Giải mã Base64 Steganography trong C# .NET

Gần đây tôi được mời tham gia một cuộc thi CTF, và trong kỳ nghỉ lễ端午节, tôi đã làm thử một số bài tập trực tuyến. Một số bài yêu cầu lập trình, phần lớn hướng dẫn trên mạng đều viết bằng Python, trong khi ngôn ngữ backend tôi quen dùng là C#. Việc đọc hiểu script Python khá vất vả vì có nhiều hàm phải tra cứu cách sử dụng, nên tôi quyết định thử viết bằng C#. Thật ra C# cũng hỗ trợ lập trình tương tác trong VSCode thông qua plugin, rất tiện lợi. Sau một buổi chiều mày mò, cuối cùng tôi cũng debug thành công - nhớ lại thì lần viết code trước đã là nửa năm trước rồi.

using System.IO;
using System.Text;

// Đọc toàn bộ các dòng từ file
string[] lines = File.ReadAllLines("flag_encode.txt");
StringBuilder resultBuilder = new StringBuilder();
string binaryData = string.Empty;

foreach (string line in lines)
{
    byte[] rawBytes = Encoding.Default.GetBytes(line);
    string processedLine = Encoding.Default.GetString(rawBytes);
    string stegoData = processedLine.Replace("\n", "");
    
    // Giải mã Base64
    byte[] decodedBytes = Convert.FromBase64String(stegoData);
    string originalText = Encoding.Default.GetString(decodedBytes);
    
    resultBuilder.Append(originalText);
    
    // Mã hóa lại để so sánh
    byte[] reEncodedBytes = Encoding.Default.GetBytes(originalText);
    string reEncodedString = Convert.ToBase64String(reEncodedBytes);
    
    // Tính toán sự khác biệt
    int difference = CalculateDifference(stegoData, reEncodedString);
    int paddingCount = line.Count(c => "=".Contains(c));
    
    if (difference != 0)
    {
        // Chuyển đổi sự khác biệt thành chuỗi nhị phân
        string bitSequence = Convert.ToString(difference, 2).PadLeft(paddingCount * 2, '0');
        binaryData += bitSequence;
    }
    else if (paddingCount >= 1)
    {
        binaryData = binaryData.PadRight(binaryData.Length + paddingCount * 2, '0');
    }
}

// Chuyển đổi chuỗi nhị phân thành mảng byte
byte[] finalBytes = new byte[binaryData.Length / 8];
for (int i = 0; i < finalBytes.Length; i++)
{
    finalBytes[i] = Convert.ToByte(binaryData.Substring(i * 8, 8), 2);
}

// Chuyển đổi mảng byte thành chuỗi ký tự
string extractedFlag = Encoding.Default.GetString(finalBytes);

Console.WriteLine(resultBuilder.ToString());
Console.WriteLine();
Console.WriteLine("flag{" + extractedFlag + "}");

/// <summary>
/// Tính toán chênh lệch giữa hai ký tự trong bảng Base64
/// </summary>
int CalculateDifference(string input1, string input2)
{
    const string base64Chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    int minLength = Math.Min(input1.Length, input2.Length);
    
    for (int i = 0; i < minLength; i++)
    {
        if (input1[i] != input2[i])
        {
            return Math.Abs(base64Chars.IndexOf(input1[i]) - base64Chars.IndexOf(input2[i]));
        }
    }
    
    return 0;
}

Tôi đã mất rất nhiều thời gian để xử lý một vấn đề nhỏ sau đây: trong chuỗi có ký tự đặc biệt "" không phải là ký tự Base64 hợp lệ, khiến cho việc giải mã và mã hóa lại bằng C# cho ra kết quả khác nhau. Tuy nhiên, script Python không gặp vấn đề này, còn trên cyberchef.org thì hiển thị một dấu chấm màu đỏ.

to themselves. Plainly visible encrypted messages&#151;no matter

Dữ liệu mẫu flag_encode (đã thay thế dấu chấm bằng khoảng trắng):

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Dữ liệu gốc chưa được thay thế (dòng 19 và 20):

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Dưới đây là script Python3 để tham khảo, chạy cho cả hai file đều cho kết quả đúng:

# Giải mã Base64 steganography
import base64

def get_diff(s1, s2):
    base64chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
    result = 0
    for i in range(len(s2)):
        if s1[i] != s2[i]:
            return abs(base64chars.index(s1[i]) - base64chars.index(s2[i]))
    return result

def b64_stego_decode():
    file = open("flag_encode.txt", "rb")
    binary_string = ''
    lines = file.readlines()
    
    for line in lines:
        line_str = str(line, encoding="utf-8")
        stego = line_str.replace('\n', '')
        
        # Giải mã Base64
        realtext = base64.b64decode(line_str)
        
        # Mã hóa lại để so sánh
        realtext = str(base64.b64encode(realtext), encoding="utf-8")
        
        # Tính toán sự khác biệt
        diff = get_diff(stego, realtext)
        
        # Đếm số ký tự padding
        n = stego.count('=')
        
        if diff:
            # Chuyển đổi diff thành chuỗi nhị phân
            bit_chunk = bin(diff)[2:].zfill(n * 2)
            binary_string += bit_chunk
        else:
            # Thêm các bit 0 nếu không có diff
            binary_string += '0' * n * 2
    
    # Giải mã chuỗi nhị phân thành văn bản
    flag = ''
    i = 0
    while i < len(binary_string):
        if int(binary_string[i:i+8], 2):
            flag += chr(int(binary_string[i:i+8], 2))
        i += 8
    
    print(flag)

if __name__ == '__main__':
    b64_stego_decode()

Kết luận

Về vấn đề đã gặp phải ở trên, nếu có ai đó có thể giải thích cho tôi hiểu nguyên nhân và cách khắc phục, tôi rất biết ơn!

Thẻ: csharp dotnet base64 steganography CTF

Đăng vào ngày 12 tháng 7 lúc 17:46

Thành phố Cuồng loạn

Giải mã Base64 Steganography trong C# .NET

Dữ liệu gốc chưa được thay thế (dòng 19 và 20):

Kết luận

Thẻ Phổ Biến