_c2pa_info({**info, "soft_binding_value": f"{_WATERMARK_ID}, {uuid.uuid4()}"}) is None