test(crypto): add MTProto init and splitter coverage

tests/test_crypto_mtproto.py

@@ -0,0 +1,85 @@
+import struct
+import unittest
+
+from proxy.crypto_backend import create_aes_ctr_transform
+from proxy.tg_ws_proxy import _MsgSplitter, _dc_from_init, _patch_init_dc
+
+
+KEY = bytes(range(32))
+IV = bytes(range(16))
+PROTO_TAG = 0xEFEFEFEF
+
+
+def _xor(left: bytes, right: bytes) -> bytes:
+    return bytes(a ^ b for a, b in zip(left, right))
+
+
+def _keystream(size: int) -> bytes:
+    transform = create_aes_ctr_transform(KEY, IV)
+    return transform.update(b"\x00" * size) + transform.finalize()
+
+
+def _build_init_packet(dc_raw: int, proto: int = PROTO_TAG) -> bytes:
+    packet = bytearray(64)
+    packet[8:40] = KEY
+    packet[40:56] = IV
+
+    plain_tail = struct.pack("<Ih", proto, dc_raw) + b"\x00\x00"
+    packet[56:64] = _xor(plain_tail, _keystream(64)[56:64])
+    return bytes(packet)
+
+
+def _encrypt_after_init(init_packet: bytes, plaintext: bytes) -> bytes:
+    transform = create_aes_ctr_transform(init_packet[8:40], init_packet[40:56])
+    transform.update(b"\x00" * 64)
+    return transform.update(plaintext) + transform.finalize()
+
+
+class CryptoBackendTests(unittest.TestCase):
+    def test_unknown_backend_raises_error(self):
+        with self.assertRaises(ValueError):
+            create_aes_ctr_transform(KEY, IV, backend="missing")
+
+
+class MtProtoInitTests(unittest.TestCase):
+    def test_dc_from_init_reads_non_media_dc(self):
+        init_packet = _build_init_packet(dc_raw=2)
+
+        self.assertEqual(_dc_from_init(init_packet), (2, False))
+
+    def test_dc_from_init_reads_media_dc(self):
+        init_packet = _build_init_packet(dc_raw=-4)
+
+        self.assertEqual(_dc_from_init(init_packet), (4, True))
+
+    def test_patch_init_dc_updates_signed_dc_and_preserves_tail(self):
+        original = _build_init_packet(dc_raw=99) + b"tail"
+
+        patched = _patch_init_dc(original, -3)
+
+        self.assertEqual(_dc_from_init(patched[:64]), (3, True))
+        self.assertEqual(patched[64:], b"tail")
+
+
+class MsgSplitterTests(unittest.TestCase):
+    def test_splitter_splits_multiple_abridged_messages(self):
+        init_packet = _build_init_packet(dc_raw=-2)
+        plain_chunk = b"\x01abcd\x02EFGH1234"
+        encrypted_chunk = _encrypt_after_init(init_packet, plain_chunk)
+
+        parts = _MsgSplitter(init_packet).split(encrypted_chunk)
+
+        self.assertEqual(parts, [encrypted_chunk[:5], encrypted_chunk[5:14]])
+
+    def test_splitter_leaves_single_message_intact(self):
+        init_packet = _build_init_packet(dc_raw=2)
+        plain_chunk = b"\x02abcdefgh"
+        encrypted_chunk = _encrypt_after_init(init_packet, plain_chunk)
+
+        parts = _MsgSplitter(init_packet).split(encrypted_chunk)
+
+        self.assertEqual(parts, [encrypted_chunk])
+
+
+if __name__ == "__main__":
+    unittest.main()