import random
from secret import FLAG
assert FLAG.startswith('n1ctf{')
assert FLAG.endswith('}')
SECRET = bytes.fromhex(FLAG[6:-1])
assert len(SECRET) == 16
p = 251
e = [1, 20, 113, 149, 219]
y = b''
for x in range(1, p):
coeff = [random.choice(e)] + list(SECRET)
y += bytes([sum(c * pow(x, i, p) for i, c in enumerate(coeff)) % p])
print(f'Token: {y.hex()}')
import sys
from secret import FLAG, REGISTER, TAPS
assert FLAG.startswith('flag')
assert len(REGISTER) == 16
assert len(TAPS) == 5
class LFSR:
def __init__(self, register, taps):
self.register = register
self.taps = taps
def next(self):
new = 0
ret = self.register[0]
for i in self.taps:
new ^= self.register[i]
self.register = self.register[1:] + [new]
return ret
def encrypt():
enc_flag = []
for char in FLAG.encode():
enc_char = 0
for binary in '{:08b}'.format(char):
enc_char <<= 1
enc_char += (int(binary) ^ lfsr.next())
enc_flag.append(enc_char)
return bytes(enc_flag)