360CTF Re wp

Posted 2020-11-16 harmonica11

这比赛唯一的一道Re??

exe，看字符串

 

找到主函数

 

看到判断的地方为loc_404600,去看一下

 

像是SMC，找修改404600处的函数

 

 此处用到了loc_404600,找到这个函数为

 

 

 

这里将loc_404600与mnbv循环的异或

写个脚本处理下

#include <idc.idc>
static decrypt(from, size) {
        auto i, x,key1,key2,key3,key4;
        key1 = 0x6D;
        key2 = 0x6E;
        key3 = 0x62;
        key4 = 0x76;
   for ( i=0; i != size; i=i+1 ) {
        if(i%4==0){
                x = Byte(from);
                x = (x^key1);
                PatchByte(from,x);
        }
        if(i%4==1){
                x = Byte(from);
                x = (x^key2);
                PatchByte(from,x);
        }
        if(i%4==2){
                x = Byte(from);
                x = (x^key3);
                PatchByte(from,x);
        }
        from = from + 1;
   }
 Message("
" + "Decrypt Complete
");
}

decrypt(0x00404600,0x260);

 

修改后重新分析创建函数得到

 

 

 看check

 

 

 

 

对输入的前16位进行转换使之等于“66733～6775”

光看ida分析这段实在是太难了，结合ollydbg分析，逻辑是将前十六个输入循环的与greatctf异或后平方再乘12345679

flag=‘‘

a=667339003789000121539302795007135856775//12345679
b=pow(a,0.5)
c=str(b)
key=‘greatctf‘
for i in range(16):
    flag+=chr(ord(c[i])^ord(key[i%8]))
print(flag)

 

再看check2

 

 先搞随机数，在解四元一次方程

#include<stdlib.h>
#include<stdio.h>
int main(){
        int v15,v8,v14,v13;
        srand(0xbc6146);
        v15 = rand() % 360;
        v8 = rand() % 360;
        v13 = rand() % 360;
        v14 = rand() % 360;
        printf("v5=%x,v8=%x,V13=%x,v14=%x
",v15,v8,v13,v14);
        return 0;
}

 在这我出了个小错误，我最开始是在Linux里跑的，结果随机数与程序里不一样（??

from z3 import*

def hex_str(x):
    temp=‘‘
    for i in range(len(x)//2):
        temp+=chr(int(x[2*i:2*i+2],16))
    return temp

f=Solver()
x=[Int(‘x%d‘%i) for i in range(4)]

f.add(x[0]+3*x[3]-1000*2 == 0x1A06491E7)
f.add(x[2]*0xc0-x[3]*0xb == 0x244BFD2B9C)
f.add(2*(x[1]+0x37a)+x[2]*0x1f == 0x71CE119D5)
f.add(x[1]*0x1f*136-0xc0*x[0] == 0x431E9A36840)


if f.check() == sat:
    for i in range(4):
        print(hex_str(hex(f.model()[x[i]].as_long())[2:])[::-1])

 输出

simp
0CTF
__36
leRe

顺序修改一下就可以了