USTC Hackergame 2020 Writeup

( •̀ ω •́ )y 耶,一星期的 Hackergame 终于结束,最终获得校内第九,总排名六十,总分3000分。
听说有跟我同是 20级 的 dalao 拿到校内第二,简直恐怖如斯 ヽ(*。>Д<)o゜

签到

直接审查元素 (Inspect Element) 将 input 的 value 改成 1

签到

点击提取即可获得 flag
flag{hR6Ku81-HappyHacking2020-58eadee9bc}

阅读更多
Hello World

Hello World

第 N 次开坑, 希望这次持久一些。 测试一下代码高亮。
预计数分期中后更新 lug-hackergame-2020 的 writeup。

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#!/usr/bin/env python3

from Crypto.Cipher import AES
import os
from hashlib import sha256
from utils import *


def talk_to_Alice():
name = bytes.fromhex(input("What's your name? "))
extra = bytes.fromhex(input("What else do you want to say? "))
msg = b"Thanks " + name + b" for taking my flag: " + flag + extra
plaintext = msg + sha256(msg).digest()
iv = os.urandom(AES.block_size)
aes = AES.new(AES_key, AES.MODE_CBC, iv)
print("This is my encrypted message, please take it to Bob:")
print((iv + aes.encrypt(pad(plaintext))).hex())


def talk_to_Bob():
try:
ciphertext = bytes.fromhex(input("Show me your message from Alice: "))
iv = ciphertext[: AES.block_size]
aes = AES.new(AES_key, AES.MODE_CBC, iv)
plaintext = unpad(aes.decrypt(ciphertext[AES.block_size :]))
assert sha256(plaintext[:-32]).digest() == plaintext[-32:]
print("Thanks")
except:
print("What's your problem???")


if __name__ == "__main__":
flag = open("flag1").read().encode()
AES_key = os.urandom(16)
while True:
choice = input("Whom do you want to talk to? ")
if choice == "Alice":
talk_to_Alice()
elif choice == "Bob":
talk_to_Bob()

… 似乎对 C 的支持不那么好

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#include <stdio.h>
#include <stdbool.h>
#include <ctype.h>
#include <limits.h>
#include <assert.h>

#define EMPTY 0
#define BLACK 1
#define WHITE 2
// Send payload to server to get flag plz
#define FLAG "\x76\x43\x49\x4C\x09\x5A\x4A\x55" \
"\x41\x41\x4E\x54\x11\x46\x5C\x14" \
"\x46\x53\x45\x4E\x5C\x48\x1B\x48" \
"\x52\x1E\x58\x25\x35\x62\x25\x28" \
"\x24\x21\x67\x38\x25\x30"

#define COMPUTER BLACK
#define HUMAN WHITE

#define MIN_SCORE -100

char flag[] = FLAG;
int board[3][3] = {};

void flag_decode(void) {
for (int i = 0; flag[i] != '\0'; i++) {
flag[i] ^= (i + 23333);
}
}

int check(void) {
for (int i = 0; i < 3; i++) {
if (board[i][0] == board[i][1] && board[i][1] == board[i][2] && board[i][0] != EMPTY) {
return board[i][0];
}
if (board[0][i] == board[1][i] && board[1][i] == board[2][i] && board[0][i] != EMPTY) {
return board[0][i];
}
}
if (board[0][0] == board[1][1] && board[1][1] == board[2][2] && board[0][0] != EMPTY) {
return board[0][0];
}
if (board[0][2] == board[1][1] && board[1][1] == board[2][0] && board[0][2] != EMPTY) {
return board[0][2];
}
return false;
}

bool full(void) {
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
if (board[i][j] == EMPTY) {
return false;
}
}
}
return true;
}

void print(void) {
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
char out = '_';
if (board[i][j] == WHITE) {
out = 'X';
} else if (board[i][j] == BLACK) {
out = 'O';
} else if (board[i][j] != EMPTY) {
out = '?'; // well if you see this something may be wrong.
}
printf("%c", out);
}
puts("");
}
}

int opp(int p) {
if (p == BLACK)
return WHITE;
else if (p == WHITE)
return BLACK;
assert(0);
}

int minimax(int player) {
int winColor = check();
if (winColor != EMPTY) {
return winColor == player ? 1 : -1;
}

int movex = -1, movey = -1;
int max_score = MIN_SCORE;
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
if (board[i][j] == EMPTY) {
board[i][j] = player;
int score = -minimax(opp(player));
if (score > max_score) {
max_score = score;
movex = i; movey = j;
}
board[i][j] = EMPTY;
}
}
}
if (movex == -1) {
return 0;
}
return max_score;
}

void ai(int *x, int *y) {
// Make sure that human cannot win
// I heard that there's an algorithm named "Minimax"
int movex = -1, movey = -1;
int max_score = MIN_SCORE;
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
if (board[i][j] == EMPTY) {
board[i][j] = COMPUTER;
int score = -minimax(HUMAN);
if (score > max_score) {
max_score = score;
movex = i; movey = j;
}
board[i][j] = EMPTY;
}
}
}
*x = movex;
*y = movey;
}

int main(void) {
setvbuf(stdin, 0, 2, 0);
setvbuf(stdout, 0, 2, 0);

bool success = false; // human wins?
char input[128] = {}; // input is large and it will be ok.

puts("Welcome to Tic Tac Toe! Computer first!");
puts("You're 'X' and I'm 'O'!");
while (!success) {
// computer: BLACK
int x, y;
puts("Now computer goes!");
ai(&x, &y);
board[x][y] = BLACK;
print();
if (check()) {
success = false;
break;
}
if (full()) {
break;
}
puts("Now human goes!");
// human: WHITE

// example input: (0,1)
while (true) {
printf("Your turn. Input like (x,y), such as (0,1): ");
gets(input);
x = input[1] - '0';
y = input[3] - '0';
printf("You wanna put X on (%d,%d)...\n", x, y);
if (!(x >= 0 && x < 3 && y >= 0 && y < 3)) {
puts("Wrong input! Please try again!");
continue;
}
if (board[x][y] != EMPTY) {
puts("This pos has already been occupied!");
continue;
}
break;
}
board[x][y] = WHITE;
print();
if (check()) {
success = true;
break;
}
if (full()) {
break;
}
}
if (success) {
puts("What? You win! Here is your flag:");
flag_decode();
puts(flag);
} else {
puts("You failed! See you next time~");
}
return 0;
}
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