SSSS sekaiCTF 2025

Mon, 18 Aug 2025 00:00:00 +0000

Challenge Description

Shamir SendS the Secret to everyone
Author : Utaha
Points : 100 pts

Source code

import random, os


p = 2 ** 256 - 189

FLAG = os.getenv("FLAG", "SEKAI{}")


def challenge(secret):

 t = int(input())

 assert 20 <= t <= 50, "Number of parties not in range"


 f = gen(t, secret)


 for i in range(t):

 x = int(input())

 assert 0 < x < p, "Bad input"

 print(poly_eval(f, x))


 if int(input()) == secret:

 print(FLAG)

 exit(0)

 else:

 print(":<")


def gen(degree, secret):

 poly = [random.randrange(0, p) for _ in range(degree + 1)]

 index = random.randint(0, degree)


 poly[index] = secret

 return poly


def poly_eval(f, x):

 return sum(c * pow(x, i, p) for i, c in enumerate(f)) % p


if __name__ == "__main__":

 secret = random.randrange(0, p)

 for _ in range(2):

 challenge(secret)

Let’s say S is the secret that we wish to share.

Sekaictf2025 on Nabil Kara — CTF Writeups

SSSS sekaiCTF 2025

Challenge Description

Source code

How Shamir’s secret sharing scheme (SSS) works ?