#!/usr/bin/env python3 from Crypto.Util.number import * from flag import flag l = len(flag) m_1, m_2 = flag[: l // 2], flag[l // 2:] x, y = bytes_to_long(m_1), bytes_to_long(m_2) k = ''' 000bfdc32162934ad6a054b4b3db8578674e27a165113f8ed018cbe9112 4fbd63144ab6923d107eee2bc0712fcbdb50d96fdf04dd1ba1b69cb1efe 71af7ca08ddc7cc2d3dfb9080ae56861d952e8d5ec0ba0d3dfdf2d12764 '''.replace('\n', '') assert((x**2 + 1)*(y**2 + 1) - 2*(x - y)*(x*y - 1) == 4*(int(k, 16) + x*y))
sageに突っ込んでfactorする。こういう感じ
sage: x, y = var("x, y")
sage: f = ((x2 + 1)*(y2 + 1) - 2*(x - y)*(x*y - 1) - 4*x*y) / 4
sage: f.factor()
1/4*(x + 1)2*(y - 1)2
すると簡単そうになるのでを計算して、のfactorをとって、候補を列挙する
s = 992752253935874779143952218845275961347009322164731344882417010624071055636710540798045985678351986133612 factors = [2, 2 , 3 , 11, 11 , 19 , 47 , 71 , 3449 , 11953 , 5485619 , 2035395403834744453 , 17258104558019725087 , 1357459302115148222329561139218955500171643099] from itertools import product from Crypto.Util.number import long_to_bytes for pat in product([0,1], repeat=len(factors)): x = 1 for i in range(len(pat)): if pat[i] == 1: x *= factors[i] flag_car = long_to_bytes(x - 1) if not flag_car.startswith(b"CCTF"): continue y = s // x flag_cdar = long_to_bytes(y + 1) print(flag_car + flag_cdar) quit()