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解题思路

cipher = C28BC39DC3A6C283C2B3C39DC293C289C2B8C3BAC29EC3A0C3A7C29A1654C3AF28C3A1C2B1215B53

len(cipher) = 80用jeb打开，能最终定位到一个关键函数，这个函数输入两个参数，第一个是flag，第二个是hellodsctf字符串的md5，输出为cipher。直接爆破每一位

import java.lang.String;
public class Main {
   public static void main(String[] args) {
       c a = new c();
       String flag = "sctf{";
       String printable = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!#$%&()*+,-.:;<=>?@[]^_{|}~";
       String ss = "C28BC39DC3A6C283C2B3C39DC293C289C2B8C3BAC29EC3A0C3A7C29A1654C3AF28C3A1C2B1215B53";
       for(int j=0;j<100;j++)
       {
           for(int i=0;i<printable.length();i++)
           {
               String now=  flag + printable.charAt(i);
               //System.out.println(now);
               String d = a.a(now,"E7E64BF658BAB14A25C9D67A054CEBE5");
               if(ss.indexOf(d) == 0)
               {
                   System.out.println("flag: " + now);
                   flag = now;
               }
           }
           //break;
       }
   }
}

FLAG

sctf{IS_IS_A_NICE_SONG}