Csharp Base64 隱寫.md

峰圣榜發表於2024-06-10

Csharp(C# .net) Base64 隱寫

最近被拉去報名參加CTF比賽，趕鴨子上架，趁著端午假期在網上做了做題，有一些題需要用到程式設計，網上介紹的一般都是Python寫的，自己平時鼓搗的後端語言只有C#，python的指令碼看得一知半解，有多函式還得去查閱用法，非常不方便，便想試著用C#寫寫試試，其實C#也支援在vscode裡透過外掛實現互動式程式設計，也非常方便。透過一個半天鼓搗，終於除錯成功，一回想，上次寫程式碼已經是半年前了。

using System.IO;
using System.Text;
string[] strs = File.ReadAllLines("flag_encode.txt");
StringBuilder sb = new StringBuilder();
string bitStrAll = string.Empty;
foreach (string line in strs)
{
    byte[] l = Encoding.Default.GetBytes(line);
    string ln = Encoding.Default.GetString(l);
    string stego = ln.Replace("\n", "");
    byte[] outputb = Convert.FromBase64String(stego);
    string orgStr = Encoding.Default.GetString(outputb);
    sb.Append(orgStr);
    byte[] outputb1 = Encoding.Default.GetBytes(orgStr);

    string BStr = Convert.ToBase64String(outputb1);
    //Console.WriteLine(stego);
    //Console.WriteLine(BStr);
    // Console.WriteLine(orgStr);
    int diff = sCompare(stego, BStr);
    //Console.WriteLine(diff);
    int n = line.Count(i => "=".Contains(i)); //"="符號的個數


    if (diff != 0)
    {
        string bitStr = Convert.ToString(diff, 2).PadLeft(n * 2, '0');

        bitStrAll = bitStrAll + bitStr;
        //Console.WriteLine(bitStrAll);
    }
    else if (n >= 1)
    {
        bitStrAll = bitStrAll.PadRight(bitStrAll.Length + n * 2, '0');
    }

}

//Console.WriteLine(bitStrAll);

//二進位制字串轉化為byte[]
byte[] bytesTest1 = new byte[bitStrAll.Length / 8];
for (int i = 0; i < bytesTest1.Length; i++)
{
    bytesTest1[i] = Convert.ToByte(bitStrAll.Substring(i * 8, 8), 2);
}
//將byte[]轉換為字串
string str = Encoding.Default.GetString(bytesTest1);


Console.WriteLine(sb.ToString());
Console.WriteLine();
Console.WriteLine("flag{"+str+"}");

/// <summary>
/// 計算差異字元的差
/// </summary>
/// <param name="s1"></param>
/// <param name="s2"></param>
/// <returns></returns>
int sCompare(string s1, string s2)
{
    string strM64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    int n = s1.Length > s2.Length ? s2.Length : s1.Length;/*獲得較短的字串的長度*/
    for (int i = 0; i < n; i++)
    {
        if (s1[i] != s2[i])
        /*同位置字元不相同*/
        {
            return Math.Abs(strM64.IndexOf(s1[i]) - strM64.IndexOf(s2[i]));
        }

    }
    return 0;

}

花了很多時間在下面這句上，中間有個“”非Base64的異常字元，使得用C#解碼和反編碼，結果不相同。但是在py指令碼上沒有問題，在cyberchef.org出現的是一個紅色的圓點

 to themselves. Plainly visible encrypted messagesno matter

flag_encode:

把圓點替換成空格後的txt

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未替換的txt(19行和20行）：

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下面是python3的指令碼，執行兩個txt都正確

# base64隱寫
import base64
def get_diff(s1, s2):
    base64chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
    res = 0
    for i in range(len(s2)):
        if s1[i] != s2[i]:
            return abs(base64chars.index(s1[i]) - base64chars.index(s2[i]))
    return res


def b64_stego_decode():
    file = open("flag_encode.txt","rb")
    x = ''                                      # x即bin_str
    lines =  file.readlines()
    
    for line in lines:
       
        l = str(line, encoding = "utf-8")
        stego = l.replace('\n','')
        #print(stego)
        realtext = base64.b64decode(l)
        #print(realtext)
        realtext = str(base64.b64encode(realtext),encoding = "utf-8")
        #print(realtext)
        diff = get_diff(stego, realtext)        # diff為隱寫字串與實際字串的二進位制差值
        #print(diff)
        n = stego.count('=')
        if diff:
            a= bin(diff)[2:].zfill(n*2)
            x+=a
        else:
            b= '0' * n*2
            x+=b
    #print(x)
    i = 0
    flag = ''
    while i < len(x):
        if int(x[i:i+8],2):
            flag += chr(int(x[i:i+8],2))
        i += 8
    print(flag)

if __name__ == '__main__':
    b64_stego_decode()

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flag_encode:

未替換的txt(19行和20行）：

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