The Slot Whisperer - Crypto
Points: 50 | Flag: 0xfun{sl0t_wh1sp3r3r_lcg_cr4ck3d} | Solved by: Smothy @ 0xN1umb
what we got
nc connection and a python file showing a slot machine. classic LCG vibes:
class SlotMachineLCG:
def __init__(self, seed=None):
self.M = 2147483647 # 2^31 - 1
self.A = 48271
self.C = 12345
self.state = seed if seed is not None else 1
def next(self):
self.state = (self.A * self.state + self.C) % self.M
return self.state
def spin(self):
return self.next() % 100server gives us 10 spins and wants us to predict the next 5. ez.
the solve
LCG = Linear Congruential Generator. literally the most predictable "random" ever lmao
the math is just: state = (A * state + C) % M
we only see state % 100 tho, so we gotta recover the full internal state. but like... we know all the parameters (A, C, M) so we can just brute force it
for each spin value (0-99), there's about 21 million possible internal states that could produce it. sounds like a lot but nah - we check if a candidate state produces ALL 10 observed outputs. most fail immediately on the 2nd check so its fast af
from pwn import *
M = 2147483647
A = 48271
C = 12345
def crack_lcg(outputs):
o0 = outputs[0]
for k in range(M // 100 + 1):
state = 100 * k + o0
if state >= M:
break
# check if this state produces all outputs
valid = True
test_state = state
for expected in outputs:
if test_state % 100 != expected:
valid = False
break
test_state = (A * test_state + C) % M
if valid:
return test_state # return state AFTER observations
return None
def predict_next(state, n=5):
predictions = []
for _ in range(n):
predictions.append(state % 100)
state = (A * state + C) % M
return predictions
r = remote('chall.0xfun.org', 48289)
# grab 10 spins
outputs = [int(r.recvline().decode().strip()) for _ in range(10)]
print(f"Observed: {outputs}")
# crack it
next_state = crack_lcg(outputs)
predictions = predict_next(next_state, 5)
print(f"Predictions: {predictions}")
# send it
r.recvuntil(b"Predict the next 5 spins (space-separated):")
r.sendline(' '.join(map(str, predictions)).encode())
print(r.recvall(timeout=3).decode())run it:
Observed spins: [79, 94, 26, 3, 2, 76, 37, 5, 97, 41]
Cracking LCG state...
Found next state: 986752626
Predictions: [26, 31, 60, 42, 28]
JACKPOT! You've mastered the slot machine!
0xfun{sl0t_wh1sp3r3r_lcg_cr4ck3d}
flag
0xfun{sl0t_wh1sp3r3r_lcg_cr4ck3d}
ngl this is why you never use LCG for anything security related. took like 2 seconds to crack lol
smothy out ✌️