deaddrop2.sage

def read_data(path):
    with open(path) as f:
        q = int(next(f))
        rest = f.read().replace('L', '')
        return q, eval(rest)

n, rest = read_data('flag.enc')
FF = GF(n)

def logarithm_mod_2(h):
    return int(h^((n-1)/2) != 1)

A = Matrix(Zmod(2), [[logarithm_mod_2(FF(x)) for x in row] for row, _ in rest])
b = vector(Zmod(2), [logarithm_mod_2(FF(x)) for _, x in rest])

# one bit is skipped
l = [0] + list(A\b)
print(bytes([int(''.join(map(str, l[k:k+8])), 2) for k in range(0, len(l), 8)]))