198 lines
5.1 KiB
Python
198 lines
5.1 KiB
Python
"""
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Lab003. Обнаружение повреждения пакета с помощью CRC-32.
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Цель:
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1. Сформировать пакет с CRC-32.
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2. Проверить исправный пакет.
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3. Изменить один бит полезной нагрузки.
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4. Убедиться, что CRC обнаруживает повреждение.
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"""
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import struct
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import zlib
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# ============================================================
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# Константы протокола
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# ============================================================
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SYNC_WORD = 0xAA55
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PROTOCOL_VERSION = 1
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MESSAGE_TYPE_TEXT = 1
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HEADER_FORMAT = ">HBBHH"
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CRC_FORMAT = ">I"
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CRC_SIZE = struct.calcsize(CRC_FORMAT)
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# ============================================================
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# Формирование исходного пакета
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# ============================================================
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message = "ПРИВЕТ SDR"
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payload = message.encode("utf-8")
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sequence_number = 1
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payload_length = len(payload)
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header = struct.pack(
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HEADER_FORMAT,
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SYNC_WORD,
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PROTOCOL_VERSION,
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MESSAGE_TYPE_TEXT,
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sequence_number,
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payload_length,
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)
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# CRC вычисляется по заголовку и полезной нагрузке.
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packet_without_crc = header + payload
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crc_value = zlib.crc32(packet_without_crc) & 0xFFFFFFFF
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crc_bytes = struct.pack(
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CRC_FORMAT,
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crc_value,
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)
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packet = packet_without_crc + crc_bytes
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print("Исходное сообщение:")
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print(message)
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print("\nCRC-32 исходного пакета:")
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print(f"0x{crc_value:08X}")
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print("\nПолный пакет:")
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print(packet.hex(" "))
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print("\nДлина полного пакета:")
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print(len(packet), "байт")
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# ============================================================
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# Проверка исправного пакета
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# ============================================================
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received_data = packet[:-CRC_SIZE]
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received_crc_bytes = packet[-CRC_SIZE:]
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(received_crc,) = struct.unpack(
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CRC_FORMAT,
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received_crc_bytes,
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)
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calculated_crc = zlib.crc32(received_data) & 0xFFFFFFFF
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print("\n--- Проверка исправного пакета ---")
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print(f"CRC из пакета: 0x{received_crc:08X}")
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print(f"CRC вычисленный: 0x{calculated_crc:08X}")
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assert received_crc == calculated_crc
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print("Результат: пакет не повреждён.")
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# ============================================================
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# Имитация повреждения одного бита
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# ============================================================
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corrupted_packet = bytearray(packet)
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header_size = struct.calcsize(HEADER_FORMAT)
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# Буква S находится по индексу 13 внутри PAYLOAD.
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payload_byte_index = 13
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packet_byte_index = header_size + payload_byte_index
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original_byte = corrupted_packet[packet_byte_index]
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# Меняем младший бит:
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# S = 0x53
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# R = 0x52
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corrupted_packet[packet_byte_index] ^= 0x01
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corrupted_byte = corrupted_packet[packet_byte_index]
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print("\n--- Повреждение одного бита ---")
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print(
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f"До повреждения: "
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f"0x{original_byte:02X} = {original_byte:08b}"
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)
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print(
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f"После повреждения: "
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f"0x{corrupted_byte:02X} = {corrupted_byte:08b}"
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)
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# ============================================================
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# Проверка повреждённого пакета
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# ============================================================
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corrupted_data = bytes(
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corrupted_packet[:-CRC_SIZE]
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)
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corrupted_crc_bytes = bytes(
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corrupted_packet[-CRC_SIZE:]
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)
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(received_crc_after_corruption,) = struct.unpack(
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CRC_FORMAT,
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corrupted_crc_bytes,
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)
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calculated_crc_after_corruption = (
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zlib.crc32(corrupted_data) & 0xFFFFFFFF
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)
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print("\n--- Проверка повреждённого пакета ---")
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print(
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f"CRC из пакета: "
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f"0x{received_crc_after_corruption:08X}"
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)
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print(
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f"CRC вычисленный: "
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f"0x{calculated_crc_after_corruption:08X}"
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)
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if received_crc_after_corruption != calculated_crc_after_corruption:
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print("\nРезультат: CRC обнаружил повреждение пакета.")
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else:
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print("\nРезультат: повреждение не обнаружено.")
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# ============================================================
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# Показываем повреждённое сообщение
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# ============================================================
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corrupted_payload = corrupted_data[header_size:]
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corrupted_message = corrupted_payload.decode("utf-8")
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print("\nСообщение внутри повреждённого пакета:")
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print(corrupted_message)
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# ============================================================
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# Автоматические проверки
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# ============================================================
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assert corrupted_message == "ПРИВЕТ RDR"
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assert (
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received_crc_after_corruption
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!= calculated_crc_after_corruption
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)
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print("\nПроверка пройдена: CRC-32 обнаружил изменение одного бита.") |