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from base64 import b64decode
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def hex_decode(hex_str):
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    """將十六進制字串轉換回壓縮單元列表"""
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    units = []
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    i = 0
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    while i < len(hex_str):
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        # 每 10 個字符為一個單元
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        unit_hex = hex_str[i:i+10]
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        if len(unit_hex) < 10:  # 處理最後不完整的單元
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            break
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        # 解析壓縮單元
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        offset = int(unit_hex[0:4], 16)  # 前 4 個字符是偏移量
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        length = int(unit_hex[4:8], 16)  # 中間 4 個字符是長度
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        next_char = bytes.fromhex(unit_hex[8:10])  # 最後 2 個字符是下一個字符
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        units.append((offset, length, next_char))
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        i += 10
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    return units
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def lz77_decompress(compressed_units):
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    """LZ77 解壓縮"""
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    result = bytearray()
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    for offset, length, next_char in compressed_units:
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        # 如果有需要複製的內容 (length > 0)
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        if length > 0:
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            # 從已解壓的內容中複製
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            start = len(result) - offset
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            for i in range(length):
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                result.append(result[start + i])
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        # 添加下一個字符
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        result.extend(next_char)
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    return bytes(result)
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def decrypt_file(input_file, output_file):
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    """解密完整文件"""
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    # 讀取加密數據
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    with open(input_file, 'rb') as f:
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        encrypted_data = f.read().decode('ascii').strip()
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    # 第一次 HEX 解碼和 LZ77 解壓縮
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    units1 = hex_decode(encrypted_data)
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    decompressed1 = lz77_decompress(units1)
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    # 第二次 HEX 解碼和 LZ77 解壓縮
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    units2 = hex_decode(decompressed1.decode('ascii'))
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    decompressed2 = lz77_decompress(units2)
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    # Base64 解碼
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    final_data = b64decode(decompressed2)
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    # 寫入結果
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    with open(output_file, 'wb') as f:
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        f.write(final_data)
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if __name__ == "__main__":
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    # 使用示例
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    decrypt_file("output", "decrypted_flag.png")