ångstromCTF 2022
Note : A JOURNEY TO GAIN KNOWLEDGE
Caesar and Desister
sulx{klgh_jayzl_lzwjw_ujqhlgyjshzwj_kume}
Randomly Sampled Algorithm
Vinegar Factory
Source
#!/usr/local/bin/python3
import string
import os
import random
with open("flag.txt", "r") as f:
flag = f.read().strip()
alpha = string.ascii_lowercase
def encrypt(msg, key):
ret = ""
i = 0
for c in msg:
if c in alpha:
ret += alpha[(alpha.index(key[i]) + alpha.index(c)) % len(alpha)]
i = (i + 1) % len(key)
else:
ret += c
return ret
inner = alpha + "_"
noise = inner + "{}"
print("Welcome to the vinegar factory! Solve some crypto, it'll be fun I swear!")
i = 0
while True:
if i % 50 == 49:
fleg = flag
else:
fleg = "actf{" + "".join(random.choices(inner, k=random.randint(10, 50))) + "}"
start = "".join(random.choices(noise, k=random.randint(0, 2000)))
end = "".join(random.choices(noise, k=random.randint(0, 2000)))
key = "".join(random.choices(alpha, k=4))
print(f"Challenge {i}: {start}{encrypt(fleg + 'fleg', key)}{end}")
x = input("> ")
if x != fleg:
print("Nope! Better luck next time!")
break
i += 1
Overview
When connect to the server:
Analysis:
Easily to see that we have to bypass 50 levels of this chall to get the flag
The length of key is 4
The strange code is {start}{encrypt(fleg + 'fleg', key)}{end}
That means, {start} and {end} are random from noise
Fake flag is random from inner, now we can determind the position of char '{', '}' and '_' because they aren't encrypted
The most important thing is: the server has a 20 second timeout so that we have to use pwntools for scripting.
Idea:
From this we can guess the last 4 characters is the cipher of 'fleg' then we can find the key easily by decrypt Vigenere, finally use the key to decrypt flag.
Solve
import string
import os
import random
import re
from itertools import permutations
from pwn import *
p= remote("challs.actf.co", 31333)
for Y in range(50):
message = p.recvuntil("\n> ",drop= True).decode()
if Y < 10:
line = message[86:]
else:
line = message[87:]
alpha = string.ascii_lowercase
inner = alpha + "_"
k = 5
head = []
head= ([i.start() for i in re.finditer('{', line)])
tail= ([i.start() for i in re.finditer('}', line)])
flag = []
for i in head:
for j in tail:
if i<2000 and j-i >= 10 and j-i <= 50 and line[i-4] in inner and line[i-3] in inner and line[i-2] in inner and line[i-1] in inner:
try:
if line[j+4] in inner and line[j+3] in inner and line[j+2] in inner and line[j+1] in inner and len(line[i+1:j]) >= 10:
if '{' not in line[i+1:j] and '}' not in line[i+1:j]:
flag.append(line[i-4:j+5])
except:
continue
def convertTuple(tup):
# initialize an empty string
str = ''
for item in tup:
str = str + item
return str
def decrypt(cipher, key):
ret = ""
i = 0
for c in cipher:
if c in alpha:
ret += alpha[alpha.index(c)-(alpha.index(key[i]) ) % len(alpha)]
i = (i + 1) % len(key)
else:
ret += c
return ret
def find_key(cipher, msg):
ret = ""
i = 0
for i in range(0,4):
ret += alpha[alpha.index(cipher[i])-(alpha.index(msg[i]) ) % len(alpha)]
return ret
may_be_flag = []
def allPermutations(str):
# Get all permutations of string 'ABC'
permList = permutations(str)
# print all permutations
for i in permList:
i = convertTuple(i)
for f in flag:
try:
may_be_flag.append(decrypt(f[:-4],i))
except:
continue
key_list = []
for i in flag:
try:
key = find_key(i[-4:],'fleg')
except:
continue
key_list.append(key)
for i in key_list:
allPermutations(i)
superplag = []
for i in may_be_flag:
if 'actf' in i:
print(i,Y)
p.sendline(i)
break
log log log
Source
from secrets import randbits
from flag import flag
flagbits = len(flag) * 8
flag = int(flag.hex(),16)
q = 127049168626532606399765615739991416718436721363030018955400489736067198869364016429387992001701094584958296787947271511542470576257229386752951962268029916809492721741399393261711747273503204896435780180020997260870445775304515469411553711610157730254858210474308834307348659449375607755507371266459204680043
p = q * 2^1024 + 1
assert p in Primes()
nbits = p.nbits()-1
e = randbits(nbits-flagbits)
e <<= flagbits
e |= flag
K = GF(p)
g = K.multiplicative_generator()
a = g^e
print(hex(p))
print(g)
print(hex(a))
print(flagbits)
#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
#3
#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
#880Analysis
p = q * 2^1024 + 1
Actually, from 3 lines we know that the 880 lowest bits of e are the bits of flag
e = randbits(nbits-flagbits) e <<= flagbits e |= flagNow the challenge is return how to solve the discrete log problem
Idea
To solve this, we have to know a little bit about Legendre symbol then i got an interesting thing:
That mean if r is even or the least significant bit is 0 then x is a quadratic residue (legendre_symbol(a, p) == 1)
Otherwise, r is odd and (legendre_symbol(a, p) == 0)
Solve
def legendre_symbol(a, p):
""" Compute the Legendre symbol a|p using
Euler's criterion. p is a prime, a is
relatively prime to p (if p divides
a, then a|p = 0)
Returns 1 if a has a square root modulo
p, -1 otherwise.
"""
ls = pow(a, (p - 1) // 2, p)
return -1 if ls == p - 1 else ls
def modular_sqrt(a, p):
""" Find a quadratic residue (mod p) of 'a'. p
must be an odd prime.
Solve the congruence of the form:
x^2 = a (mod p)
And returns x. Note that p - x is also a root.
0 is returned is no square root exists for
these a and p.
The Tonelli-Shanks algorithm is used (except
for some simple cases in which the solution
is known from an identity). This algorithm
runs in polynomial time (unless the
generalized Riemann hypothesis is false).
"""
# Simple cases
#
if legendre_symbol(a, p) != 1:
return 0
elif a == 0:
return 0
elif p == 2:
return p
elif p % 4 == 3:
return pow(a, (p + 1) // 4, p)
# Partition p-1 to s * 2^e for an odd s (i.e.
# reduce all the powers of 2 from p-1)
#
s = p - 1
e = 0
while s % 2 == 0:
s //= 2
e += 1
# Find some 'n' with a legendre symbol n|p = -1.
# Shouldn't take long.
#
n = 2
while legendre_symbol(n, p) != -1:
n += 1
# Here be dragons!
# Read the paper "Square roots from 1; 24, 51,
# 10 to Dan Shanks" by Ezra Brown for more
# information
#
# x is a guess of the square root that gets better
# with each iteration.
# b is the "fudge factor" - by how much we're off
# with the guess. The invariant x^2 = ab (mod p)
# is maintained throughout the loop.
# g is used for successive powers of n to update
# both a and b
# r is the exponent - decreases with each update
#
x = pow(a, (s + 1) // 2, p)
b = pow(a, s, p)
g = pow(n, s, p)
r = e
while True:
t = b
m = 0
for m in range(r):
if t == 1:
break
t = pow(t, 2, p)
if m == 0:
return x
gs = pow(g, 2 ** (r - m - 1), p)
g = (gs * gs) % p
x = (x * gs) % p
b = (b * g) % p
r = m
p = 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
g = 3
a = 0xaf99914e5fb222c655367eeae3965f67d8c8b3a0b3c76c56983dd40d5ec45f5bcde78f7a817dce9e49bdbb361e96177f95e5de65a4aa9fd7eafec1142ff2a58cab5a755b23da8aede2d5f77a60eff7fb26aec32a9b6adec4fe4d5e70204897947eb441cc883e4f83141a531026e8a1eb76ee4bff40a8596106306fdd8ffec9d03a9a54eb3905645b12500daeabdb4e44adcfcecc5532348c47c41e9a27b65e71f8bc7cbdabf25cd0f11836696f8137cd98088bd244c56cdc2917efbd1ac9b6664f0518c5e612d4acdb81265652296e4471d894a0bd415b5af74b9b75d358b922f6b088bc5e81d914ae27737b0ef8b6ac2c9ad8998bd02c1ed90200ad6fff4a37
flagbits = 880
flag = ''
for _ in range(flagbits):
print(_)
if legendre_symbol(a, p) == 1:
flag += '0'
a = modular_sqrt(a, p)
elif legendre_symbol(a, p) == -1:
flag += '1'
a = modular_sqrt(a * pow(g, -1, p), p)
print(flag)Last updated
