GDOUCTF 2023 CTF Writeup

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2023-04-17 2023-04-20 约 2400 字预计阅读 5 分钟

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本文最后更新于 2023-04-20，文中内容可能已过时。

rank: 27

misc完整wp

以及取证题不适用取证软件的手工做法

MISC

pixelart

提取像素点

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from PIL import Image


im = Image.open('arcaea.png')
# print(im.height)
img_new = Image.new('RGB',(320,180))

for i in range(0,im.width,12):
    for j in range(0,im.height,12):

        rgb = im.getpixel((i,j))
        img_new.putpixel((i//12,j//12),rgb)

# img_new.show()
img_new.save('fake_flag.png')

然后在lsb隐写里

Matryoshka

压缩包套娃，不过密码要解算式

最麻烦的地方是从左向右计算，

然后两个坑是十进制不能以0开头和答案是负数要去掉符号

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import os
# 去掉十进制数字开头的0
def not_start_zero(tmp):
    while True:
        if tmp.startswith("0"):
            tmp = tmp[1:]
        else:
            break
    return tmp
# 从左到右计算算式
def Calculate(calc:str)->str:
    tmp = ""
    f = []
    for index,i in enumerate(calc):
        if i.isdigit():
            tmp += i
        if i.isdigit()==False:
            f.append(not_start_zero(tmp))
            f.append(i)
            tmp = ""
        if index == (len(calc)-1):
            
            f.append(not_start_zero(tmp))

    result = int("".join(f[:1]))
    for i in range(1,len(f),2):
        result = eval(str(result)+"".join(f[i:i+2]))

    return str(result)
# 循环解压缩
while True:
    zip_name = "".join([i for i in os.listdir() if i.endswith('.zip')])
    if zip_name == "":
        break
    print(zip_name)
    

    pwdtxt="".join([i for i in os.listdir() if i.endswith('txt')])
    passwd =open(pwdtxt,'r').read()
    passwd=passwd.replace('zero','0').replace('one','1').replace('two','2').replace('three','3').replace('four','4').replace('five','5').replace('six','6').replace('seven','7').replace('eight','8').replace('nine','9')
    passwd = passwd.replace('plus','+').replace('times','*').replace('mod','%').replace('minus','-')
    print(passwd)
    # 答案中如果有符号 也要去掉
    cal_passwd = Calculate(passwd).replace('-','')
    print(cal_passwd)
    # zipfile模块太慢了 
    os.system(f'unzip -P {cal_passwd} {zip_name}')

    os.remove(zip_name)
    os.remove(pwdtxt)

zipfile太慢了，这里用的unzip命令解压

getnopwd

明文攻击，攻击流量包

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bkcrack -C getnopwd.zip -c final.pcapng  -x 8 4D3C2B1A01000000FFFFFFFFFFFFFFFF

解压得到一个流量包，另一个文件是doc

在document.xml中可以看到

流量包根据品牌可以知道是数位板流量

用knm提取USB流量

然后处理流量，Z为0的时候为没有下笔

参考B神脚本

https://byxs20.github.io/posts/43085.html#4-getnopwd

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import binascii
import matplotlib.pyplot as plt


with open("./out.data", "r") as f:
    data = f.read().splitlines()

def big_to_small(data):
    return binascii.hexlify(binascii.unhexlify(data)[::-1]).decode()

x_lis, y_lis = [], []
for line in data:
    x,y,z = int((big_to_small(line[4:8])),16),-int((big_to_small(line[8:12])),16),int((big_to_small(line[12:16])),16)

    if z == 0:
        continue
    
    x_lis.append(x)
    y_lis.append(y)


plt.scatter(x_lis, y_lis)
plt.show()

t3stify

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import scipy.io.wavfile as wav
import numpy as np 
import matplotlib.pyplot as plt

sample_rate, data = wav.read('flagg.wav')

left = data[:, 0::2] 
right = data[:, 1::2]

diff = np.abs(left - right)

plt.plot(diff)
plt.show()

左右声道差分

中间明显有不同，放大细看可以看出摩斯

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.- .-. -.-. .- . .- .---- ..-. .---- . ...-- ...--

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ARCAEA1F1E33

是deepsound的密码

解得flag

Ez Forensics

数据库版本+字符集格式+最长列名示例：NSSCTF{v0.0.1+GBK+1}

diskgenius挂载，可以找到1，2，4三个zip，还有个看着是exe实际上是zip的MySQL.exe

3.zip就在里面

1234ZIP里面三个文件刚好是数据表的数据

file命令可以发现分别的frm MYD MYI文件

50726对应MySQL 5.7.26版本

新建一个数据库导入

这个数据库不是UTF8的，但是表数据是utf8的默认，所以字符集格式是UTF8

比较发现最长的列名是表2的listen

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NSSCTF{v5.7.26+UTF-8+listen}

crypto

Absolute_Baby_Encrytpion

逆

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let messagetoEncrypt = prompt("Enter a string: ").toLowerCase();
let charArray = messagetoEncrypt.split("");
let encryptedString = "";
let hasInvalidCharacter = false;

for (let i = 0; i < charArray.length; i++) {
    switch (charArray[i]) {
        case '!':
            encryptedString = encryptedString.concat('a')
            break;
        case '1':
            encryptedString = encryptedString.concat('b')
            break;
        case ')':
            encryptedString = encryptedString.concat('c')
            break;
        case 'v':
            encryptedString = encryptedString.concat('d')
            break;
        case 'm':
            encryptedString = encryptedString.concat('e')
            break;
        case '+':
            encryptedString = encryptedString.concat('f')
            break;
        case 'q':
            encryptedString = encryptedString.concat('g')
            break;
        case '0':
            encryptedString = encryptedString.concat('h')
            break;
        case 'c':
            encryptedString = encryptedString.concat('i')
            break;
        case ']':
            encryptedString = encryptedString.concat('j')
            break;
        case '(':
            encryptedString = encryptedString.concat('k')
            break;
        case '}':
            encryptedString = encryptedString.concat('l')
            break;
        case '[':
            encryptedString = encryptedString.concat('m')
            break;
        case '8':
            encryptedString = encryptedString.concat('n')
            break;
        case '5':
            encryptedString = encryptedString.concat('o')
            break;
        case '$':
            encryptedString = encryptedString.concat('p')
            break;
        case '*':
            encryptedString = encryptedString.concat('q')
            break;
        case 'i':
            encryptedString = encryptedString.concat('r')
            break;
        case '>':
            encryptedString = encryptedString.concat('s')
            break;
        case '#':
            encryptedString = encryptedString.concat('t')
            break;
        case '<':
            encryptedString = encryptedString.concat('u')
            break;
        case '?':
            encryptedString = encryptedString.concat('v')
            break;
        case 'o':
            encryptedString = encryptedString.concat('w')
            break;
        case '^':
            encryptedString = encryptedString.concat('x')
            break;
        case '-':
            encryptedString = encryptedString.concat('y')
            break;
        case '_':
            encryptedString = encryptedString.concat('z')
            break;
        case 'h':
            encryptedString = encryptedString.concat('0')
            break;
        case 'w':
            encryptedString = encryptedString.concat('1')
            break;
        case 'e':
            encryptedString = encryptedString.concat('2')
            break;
        case '9':
            encryptedString = encryptedString.concat('3')
            break;
        case 'g':
            encryptedString = encryptedString.concat('4')
            break;
        case 'z':
            encryptedString = encryptedString.concat('5')
            break;
        case 'd':
            encryptedString = encryptedString.concat('6')
            break;
        case '~':
            encryptedString = encryptedString.concat('7')
            break;
        case '=':
            encryptedString = encryptedString.concat('8')
            break;
        case 'x':
            encryptedString = encryptedString.concat('9')
            break;
        case 'j':
            encryptedString = encryptedString.concat('!')
            break;
        case ':':
            encryptedString = encryptedString.concat('@')
            break;
        case '4':
            encryptedString = encryptedString.concat('#')
            break;
        case 'b':
            encryptedString = encryptedString.concat('$')
            break;
        case '`':
            encryptedString = encryptedString.concat('~')
            break;
        case 'l':
            encryptedString = encryptedString.concat('^')
            break;
        case '3':
            encryptedString = encryptedString.concat('&')
            break;
        case 't':
            encryptedString = encryptedString.concat('*')
            break;
        case '6':
            encryptedString = encryptedString.concat('(')
            break;
        case 's':
            encryptedString = encryptedString.concat(')')
            break;
        case 'n':
            encryptedString = encryptedString.concat('_')
            break;
        case ';':
            encryptedString = encryptedString.concat('+')
            break;

        case '\'':
            encryptedString = encryptedString.concat('-')
            break;
        case 'r':
            encryptedString = encryptedString.concat('=')
            break;
        case 'k':
            encryptedString = encryptedString.concat('`')
            break;
        case 'p':
            encryptedString = encryptedString.concat('~')
            break;
        case '\"':
            encryptedString = encryptedString.concat('{')
            break;
        case '&':
            encryptedString = encryptedString.concat('}')
            break;
        case '/':
            encryptedString = encryptedString.concat('[')
            break;
        case '\\':
            encryptedString = encryptedString.concat(']')
            break;
        case '2':
            encryptedString = encryptedString.concat('|')
            break;
        case '.':
            encryptedString = encryptedString.concat(':;')
            break;
        case '%':
            encryptedString = encryptedString.concat(';')
            break;
        case '|':
            encryptedString = encryptedString.concat('\"')
            break;
        case ',':
            encryptedString = encryptedString.concat('\'')
            break;
        case '@':
            encryptedString = encryptedString.concat('<')
            break;
        case '{':
            encryptedString = encryptedString.concat('>')
            break;
        case 'u':
            encryptedString = encryptedString.concat(',')
            break;
        case '7':
            encryptedString = encryptedString.concat('.')
            break;
        case 'y':
            encryptedString = encryptedString.concat('?')
            break;
        case 'a':
            encryptedString = encryptedString.concat('/')
            break;

        default:
            hasInvalidCharacter = true;
    }
}

if (hasInvalidCharacter) {
    encryptedString = "Invalid String!";
} else {
    console.log(`Your encoded string is ${encryptedString}`);
}

babylua

这段代码实现了一个简单的加密算法。它的工作流程如下:

它首先生成一个4位的随机种子seed,然后使用md5算法计算key = md5(md5(seed))。它打印key的前10位,在例子中是b5e62abe84。

它将要加密的字符串flag转换为字符的ASCII码,存储在secret数组中。

它将key中的每个字符的ASCII码和flag中的对应字符的ASCII码相加,得到加密后的ASCII码,存储回secret数组。

它将secret数组中的值打印出来,这些值就是加密后的字符串。

所以为了解密,我们需要:

拿到key的前10位,在例子中是b5e62abe84。

拿到加密后的字符串,在例子中是200 161 198 157 173 169 199 150 105 163 193 175 173 194 135 131 135 225。

对每个值减去key中的对应字符的ASCII码,就可以得到flag的ASCII码。

将ASCII码转换回字符,拼接起来就是flag。

这是ai答的

有一点不对，就是key需要通过爆破还原完整的key

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from hashlib import md5 
import itertools
import string

def md5_(string):
    return md5(string.encode()).hexdigest()

for i in itertools.product(string.ascii_letters,repeat=4):
    seed = "%s%s%s%s"%i
    # print(key)
    if "b5e62abe84" in md5_(md5_(seed)):
        print(seed)
        key=md5_(md5_(seed))
        print(key)
        break


encrypted = [200, 161, 198, 157, 173, 169, 199, 150, 105, 163, 193, 175, 173, 194, 135, 131, 135, 225]

flag = ''
for i in range(len(encrypted)):
    ascii_code = encrypted[i] - ord(key[i])
    flag += chr(ascii_code)

print(flag)
# flag{He11o_Lua!!!}

Magic of Encoding

数据清洗

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import base64
import zipfile
confuse_flags = ["flag{Xd_fake_flag_xD}", "find_me_if_you_can", "flag{not_the_correct_flag_lol}","\nflag{not_the_correct_flag_lol}\nflag{not_the_correct_flag_lol}\n"]
base64_lis = [base64.b64encode(i.encode()).decode() for i in confuse_flags]

data = open("Magic_Of_Encoding.txt", "r").read()

for i in base64_lis:
    data = data.replace(i, "")

print(data)

with open("flag.zip", "wb") as f:
    f.write(base64.b64decode(data))


# zipfile.ZipFile

zip_load = zipfile.ZipFile('flag.zip')
zip_load.extractall()
zip_load.close()
print(open('Magic of Encoding.txt').read())

# flag{h0p3_y0u_lik3d_the_M4gic_7rick}

Math Problem

已知ecc的a,b和基点的y轴

椭圆曲线的p就是rsa的p

所以求出p的值就可以解出flag

用sage在有理数环上带入椭圆曲线的函数表达式$x^3+a*x+b-y^2$求解x

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from Crypto.Util.number import *
from sage.all import *

e = 65537
n = 79239019133008902130006198964639844798771408211660544649405418249108104979283858140199725213927656792578582828912684320882248828512464244641351915288069266378046829511827542801945752252863425605946379775869602719406340271702260307900825314967696531175183205977973427572862807386846990514994510850414958255877
c = 45457869965165575324534408050513326739799864850578881475341543330291990558135968254698676312246850389922318827771380881195754151389802803398367341521544667542828862543407738361578535730524976113729406101764290984943061582342991118766322793847422471903811686775249409300301726906738475446634950949059180072008
a = 9303981927028382051386918702900550228062240363697933771286553052631411452412621158116514735706670764224584958899184294505751247393129887316131576567242619
b = 9007779281398842447745292673398186664639261529076471011805234554666556577498532370235883716552696783469143334088312327338274844469338982242193952226631913
y = 970090448249525757357772770885678889252473675418473052487452323704761315577270362842929142427322075233537587085124672615901229826477368779145818623466854

R.<x> = PolynomialRing(Zmod(n))
f = x^3+a*x+b-y^2
ans = f.monic().small_roots(X=2^64, beta=0.4, epsilon=0.02)

p = gcd(n, ZZ(f(ans[0])))
q = n//p
d = inverse(e, (p-1)*(q-1))
m = pow(c, d, n)
print(long_to_bytes(int(m)).decode())

# flag{c4edd6d0-d1b3-cbda-95e3-a323edc35be5}

epsilon取0.02算的比较快