2021NSCTF RE WP

题目

链接：https://pan.baidu.com/s/1yt4LNXsk6kdfnaVjT4AsNQ
提取码：py32

ViolentScript.apk

做法一

AndroidKiller搜索flag{定位主函数

一般题目字符串中没有flag时 从smali->com中找
apk中original存放的是配置信息，res存放资源文件（图片视频之类），smali存放字节码文件，其中Android和Androidx为安卓库文件，com中存放的是用户自定义的包

查看java源码

package com.liyasoft.crackmea;

import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;
import android.os.Bundle;
import android.support.v7.app.AlertDialog.Builder;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class MainActivity
  extends AppCompatActivity
  implements View.OnClickListener
{
  private static int[] mask = { 23, 1, 0, 1, 10, 17, 7, 90, 20 };
  private Button mButton;
  private EditText mPassword;
  private EditText mUsername;
  
  private static boolean checkUsername(String paramString)
  {
    boolean bool1 = true;
    int i = paramString.length();
    int j = 0;
    if (i == 9)
    {
      boolean bool2;
      for (;;)
      {
        bool2 = bool1;
        if (j >= 9) {
          break;
        }
        if ((paramString.charAt(j) ^ "miaomiao!".charAt(j)) != mask[j])
        {
          bool2 = false;
          break;
        }
        j++;
      }
      return bool2;
    }
    return false;
  }
  
  private static String getString(byte[] paramArrayOfByte)
  {
    StringBuilder localStringBuilder = new StringBuilder();
    for (int i = 0; i < paramArrayOfByte.length; i++) {
      localStringBuilder.append(paramArrayOfByte[i]);
    }
    return localStringBuilder.toString();
  }
  
  public static String hash(String paramString)
  {
    try
    {
      paramString = MessageDigest.getInstance("MD5").digest(paramString.getBytes());
      StringBuilder localStringBuilder = new java/lang/StringBuilder;
      localStringBuilder.<init>(40);
      int i = paramString.length;
      for (int j = 0; j < i; j++)
      {
        int k = paramString[j];
        if ((k & 0xFF) >> 4 == 0)
        {
          localStringBuilder.append("0");
          localStringBuilder.append(Integer.toHexString(k & 0xFF));
        }
        else
        {
          localStringBuilder.append(Integer.toHexString(k & 0xFF));
        }
      }
      paramString = localStringBuilder.toString();
      return paramString;
    }
    catch (NoSuchAlgorithmException paramString) {}
    return null;
  }
  
  private void initView()
  {
    this.mButton = ((Button)findViewById(2131230814));
    this.mUsername = ((EditText)findViewById(2131230915));
    this.mPassword = ((EditText)findViewById(2131230835));
    this.mButton.setOnClickListener(this);
  }
  
  private static boolean snCheck(String paramString)
  {
    if (paramString.length() != 32) {
      return false;
    }
    if ((paramString.toLowerCase().startsWith("27a3c22d01eb")) && (paramString.toLowerCase().endsWith("6ee8866aa5e5e7820"))) {
      try
      {
        boolean bool = hash(paramString).equalsIgnoreCase("271373f323d9f76dc31204ff19823fdf");
        return bool;
      }
      catch (Exception paramString)
      {
        return false;
      }
    }
    return false;
  }
  
  public void onClick(View paramView)
  {
    if (paramView.getId() == 2131230814)
    {
      AlertDialog.Builder localBuilder = new AlertDialog.Builder(this);
      Object localObject = this.mUsername.getText().toString();
      paramView = this.mPassword.getText().toString();
      if ((checkUsername((String)localObject)) && (snCheck(paramView)))
      {
        localObject = new StringBuilder();
        ((StringBuilder)localObject).append("恭喜，您登陆成功:flag{");
        ((StringBuilder)localObject).append(paramView);
        ((StringBuilder)localObject).append("}");
        localBuilder.setMessage(((StringBuilder)localObject).toString());
        localBuilder.setNegativeButton("确定", new DialogInterface.OnClickListener()
        {
          public void onClick(DialogInterface paramAnonymousDialogInterface, int paramAnonymousInt)
          {
            paramAnonymousDialogInterface.dismiss();
          }
        });
      }
      else
      {
        localBuilder.setMessage("用户名或密码不正确 密码就是答案哦!");
        localBuilder.setNegativeButton("确定", new DialogInterface.OnClickListener()
        {
          public void onClick(DialogInterface paramAnonymousDialogInterface, int paramAnonymousInt)
          {
            paramAnonymousDialogInterface.dismiss();
          }
        });
      }
      localBuilder.show();
    }
  }
  
  protected void onCreate(Bundle paramBundle)
  {
    super.onCreate(paramBundle);
    setContentView(2131427356);
    initView();
  }
}

根据提示密码就是flag，所以着重看snCheck

    if (paramString.length() != 32) {
      return false;
    }
    if ((paramString.toLowerCase().startsWith("27a3c22d01eb")) && (paramString.toLowerCase().endsWith("6ee8866aa5e5e7820"))) {
      try
      {
        boolean bool = hash(paramString).equalsIgnoreCase("271373f323d9f76dc31204ff19823fdf");
        return bool;
      }

也就是paramString以27a3c22d01eb开头，6ee8866aa5e5e7820结尾，且长度为32位，还有中间3位需要爆破
爆破脚本如下
python：

import hashlib

nums1 = ["27a3c22d01eb"]
nums2 = [0]*3
nums3 = ["6ee8866aa5e5e7820"]
str = "271373f323d9f76dc31204ff19823fdf"
str2 = "0123456789abcdefjhigklmnopqrstuvwxyz"

def computeMD5(message):
    m = hashlib.md5()
    m.update(message.encode(encoding='utf-8'))
    return m.hexdigest()

def check():
    for i in str2:
        for j in str2:
            for k in str2:
                nums2[0] = i
                nums2[1] = j
                nums2[2] = k
                nums4 = nums1 + nums2 + nums3
                if computeMD5(''.join('%s' %id for id in nums4)) == str:
                    print( ''.join(nums4) )
                    return

check()

java：
t.charAt(i)是提取t中第i个位置的字符

public static void main(String[] args) {
        String t = "0123456789abcdefghijklmnopqrstuvwxyz";
        String flagA  = "27a3c22d01eb";
        String flagB = "6ee8866aa5e5e7820";
        for (int i = 0; i < t.length(); i++) {
            char x = t.charAt(i);
            for (int j = 0; j < t.length(); j++) {
                char y = t.charAt(j);
                for (int k = 0; k < t.length(); k++) {
                    char z = t.charAt(k);
                    String flag = flagA + x + y + z + flagB;
                    if ("271373f323d9f76dc31204ff19823fdf".equals(MD5Utils.stringToMD5(flag))){
                        System.out.println("flag: "+flag);
                    }
                }
            }
        }
}

做法二

直接把271373f323d9f76dc31204ff19823fdf进行md5解密，得到wgwl$5000!

然后再将wgwl$5000! md5加密得到27a3c22d01eb0466ee8866aa5e5e7820
这里强推网站https://www.somd5.com/
可以自动解两层md5

Reverse02

做法一

一个exe，看一下界面

到md5都是可读的，需要的就是后面的部分
查壳

查找字符串

发现两个函数
这里是表明在文本框中输入什么才会弹出flag提示框

  int v1; // edx
  int v2; // esi
  int result; // eax

  if ( ((unsigned __int8)a13 ^ 7) == *this
    && ((unsigned __int8)a12 ^ 7) == this[1]
    && ((unsigned __int8)a11 ^ 7) == this[2]
    && ((unsigned __int8)a10 ^ 7) == this[3]
    && ((unsigned __int8)a9 ^ 7) == this[4]
    && ((unsigned __int8)a8 ^ 7) == this[5]
    && ((unsigned __int8)a7 ^ 7) == this[6] )
  {
    v1 = dword_403380 + 2;
    v2 = dword_403018 - 1;
    dword_403380 += 2;
    --dword_403018;
  }
  else
  {
    v1 = dword_403380;
    v2 = dword_403018;
  }
  if ( ((unsigned __int8)a6 ^ 0x33) == this[7]
    && ((unsigned __int8)a5 ^ 0x33) == this[8]
    && ((unsigned __int8)a4 ^ 0x33) == this[9]
    && ((unsigned __int8)a3 ^ 0x33) == this[10]
    && ((unsigned __int8)a2 ^ 0x33) == this[11]
    && ((unsigned __int8)a1 ^ 0x33) == this[12] )
  {
    --v1;
    v2 += 2;
    dword_403380 = v1;
    dword_403018 = v2;
  }
  if ( v2 + v1 == 3 )
    result = sub_401000();
  else
    result = MessageBoxA(0, "flag:{NSCTF_md57e0cad17016b0>?45?f7c>0>4a>1c3a0}", "Flag", 0);
  dword_403018 = 1;
  dword_403380 = 0;
  return result;
}

这里是flag加密原理

int sub_401000()
{
  char *i; // eax
  char Destination; // [esp+0h] [ebp-38h] BYREF
  char v3[51]; // [esp+1h] [ebp-37h] BYREF

  Destination = 0;
  memset(v3, 0, 0x30u);
  strncpy_s(&Destination, 0x31u, "flag:{NSCTF_md57e0cad17016b0>?45?f7c>0>4a>1c3a0}", 0x30u);
  for ( i = &v3[14]; *i != 125; ++i )
    *i ^= 7u;
  return MessageBoxA(0, &Destination, "Flag", 0);

实际上按照这个^7就可以了

#include <stdio.h>
main(){
	char a[33] = "7e0cad17016b0>?45?f7c>0>4a>1c3a0";
	int i;
	for (i=0;i<32;i++){
		printf("%c",a[i] ^ 7);
	}
}

得到0b7dfc60761e798328a0d9793f96d4f7
可以获取输入验证一下

#include <stdio.h>
main(){
	char a[] ={0x4A, 0x50, 0x46, 0x6A, 0x58, 0x6A, 0x32, 0x3B, 0x39, 0x47, 0x57, 0x62, 0x4E};
	int i;
	for (i=12;i>=6;i--){
		printf("%c",a[i] ^ 7);
	}
	for (i=5;i>=0;i--){
		printf("%c",a[i] ^ 0x33);
	}
}

得到输入为IeP@><5YkYucy

做法二

摁下按钮，flag弹框出现，可以选择用破解，将输入判定nop掉
od打开，先查字符串，两个flag{

区分一下
第一个下面先调用了strncpy，后调用了message，所以是输出正确flag的函数

第二个只调用了message，是检验输入的函数

这里只要把调用第一个函数的判断条件nop掉，即可直接输出
直接运行

re

主函数为

有两个key
key1

dwfsxe *__fastcall key1(dwfsxe *this)
{
  int i; // [rsp+14h] [rbp-2Ch]
  unsigned __int64 v3; // [rsp+18h] [rbp-28h] BYREF
  const char *v4; // [rsp+20h] [rbp-20h]
  size_t size; // [rsp+28h] [rbp-18h]
  char *v6; // [rsp+30h] [rbp-10h]
  unsigned __int64 v7; // [rsp+38h] [rbp-8h]

  v7 = __readfsqword(0x28u);
  v4 = "OFG{OxS3Lha6MUDk[0PnXofmcUrp`E3w`1@zalL2fZX1gJn4SWHFPGTEP2jHQivOVW7RWDDQW3PTTnf[UTmjSAOiHT6oIkerZ{q?";
  size = 192LL;
  v6 = (char *)malloc(0xC0uLL);
  for ( i = 0; i < size; ++i )
    v6[i] = v4[i] ^ 2;
  *(_QWORD *)this = ohfewfynewnfew(v6, size, &v3);
  return (dwfsxe *)(__readfsqword(0x28u) ^ v7);

key1解出来后base64解密为0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()_+
key2

handvfiu *__fastcall key2(handvfiu *this)
{
  int i; // [rsp+14h] [rbp-2Ch]
  unsigned __int64 v3; // [rsp+18h] [rbp-28h] BYREF
  const char *v4; // [rsp+20h] [rbp-20h]
  size_t size; // [rsp+28h] [rbp-18h]
  char *v6; // [rsp+30h] [rbp-10h]
  unsigned __int64 v7; // [rsp+38h] [rbp-8h]

  v7 = __readfsqword(0x28u);
  v4 = "bEBn`GBkMV{fJyMLTF{yR@sQVjUNIoULJVtsN@UQ[d>>";
  size = 44LL;
  v6 = (char *)malloc(0x2CuLL);
  for ( i = 0; i < size; ++i )
    v6[i] = v4[i] ^ 3;
  *(_QWORD *)this = ohfewfynewnfew(v6, size, &v3);
  return (handvfiu *)(__readfsqword(0x28u) ^ v7);
}

key2解出来是hP&p0!5L#3NXLs@*QR%L&UN!L)0%Q
check函数

__int64 __fastcall check(B *this, dwfsxe *a2, handvfiu *a3)
{
  int i; // [rsp+2Ch] [rbp-14h]

  if ( strlen(*(const char **)this) != 31 )
    wrong(this);
  for ( i = 0; i < strlen(*(const char **)this); ++i )
  {
    if ( *(_BYTE *)(*(_QWORD *)a2 + *(char *)(*(_QWORD *)this + i) - 48LL) != *(_BYTE *)(*(_QWORD *)a3 + i) )
      closeto(this);
  }
  return right(this);
}

check函数表明flag31位且a2[this[i] - 48 ] == a3[i]，其中this是我们输入的部分
写出脚本

a2 = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!@#$%^&*()_+"
a3 = "hP&p0!5L^#3NXLs@*QR%L&UN!L)0%Q^"


for i in range(len(a3)):           #一共循环31次
    for j in range(len(a2)):
        if a3[i] == a2[j]:
            print(chr(j + 48),end="")

flag为ActI0n5_sp3ak_Louder_than_w0rds

参考链接

NSCTF练习平台逆向题WP

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原文地址：https://www.cnblogs.com/XXX-Echoed/p/15083065.html