ISITDTU CTF 2024

This is my write-up for 3 challenges in crypto category I solved in ISITDTU CTF 2024. 2 unsolved challenges may be updated in the future 🤔. Thanks to all authors for very nice challenges 😁

ShareMixer1

import random   # TODO: heard that this is unsafe but nvm
from Crypto.Util.number import getPrime, bytes_to_long
flag = bytes_to_long(b'ISITDTU{test_flag_dkosakdisaj}')
# flag = bytes_to_long(open("flag.txt", "rb").read())
p = getPrime(256)
assert flag < p
l = 32


def share_mixer(xs):
    cs = [random.randint(1, p - 1) for _ in range(l - 1)]
    cs.append(flag)

    # mixy mix
    random.shuffle(xs)
    random.shuffle(cs)

    shares = [sum((c * pow(x, i, p)) % p for i, c in enumerate(cs)) % p for x in xs]
    return shares


if __name__ == "__main__":
    try:
        print(f"{p = }")
        queries = input("Gib me the queries: ")
        xs = list(map(lambda x: int(x) % p, queries.split()))

        if 0 in xs or len(xs) > 256:
            print("GUH")
            exit(1)

        shares = share_mixer(xs)
        print(f"{shares = }")
    except:
        exit(1)

• This challenge, we are asked to give a "query" containing a list of integers xs, which its length must be less or equal than 256 and it doesn't contain 0 (mod p) element. Then the server will calculate
  $f(x)$ with random coefficients (one of them is flag) with
  $x$ in xs. The output is shuffled before giving to us.
• Because the output is shuffled, we can not differentiate which value of
  $f(x)$ belongs to
  $x$. So we have to do a trick: Sending a sequence of value like
  $1,2,2,3,3,3,...,n,...,n$, then we based on the frequency of occurrence of elements to determine which value of
  $f(x)$ belongs to
  $x$. Then solve a system of equations to find flag.
• To be able to solve the system of equations, we have to have enough 32 value of
  $f(x)$, which means we need 32 value of
  $x$. But we can recognize that with this method of building the query, its length is too large. So we have another option to build this:
  • With i from 1 to 15: payload += [xs[i]] * i
  • With i from 16 to 30: payload += [xs[i]]*(i-15)
  • payload += [xs[31]]
  • payload += 2*[xs[32]]

xs = [random.randint(0,p) for i in range(33)]

payload = []
for i in range(1,16):
    payload += [xs[i]] * i 
for i in range(16,31):
    payload += [xs[i]]*(i-15)
payload += [xs[31]]
payload += 2*[xs[32]]
payload = ' '.join(str(i) for i in payload)

• After having the outputs, we have to brute all case of permutations (totally
  $3!\ast 3!\ast 2^{13}$), it's feasible to brute-force.
• Solve script:

from pwn import * 
from itertools import permutations
from sage.all import * 
from Crypto.Util.number import *
from tqdm import *
from collections import Counter
import random
io = process(["python3", "chall.py"])
# io = remote("35.187.238.100", 5001)

def find(n, ctr):
	return [element for element, count in ctr.items() if count == n]


def combine(arr):
    perms = [list(permutations(x)) for x in arr]
    
    result = [[]]
    for options in perms:
        result = [x + list(y) for x in result for y in options]
    
    return result
p = int(io.recvlineS().strip().split()[2])
print(p)
xs = [random.randint(0,p) for i in range(33)]

payload = []
for i in range(1,16):
    payload += [xs[i]] * i 
for i in range(16,31):
    payload += [xs[i]]*(i-15)
payload += [xs[31]]
payload += 2*[xs[32]]
payload = ' '.join(str(i) for i in payload)

io.sendlineafter(b'Gib me the queries: ', payload.encode())

shares = eval(io.recvlineS().strip().split('=')[1])
ctr = Counter(shares)
res = []
for i in range(1, 16):
    res.append(find(i, ctr))

candidate = combine(res)
F = GF(p)
mtx = []
x = [xs[1], xs[16],xs[31],xs[2],xs[17],xs[32]]
for i in range(3, 16):
    x += [xs[i], xs[i + 15]]
assert len(x) == 32
for i in x:
	row = []
	for j in range(32):
		row.append(pow(i,j,p))
	mtx.append(row)
# print(mtx)
mtx = Matrix(GF(p), mtx)
inv = ~mtx
for arr in tqdm(candidate):	
	# print(res)
	x = inv * vector(F, arr)
	x = [long_to_bytes(int(i)) for i in x]
	for i in x:
		if b'ISITDTU' in i:
			print(i)
			quit()

ShareMixer2

import random   # TODO: heard that this is unsafe but nvm
from Crypto.Util.number import getPrime, bytes_to_long
flag = bytes_to_long(b'ISITDTU{test_flag_dkosakdisaj}')

# flag = bytes_to_long(open("flag.txt", "rb").read())
# p = getPrime(256)
p = 113862595042194342666713652805343274098934957931279886727932125362984509580161
assert flag < p
l = 32

def share_mixer(xs):
    cs = [random.randint(1, p - 1) for _ in range(l - 1)]
    cs.append(flag)

    # mixy mix
    random.shuffle(xs)
    random.shuffle(cs)
    print(cs[0])
    shares = [sum((c * pow(x, i, p)) % p for i, c in enumerate(cs)) % p for x in xs]
    return shares


if __name__ == "__main__":
    try:
        print(f"{p = }")
        queries = input("Gib me the queries: ")
        xs = list(map(lambda x: int(x) % p, queries.split()))

        if 0 in xs or len(xs) > 32:
            print("GUH")
            exit(1)

        shares = share_mixer(xs)
        print(f"{shares = }")
    except:
        exit(1)

• This challenge, the author limits the length of query to 32, exactly equal to the number of values we need.
• After thinking a while, I realize that if we send -1 and 1 to get
  $f(1)$ and
  $f(-1)$, we can calculate
  $$\sum _{i=2k}^{}{c}_i\ast x^i=\frac{f(-1)+f(1)}{2}$$ Note that
  $-1$ and
  $1$ are the 2nd roots of unity in GF(p). If we send 4th-roots of unity, we can see that
  $$\sum _{i=4k}^{}{c}_i\ast x^i=\frac{f(x_1)+f(x_2)+f(x_3)+f(x_4)}{4}$$
• Therefore, my idea is sending 32nd-roots of unity in GF(p), with the condition that
  $p-1$ must devide to 32. Then we calculate the value of
  $\frac{1}{32}\sum _{}^{}f(x_i)$ and "hope" that the flag is
  $c_0$.
• Solve script:

from pwn import * 
from sage.all import * 
from Crypto.Util.number import *
while True:
    io = remote("35.187.238.100", 5002)
    p = int(io.recvlineS().strip().split()[2])
    if (p-1)%32 != 0:
        io.close()
        continue
    g = int(GF(p).multiplicative_generator())
    g = pow(g, (p-1)//32, p)
    xs = [pow(g, i, p) for i in range(32)]
    payload = ' '.join(str(i) for i in xs)
    io.sendlineafter(b'Gib me the queries: ', payload.encode())
    shares = eval(io.recvlineS().strip().split('=')[1])

    flag = long_to_bytes(sum(shares) * pow(32, -1, p) % p)
    if b'ISITDTU' in flag:
        print(flag)
        quit()
# ISITDTU{M1x_4941n!_73360d0e5fb4}  

Because the author doesn't put PoW on server, we can easily "gacha" this.

Sign

#!/usr/bin/env python3

import os

from Crypto.Util.number import *
from Crypto.Signature import PKCS1_v1_5
from Crypto.PublicKey import RSA
from Crypto.Hash import SHA256

flag = b'ISITDTU{aaaaaaaaaaaaaaaaaaaaaaaaaa}'
flag = os.urandom(255 - len(flag)) + flag


def genkey(e=11):
    while True:
        p = getPrime(1024)
        q = getPrime(1024)
        if GCD(p-1, e) == 1 and GCD(q-1, e) == 1:
            break
    n = p*q
    d = pow(e, -1, (p-1)*(q-1))
    return RSA.construct((n, e, d))


def gensig(key: RSA.RsaKey) -> bytes:
    m = os.urandom(256)
    h = SHA256.new(m)
    s = PKCS1_v1_5.new(key).sign(h)
    ss = bytes_to_long(s)

    return s


def getflagsig(key: RSA.RsaKey) -> bytes:
    return long_to_bytes(pow(bytes_to_long(flag), key.d, key.n))


key = genkey()

while True:
    print(
        """=================
1. Generate random signature
2. Get flag signature
================="""
    )

    try:
        choice = int(input('> '))
        if choice == 1:
            sig = gensig(key)
            print('sig =', sig.hex())
        elif choice == 2:
            sig = getflagsig(key)
            print('sig =', sig.hex())
    except Exception as e:
        print('huh')
        exit(-1)

• In this challenge, the server allows us to sign a random message or sign flag with unlimited times. It uses the PKCS1_v1_5 algorithm to sign. Its python implementation in pycryptodome module is here
• Before RSA decryption, the hashed message is encoded by _EMSA_PKCS1_V1_5_ENCODE function, which will add a fixed padding for a type of hash algorithm. With SHA256 hash algorithm, we can see this padding is:

prefix = b'\x01\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00010\r\x06\t`\x86H\x01e\x03\x04\x02\x01\x05\x00\x04 '

This padding can be easily found with this code

• We can see that: prefix + msg + k*n = sig^e with e=11, so small :v. Note that we can submit to server unlimited time, we can request more random signatures to construct more functions, then use AGCD (Approximate gcd) to find n, then we can easily recover flag.
• Solve script:

from Crypto.PublicKey import RSA
from Crypto.Signature import PKCS1_v1_5
from Crypto.Util.number import *
from Crypto.Hash import *
from pwn import *
import sys
sys.path.append("/mnt/e/tvdat20004/CTF/tools/attacks/acd/")
from ol import * 
io = remote("35.187.238.100", 5003)
io.recvuntil(b'Suffix: ')
poW = input("cccc: ")
io.sendline(poW.encode())
# io = process(["python3", "chall.py"])
def random_sign():
	io.sendlineafter(b'> ', b'1')
	return int(io.recvlineS().strip().split(' ')[2], 16)

prefix = b'\x01\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\x00010\r\x06\t`\x86H\x01e\x03\x04\x02\x01\x05\x00\x04 '
pre = bytes_to_long(prefix) * 2**256 
xs = [(random_sign()**11 - pre) for _ in range(30)]
# print(xs)

n = attack(xs, 256)[0]
io.sendlineafter(b'> ', b'2')
enc = int(io.recvlineS().strip().split(' ')[2], 16)
print(long_to_bytes(pow(enc, 11, n)))

attack function is from here