攻防世界 reverse002：logmein

提示是一个基本的算法逆向

先给文件权限

chmod 777 文件名

再尝试运行一下

拽入IDA，main函数F5

void __fastcall __noreturn main(__int64 a1, char **a2, char **a3)
{
  size_t v3; // rsi
  int i; // [rsp+3Ch] [rbp-54h]
  char s[36]; // [rsp+40h] [rbp-50h]
  int v6; // [rsp+64h] [rbp-2Ch]
  __int64 v7; // [rsp+68h] [rbp-28h]
  char v8[8]; // [rsp+70h] [rbp-20h]
  int v9; // [rsp+8Ch] [rbp-4h]

  v9 = 0;
  strcpy(v8, ":\"AL_RT^L*.?+6/46");
  v7 = 28537194573619560LL;
  v6 = 7;
  printf("Welcome to the RC3 secure password guesser.\n", a2, a3);
  printf("To continue, you must enter the correct password.\n");
  printf("Enter your guess: ");
  __isoc99_scanf("%32s", s);
  v3 = strlen(s);
  if ( v3 < strlen(v8) )
    sub_4007C0(v8);
  for ( i = 0; i < strlen(s); ++i )
  {
    if ( i >= strlen(v8) )
      ((void (*)(void))sub_4007C0)();
    if ( s[i] != (char)(*((_BYTE *)&v7 + i % v6) ^ v8[i]) )
      ((void (*)(void))sub_4007C0)();
  }
  sub_4007F0();
}

这里看一下sub_4007C0()

sub_4007F0()

分析程序逻辑，可以分析出s[i]是由26行的算法产生

用C语言来实现

#include "stdafx.h"
#include <stdio.h>
#include <windows.h>
#include <string.h>

int main(int argc, char* argv[])
{
	__int64 v7;
	v7 = 28537194573619560;//LL是Long Long的意思

	int v6=7;
	char v8[]=":\"AL_RT^L*.?+6/46";

	char s[36]={0};

	for(int i=0;i < strlen(v8);++i)
	{
		s[i]=(char)(*((BYTE *)&v7+i%v6)^v8[i]);
	}
	printf("flag:%s\n",s);

	return 0;
}

获取到flag