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()
对上面遇到的问题，有哪位大神指点一下？
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