看雪 2016 CTF 第二十四题 点评和解析

看雪 2016 CTF 第二十四题 点评和解析

a4 为固定值 0x9C。

函数 sub_403910 算法思想为，a1 的每一个字节值等于 a3 的每一字节值加上 0x30（48）。

结合 sub_402E20、sub_403910 两个函数，反推注册算法，程序设计如下：

unsigned char aa[0x9c] = {0x01,0x04,0x08,0x02,0x04,0x08,0x03,0x04,0x08,0x04,0x04,0x08,0x05,0x04,0x08,0x06,

0x04,0x08,0x07,0x04,0x08,0x08,0x04,0x08,0x09,0x04,0x08,0x01,0x04,0x08,0x02,0x04,

0x08,0x03,0x04,0x08,0x04,0x04,0x08,0x05,0x04,0x08,0x06,0x04,0x08,0x07,0x04,0x08,

0x08,0x04,0x08,0x09,0x04,0x08,0x01,0x04,0x08,0x02,0x04,0x08,0x03,0x04,0x08,0x04,

0x04,0x08,0x05,0x04,0x08,0x06,0x04,0x08,0x07,0x04,0x08,0x08,0x04,0x08,0x09,0x04,

0x08,0x01,0x04,0x08,0x02,0x04,0x08,0x03,0x04,0x08,0x04,0x04,0x08,0x05,0x04,0x08,

0x06,0x04,0x08,0x07,0x04,0x08,0x08,0x04,0x08,0x09,0x04,0x08,0x01,0x04,0x08,0x02,

0x04,0x08,0x03,0x04,0x08,0x04,0x04,0x08,0x05,0x04,0x08,0x06,0x04,0x08,0x07,0x04,

0x08,0x08,0x04,0x08,0x09,0x04,0x08,0x01,0x04,0x08,0x02,0x04,0x08,0x03,0x04,0x08,

0x04,0x04,0x08,0x05,0x04,0x08,0x06,0x04,0x08,0x00,0x00,0x00};

printf("pediy");

for(int i=0;i<0x9c;i++)< p="">

{

unsigned char bb = (aa[i]+0x30)^0x86;

unsigned char cc = bb>>4;

if(cc<=9) cc+=48;

else cc+=55;

unsigned char dd = bb&0x0f;

if(dd<=9) dd+=48;

else dd+=55;

printf("%c%c",cc,dd);

}

printf("\n");

运行得到