Asemoon - CryptoCTF - writeup

Tue, 06 Jan 2026 00:00:00 +0000

In this challenge, we are given a file called asemoon.sage. It starts by generating some parameters:

def gen_CP(nbit):
	R.<x> = PolynomialRing(GF(2))
	while True:
		c = ''.join([hex(randint(0, 15))[2:] for _ in range(nbit >> 2)])
		G = R(Integer(int(c, 16)).bits()[::-1]) + x ^ nbit
		if G.is_irreducible():
			return int(c, 16)

nbit = 64
skey = getRandomNBitInteger(nbit)
CP, CT = gen_CP(nbit), []
for _ in range(256):
    cb = _
    for _ in range(8):
        if cb & 1:
            cb = (cb >> 1) ^^ CP
        else:
            cb >>= 1
    CT.append(cb & 0xFFFFFFFFFFFFFFFF)

For now, it is enough to know that CP is a random integer of less than 64 bits and CT is generated from CP deterministically. Then, we have the main loop of the challenge:

CRC on manbolq's Blog

Asemoon - CryptoCTF - writeup