Csharp(C# .net) Base64 隐写
最近被拉去报名参加CTF比赛，赶鸭子上架，趁着端午假期在网上做了做题，有一些题需要用到编程，网上介绍的一般都是Python写的，自己平时鼓捣的后端语言只有C#，python的脚本看得一知半解，有多函数还得去查阅用法，非常不方便，便想试着用C#写写试试，其实C#也支持在vscode里通过插件实现交互式编程，也非常方便。通过一个半天鼓捣，终于调试成功，一回想，上次写代码已经是半年前了。
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)); //"="符号的个数
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);
//二进制字符串转化为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);
}
//将byte[]转换为字符串
string str = Encoding.Default.GetString(bytesTest1);
Console.WriteLine(sb.ToString());
Console.WriteLine();
Console.WriteLine("flag{"+str+"}");
/// <summary>
/// 计算差异字符的差
/// </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;/*获得较短的字符串的长度*/
for (int i = 0; i < n; i++)
{
if (s1[i] != s2[i])
/*同位置字符不相同*/
{
return Math.Abs(strM64.IndexOf(s1[i]) - strM64.IndexOf(s2[i]));
}
}
return 0;
}
花了很多时间在下面这句上，中间有个“”非Base64的异常字符，使得用C#解码和反编码，结果不相同。但是在py脚本上没有问题，在cyberchef.org出现的是一个红色的圆点
to themselves. Plainly visible encrypted messagesno matter
flag_encode:
把圆点替换成空格后的txt
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未替换的txt(19行和20行）：
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下面是python3的脚本，运行两个txt都正确
# base64隐写
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为隐写字串与实际字串的二进制差值
#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()