2017HCTF-Writeupippatsu-nyuukon
0x00 寫在前面
設計思路：
應用層與驅動層通信，在驅動層加密由應用層發送過來的明文后比較flag，并輸出結果
部分細節：
- 驅動層的分發函數分為2部分，SEND和RECV；
SEND：接受從應用層發來的明文并加密，其本體是DES
RECV：比較加密后的明文和加密flag- 驅動層接受的明文，實際上只有第一次加密結果是正確的
- DES后將不可視數據轉為hex
- 加密數據與加密flag比較前先異或同一個隨機字節
0x01 wp
跟到分發函數的SEND, 定位加密算法
因為DES對稱加密算法，所以從ida中摳出來，修改小部分并添加密文+key就可以跑解密腳本了
// desrypt_des.cpp
#include <stdio.h>
#include <string.h>
#define maxn 0x8000 // 理論支持明文長度
//#define ENCODE 0,16,1 // 加密用的宏
#define DECODE 15,-1,-1 // 解密用的宏
// 明文初始置換
char msg_ch[64] = {
58, 50, 42, 34, 26, 18, 10, 2, 60, 52, 44, 36, 28, 20, 12, 4,
62, 54, 46, 38, 30, 22, 14, 6, 64, 56, 48, 40, 32, 24, 16, 8,
57, 49, 41, 33, 25, 17, 9, 1, 59, 51, 43, 35, 27, 19, 11, 3,
61, 53, 45, 37, 29, 21, 13, 5, 63, 55, 47, 39, 31, 23, 15, 7
};
// 密鑰初始置換
char key_ch[56] = {
57, 49, 41, 33, 25, 17, 9, 1, 58, 50, 42, 34, 26, 18,
10, 2, 59, 51, 43, 35, 27, 19, 11, 3, 60, 52, 44, 36,
63, 55, 47, 39, 31, 23, 15, 7, 62, 54, 46, 38, 30, 22,
14, 6, 61, 53, 45, 37, 29, 21, 13, 5, 28, 20, 12, 4
};
// 擴展置換
char msg_ex[48] = {
32, 1, 2, 3, 4, 5, 4, 5, 6, 7, 8, 9,
8, 9, 10, 11, 12, 13, 12, 13, 14, 15, 16, 17,
16, 17, 18, 19, 20, 21, 20, 21, 22, 23, 24, 25,
24, 25, 26, 27, 28, 29, 28, 29, 30, 31, 32, 1
};
// 每輪密鑰的位移
char key_mov[16] = {
1, 1, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 1
};
// 壓縮置換
char key_cmprs[48] = {
14, 17, 11, 24, 1, 5, 3, 28, 15, 6, 21, 10,
23, 19, 12, 4, 26, 8, 16, 7, 27, 20, 13, 2,
41, 52, 31, 37, 47, 55, 30, 40, 51, 45, 33, 48,
44, 49, 39, 56, 34, 53, 46, 42, 50, 36, 29, 32
};
// S 盒置換
char s_box[8][6][16] = {
// S1
14, 4, 13, 1, 2, 15, 11, 8, 3, 10, 6, 12, 5, 9, 0, 7,
0, 15, 7, 4, 14, 2, 13, 1, 10, 6, 12, 11, 9, 5, 3, 8,
4, 1, 14, 8, 13, 6, 2, 11, 15, 12, 9, 7, 3, 10, 5, 0,
15, 12, 8, 2, 4, 9, 1, 7, 5, 11, 3, 14, 10, 0, 6, 13,
// S2
15, 1, 8, 14, 6, 11, 3, 4, 9, 7, 2, 13, 12, 0, 5, 10,
3, 13, 4, 7, 15, 2, 8, 14, 12, 0, 1, 10, 6, 9, 11, 5,
0, 14, 7, 11, 10, 4, 13, 1, 5, 8, 12, 6, 9, 3, 2, 15,
13, 8, 10, 1, 3, 15, 4, 2, 11, 6, 7, 12, 0, 5, 14, 9,
// S3
10, 0, 9, 14, 6, 3, 15, 5, 1, 13, 12, 7, 11, 4, 2, 8,
13, 7, 0, 9, 3, 4, 6, 10, 2, 8, 5, 14, 12, 11, 15, 1,
13, 6, 4, 9, 8, 15, 3, 0, 11, 1, 2, 12, 5, 10, 14, 7,
1, 10, 13, 0, 6, 9, 8, 7, 4, 15, 14, 3, 11, 5, 2, 12,
// S4
7, 13, 14, 3, 0, 6, 9, 10, 1, 2, 8, 5, 11, 12, 4, 15,
13, 8, 11, 5, 6, 15, 0, 3, 4, 7, 2, 12, 1, 10, 14, 9,
10, 6, 9, 0, 12, 11, 7, 13, 15, 1, 3, 14, 5, 2, 8, 4,
3, 15, 0, 6, 10, 1, 13, 8, 9, 4, 5, 11, 12, 7, 2, 14,
// S5
2, 12, 4, 1, 7, 10, 11, 6, 8, 5, 3, 15, 13, 0, 14, 9,
14, 11, 2, 12, 4, 7, 13, 1, 5, 0, 15, 10, 3, 9, 8, 6,
4, 2, 1, 11, 10, 13, 7, 8, 15, 9, 12, 5, 6, 3, 0, 14,
11, 8, 12, 7, 1, 14, 2, 13, 6, 15, 0, 9, 10, 4, 5, 3,
// S6
12, 1, 10, 15, 9, 2, 6, 8, 0, 13, 3, 4, 14, 7, 5, 11,
10, 15, 4, 2, 7, 12, 9, 5, 6, 1, 13, 14, 0, 11, 3, 8,
9, 14, 15, 5, 2, 8, 12, 3, 7, 0, 4, 10, 1, 13, 11, 6,
4, 3, 2, 12, 9, 5, 15, 10, 11, 14, 1, 7, 6, 0, 8, 13,
// S7
4, 11, 2, 14, 15, 0, 8, 13, 3, 12, 9, 7, 5, 10, 6, 1,
13, 0, 11, 7, 4, 9, 1, 10, 14, 3, 5, 12, 2, 15, 8, 6,
1, 4, 11, 13, 12, 3, 7, 14, 10, 15, 6, 8, 0, 5, 9, 2,
6, 11, 13, 8, 1, 4, 10, 7, 9, 5, 0, 15, 14, 2, 3, 12,
// S8
13, 2, 8, 4, 6, 15, 11, 1, 10, 9, 3, 14, 5, 0, 12, 7,
1, 15, 13, 8, 10, 3, 7, 4, 12, 5, 6, 11, 0, 14, 9, 2,
7, 11, 4, 1, 9, 12, 14, 2, 0, 6, 10, 13, 15, 3, 5, 8,
2, 1, 14, 7, 4, 10, 8, 13, 15, 12, 9, 0, 3, 5, 6, 11
};
// P 盒置換
char p_box[32] = {
16, 7, 20, 21, 29, 12, 28, 17, 1, 15, 23, 26, 5, 18, 31, 10,
2, 8, 24, 14, 32, 27, 3, 9, 19, 13, 30, 6, 22, 11, 4, 25
};
// 末置換
char last_ch[64] = {
40, 8, 48, 16, 56, 24, 64, 32, 39, 7, 47, 15, 55, 23, 63, 31,
38, 6, 46, 14, 54, 22, 62, 30, 37, 5, 45, 13, 53, 21, 61, 29,
36, 4, 44, 12, 52, 20, 60, 28, 35, 3, 43, 11, 51, 19, 59, 27,
34, 2, 42, 10, 50, 18, 58, 26, 33, 1, 41, 9, 49, 17, 57, 25
};
// hash 置換，將加密后的密文置換為可讀明文
char hs_ch[20] = "0123456789abcdef";
char sh_ch[128];
void init_trans() {
char i;
for (i = 0; i < 16; i++)
sh_ch[hs_ch[i]] = i; // 完成hash轉換的對應
}
char msg[maxn] = "aed3899df15bd7babb99acf5ebb9f5cd8cd44a77c53263de46ef9f3d773fe908";
char res[32];
char msgb[72], msgbt[72], keyb[18][72];
char key[16] = "deadbeef";
// 字符轉成二進制
void ChToBit(char* dest, char* src, int length) {
int i, j;
char t;
for (i = 0; i < length; i++) {
for (j = 8, t = src[i]; j > 0; j--) {
dest[(i << 3) + j] = t & 1; // 取字符末位
t >>= 1;
}
}
}
// 二進制轉成字符
void BitToCh(char* dest, char* src, int length) {
int i;
for (i = 0; i < length << 3; i++) {
dest[i >> 3] <<= 1;
dest[i >> 3] |= src[i + 1]; // 添加到末位
}
dest[length] = 0;
}
// 批置換，以offset為偏移，以count為長度
void BatchSet(char* dest, char* src, char* offset, int count) {
int i;
for (i = 0; i < count; i++)
dest[i + 1] = src[offset[i]];
}
// 得到16輪所需的密鑰
void getKeys() {
char tk[128], bk[72];
char* ptk = tk;
int i, j;
for (i = 0; i < 8; i++)
key[i] <<= 1; // 跳過奇偶校驗位
ChToBit(bk, key, 8);
BatchSet(tk, bk, key_ch, 56);
for (i = 0; i < 16; i++) {
for (j = 0; j < key_mov[i]; j++, ptk++) {
ptk[57] = ptk[28];
ptk[28] = ptk[1];
}
BatchSet(keyb[i], ptk, key_cmprs, 48);
}
}
// 將密文轉換為真正的密文
void dropMsg(char* dest, char* src) {
int i;
for (i = 0; i < 16; i++) {
dest[i >> 1] = (dest[i >> 1] << 4) | sh_ch[src[i]];
}
}
void DES(char* pmsg, int st, int cl, int step) {
int i, row, col;
char r[64], rt[48], s[8];
ChToBit(msgbt, pmsg, 8);
BatchSet(msgb, msgbt, msg_ch, 64); // 初始置換
for (; st != cl; st += step) {
memcpy(rt, msgb + 33, 32);
BatchSet(r, msgb + 32, msg_ex, 48); // 擴展置換
for (i = 1; i <= 48; i++)
r[i] ^= keyb[st][i]; // 異或操作
// s_box 代替
for (i = 0; i < 48; i += 6) {
row = col = 0;
row = r[i + 1] << 1 | r[i + 6];
col = (r[i + 2] << 3) | (r[i + 3] << 2) | (r[i + 4] << 1) | r[i + 5];
s[i / 12] = (s[i / 12] <<= 4) | s_box[i / 6][row][col];
}
ChToBit(r, s, 4);
BatchSet(msgb + 32, r, p_box, 32); // p_box 置換
for (i = 1; i <= 32; i++)
msgb[i + 32] ^= msgb[i]; // 異或
memcpy(msgb + 1, rt, 32);
}
memcpy(msgbt + 33, msgb + 1, 32);
memcpy(msgbt + 1, msgb + 33, 32);
BatchSet(msgb, msgbt, last_ch, 64); // 末置換
BitToCh(res, msgb, 8); // 轉為原明文
}
int main(int arg, char* arv[]) {
init_trans();
char mode = 'd';
getKeys(); // 得到16輪要用到的密鑰
int i;
printf("dec: ");
for (i = 0; msg[i]; i += 16) {
dropMsg(res, msg + i); // 將密文轉換為真正的密文
DES(res, DECODE); // 解密
printf("%s", res);
}
printf("\n");
return 0;
}簡單來說 只要把加密宏[0, 16 ,1]替換為[15, -1, -1]解密宏即可。
flag: hctf{Dr1v5r_M5ngM4n_2Oi7}
ps: 附一張成功cm的截圖
