概述
光滑数 (Smooth number)：指可以分解为多个小素数乘积的正整数
当p是N 的因数，并且p−1是光滑数，可以考虑使用Pollard's p-1算法来分解N
当p是N的因数，并且p+1是光滑数，可以考虑使用Williams's p+1算法来分解N
这里只介绍pollard p-1算法
算法核心
假设存在素数p，p是合数n的因子，p-1可以被分解成多个小素数k1,k2,k3......kn<B(其中B是我们选取的一个上界）
p-1=k1*k2*k3...*kn
那么存在 p-1 | B！
由费马小定理可知：
因为p-1 | B！所以
所以
而我们的目的就是求出p
于是乎
代码实现：这里m取最简单的值：2
#脚本1
def Pollard(B, n):
a = pow(2, math.factorial(B), n)
return math.gcd(a-1, n)
math.factorial(B)是求B的阶乘
a = pow(2, math.factorial(B), n) 这里模n可以减小数值，加快计算，否则脚本执行会很慢很慢
#脚本2
m = 2
i = 2
while True:
a = pow(m, i, n)
p = gmpy2.gcd(a-1, n)
if p != 1 and p != n:
q = n // p
print("p=",p)
print("q=",q)
break
i += 1
这里没用上界B，而是不断用m乘i，构成阶乘，从而找到上界
案例
2024_newstar_crypto
from Crypto.Util.number import *
from random import choice
from enc import flag
m = bytes_to_long(flag)
def get_primes(limit):
primes = []
is_prime = [True] * (limit + 1)
for num in range(2, limit + 1):
if is_prime[num]:
primes.append(num)
for multiple in range(num * num, limit + 1, num):
is_prime[multiple] = False
return primes
def get_Prime(bits):
while True:
n = 2
while n.bit_length() < bits:
n *= choice(primes)
if isPrime(n + 1):
return n + 1
e = 65537
primes = get_primes(e)
p = get_Prime(512)
q = get_Prime(512)
n = p*q
c = pow(m,e,n)
print(f"n = {n}")
print(f"e = {e}")
print(f"c = {c}")
'''
n = 11039465757321779749699115271175512667139900174718736810369447320076323722255434292310358024561522508796910115450323496447022239437773990981536427837406218598107069494432101729405702532145398564394815485237091250665627736733029998539907483103936019387506766847901269370907583069348340970264589547521878184740704877
e = 65537
c = 999985242089285281573358992862050772171553905758195933815934618530062196165616033686130348967500088738894025554769544651219372069639483611911419044407380282591769389311004388591587725946534597210624224229109408954620531714696257219047380587652138116530227788392134058889161962899416080258311278619324733722515967
'''这里的光滑数是p+1，但本质都是一样的
脚本代码
from Crypto.Util.number import *
from random import choice
from sympy import factorint
import math
import gmpy2
n = 11039465757321779749699115271175512667139900174718736810369447320076323722255434292310358024561522508796910115450323496447022239437773990981536427837406218598107069494432101729405702532145398564394815485237091250665627736733029998539907483103936019387506766847901269370907583069348340970264589547521878184740704877
e = 65537
c = 999985242089285281573358992862050772171553905758195933815934618530062196165616033686130348967500088738894025554769544651219372069639483611911419044407380282591769389311004388591587725946534597210624224229109408954620531714696257219047380587652138116530227788392134058889161962899416080258311278619324733722515967
def Pollard(B, n):
a = pow(2, math.factorial(B),n)
return math.gcd(a-1, n)
p=Pollard(e,n)
q=n//p
phi=(p-1)*(q-1)
d=gmpy2.invert(e,phi)
flag=pow(c,d,n)
print(long_to_bytes(flag))
