chall.sage

#!/usr/bin/sage
import hashlib
import re

with open("flag.txt", "rb") as f:
    FLAG = f.read()
    assert re.match(rb"Hero{[0-9a-zA-Z_]{90}}", FLAG)

F = FiniteField(2**256 - 189)
R = PolynomialRing(F, "x")
H = lambda n: int(hashlib.sha256(n).hexdigest(), 16)
C = lambda x: [H(x[i : i + 4]) for i in range(0, len(FLAG), 4)]

f = R(C(FLAG))

points = []
for _ in range(f.degree()):
    r = F.random_element()
    points.append([r, f(r)])
print(points)

try:
    flag = input(">").encode().ljust(len(FLAG))

    g = R(C(flag))

    for p in points:
        if g(p[0]) != p[1]:
            print("Wrong flag!")
            break
    else:
        print("Congrats!")
except Exception:
    pass