H&NCTF

1.lcgp

常规的LCG问题，

观察函数generate

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def generate(self):
        self.seed = (self.a * self.seed + self.b) % self.m  # %代表取余运算
        return self.seed

输入seed，获得一个新的seed 接着循环了5次，产生了5次种子

换而言之，我们需要利用这5个种子，求出一开始的种子

不妨设，一开始的seed为x1，然后我们写成数列的形式 已知求 接着来看算法的关键 这里面a，b，m都是未知的，b好处理，因为在常数的位置，两个式子相减就能消掉

考虑 这时候，我们以作为一个新数列，那么这时候只剩下两个未知数了

设

那么

于是构造了一个等比数列 说到等比数列，等比中项就是一个有趣的性质，那么，对于这个同余的等比数列，是否有着等比中项呢，可以一试 a消失了，那么此时，只剩下m这一个未知数

把同余式写成等式 可以求出k1m,k2m,等等

如果两个k没有公因数的话，那么m就是它们的最大公因数

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from Crypto.Util.number import *
y = []
km = []
possiblem = []
for i in range(4):
    y.append(x[i+1]-x[i])
for j in range(2):
    km.append(y[j+1]*y[j+1]-y[j]*y[j+2])
    if j >= 1:
        if isPrime(GCD(km[j],km[j-1])): 
            possiblem.append(GCD(km[j],km[j-1]))

这样我们就得到了可能为m的possiblem

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17685238729089629081000199791073775580173848219782600577303616658328942786376878589055722139055973814631247947507009292556525580971952751595622172013382374577299734041726162411213993858497140178230251563824957482883763872967881950049985507310408962206729214890421322185752340466578359217999838551622613116097987673746844913920603425686103175874552686869764028906518131374266999714020645422015426801070343771422250037085635856203202935123956520415086942807824199018302630540169113637093488756273183240186533936637562954227188102214396705205042491881526862572569275650908415160529650027016717502470758797514488134766043

只有一个，这非常好，既然m求出来了，那么就可以求a了 

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a = y[1]*inverse(y[0],m) % m

我们先考虑求出a之后如何解决 

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x_1 = (x[0] + inverse(a,m)*(x[0]-x[1])) % m 
print(long_to_bytes(x_1))

exp:

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from Crypto.Util.number import *
y = []
km = []
possiblem = []
for i in range(4):
    y.append(x[i+1]-x[i])
for j in range(2):
    km.append(y[j+1]*y[j+1]-y[j]*y[j+2])
    if j >= 1:
        if isPrime(GCD(km[j],km[j-1])): 
            possiblem.append(GCD(km[j],km[j-1]))
m = 17685238729089629081000199791073775580173848219782600577303616658328942786376878589055722139055973814631247947507009292556525580971952751595622172013382374577299734041726162411213993858497140178230251563824957482883763872967881950049985507310408962206729214890421322185752340466578359217999838551622613116097987673746844913920603425686103175874552686869764028906518131374266999714020645422015426801070343771422250037085635856203202935123956520415086942807824199018302630540169113637093488756273183240186533936637562954227188102214396705205042491881526862572569275650908415160529650027016717502470758797514488134766043
a = y[1]*inverse(y[0],m) % m
print(a)
x_1 = (x[0] + inverse(a,m)*(x[0]-x[1])) % m 
print(long_to_bytes(x_1))

哈基coke

逆向Arnold即可

exp:

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import numpy as np
import cv2

def arnold_decode(encoded_image, shuffle_times, a, b):
    """
    Arnold逆变换解密图像
    
    参数:
        encoded_image: 加密后的图像
        shuffle_times: 加密时使用的迭代次数
        a, b: 加密时使用的参数
        
    返回:
        解密后的图像
    """
    decoded_image = np.zeros(shape=encoded_image.shape)
    h, w = encoded_image.shape[0], encoded_image.shape[1]
    N = h  # 假设图像是正方形
    
    # 计算逆变换的参数
    det = (1 * (a*b + 1) - b * a)  # 行列式值
    inv_det = pow(det, -1, N)  # 模逆元
    
    # 逆变换矩阵
    inv_a = (a*b + 1) * inv_det % N
    inv_b = (-b) * inv_det % N
    inv_c = (-a) * inv_det % N
    inv_d = 1 * inv_det % N
    
    for time in range(shuffle_times):
        for new_x in range(h):
            for new_y in range(w):
                # 计算原始位置
                ori_x = (inv_a * new_x + inv_b * new_y) % N
                ori_y = (inv_c * new_x + inv_d * new_y) % N
                
                decoded_image[ori_x, ori_y, :] = encoded_image[new_x, new_y, :]
        
        encoded_image = np.copy(decoded_image)
    
    return decoded_image

# 使用示例
if __name__ == "__main__":
    # 读取加密后的图像
    encrypted_img = cv2.imread(r"C:\Users\infinite\Desktop\python\hnctf\en_flag.png")
    
    # 解密参数必须与加密时一致
    a = 9
    b = 1
    shuffle_times = 6  # 加密时使用的迭代次数
    
    # 解密图像
    decrypted_img = arnold_decode(encrypted_img, shuffle_times, a, b)
    
    # 保存解密后的图像
    cv2.imwrite('decrypted_flag.png', decrypted_img, [int(cv2.IMWRITE_PNG_COMPRESSION), 0])
    
    print("图像解密完成，已保存为 decrypted_flag.png")

image-20250608123628924

为什么出题人的rsa总是ez

分为两个挑战

part1涉及到common prime的生成 注意到 注意到 那么 注意到p,q相对于n来说都是小根,可以考虑作为短向量

构造 那么 就得到了一个短向量

使用LLLcvp

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L = matrix([[n, 0, 0, 0], [h1 * h2, 1, 0, 0], [-h2, 0, 1, 0], [-h1, 0, 0, 1]])
lb = [0, 1, 0, 0]
ub = [0, 1, 2**1024, 2**1024]
sol = solve_inequality(L, lb, ub)
_, _, p, q = map(int, sol)
assert p * q == n
phi = (p - 1) * (q - 1)
d = pow(0x10001, -1, phi)
m = pow(c, d, n)
print(long_to_bytes(m))

得到flag{e_is_xevaf-cityf-fisof-ketaf-metaf-disef-nuvaf-cysuf-dosuf-getuf-cysuf-dasix,bubbleBabble}

提示了bubbleBabble加密

用网站解码得到

e = 81733668723981020451323

part2 特殊素数生成

common prime已知g

强网杯的题，稍微修改一下

第八届强网杯-wp_第八届强网杯wp-CSDN博客

里ezrsa的脚本

exp:

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def Solve_c():
    sqrt_N = int(sqrt(N))
    C = sqrt_N//g^2
    a = 2
    b = pow(a,g,N)

    for i in range(2,C):
        D = (int(sqrt(C)) + 1) * i
        final = pow(int(b),int(u),int(N))
        tmp = 1
        gs = pow(int(b), int(D), N)
        for r in tqdm(range(D)):
            for s in range(D):
                #if powmod(int(b),int(r*D+s),N) == final:
                if tmp == final:
                    print("r =",r,"s =",s,"i =",i)
                    return r * D + s
                tmp = (tmp * int(b)) % N
c = Solve_c()
A = u - c  # x * y = u - c

B = v + c * g  # x + y = v + c * g

delta = iroot((B * B - 4 * A), 2)[0]

x = (B + delta) // 2

y = (B - delta) // 2

a = x // 2

b = y // 2

p = 2 * g * a + 1

q = 2 * g * b + 1

d = invert(e, (p - 1) * (q - 1))

m = powmod(C, d, N)

print(long_to_bytes(m))

数据处理

简单的求个离散对数

然后这里数字被替换了

直接遍历一遍所有组合，爆破就行了

exp:

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from tqdm import *
Z = Zmod(2^512)
m = 5084057673176634704877325918195984684237263100965172410645544705367004138917087081637515846739933954602106965103289595670550636402101057955537123475521383
c = 2989443482952171039348896269189568991072039347099986172010150242445491605115276953489889364577445582220903996856271544149424805812495293211539024953331399
new_flag=str(discrete_log(Z(c), Z(m)))
lowercase = '0123456789' 
uppercase = '7***4****5' 
from itertools import permutations
for i in tqdm(permutations("0123689")):
    up = f"7{''.join(i[:3])}4{''.join(i[3:])}5"
    table = ''.maketrans(up, lowercase) 
    flag = new_flag.translate(table)
    try:
        res = (long_to_bytes(int(flag)).decode())
        if res.startswith("H&NCTF"):
            print(res)
    except:
        pass

ez-factor

注意到 其中k是512位，r是248位，r远远小于hint

那么我们有 r是一个小根，使用CopperSmith慢慢调参数即可

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R = 2^248  # r 的上界
e=0x10001
P.<x> = PolynomialRing(Zmod(N))
f = hint -x
f = f.monic()
# r = f.small_roots(X=R, beta=0.495, epsilon=0.015)[0] 
# print("Found r =", r)
r = 310384729555967603261671853388867753979360895944109353196595111340924855459
p = gcd(hint - r, N)
q = N // p
assert p * q == N
print("Found p =", p)
print("Found q =", q)
d = inverse(e,(p-1)*(q-1))
m = pow(c,d,N)
print(long_to_bytes(m))

ez-factor-pro

后面只是障眼法，关键还是求r

这里r变成了252位，怎么调参发现都不太行

所以考虑爆破几位r

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R = 2^252  # r 的上界
e=0x10001
P.<x> = PolynomialRing(Zmod(N))
high_bits = 12 #爆破高位
X = 2^(rbits - high_bits) - 1  

for high in trange(2^high_bits,-1,-1):
    r_high = high << (rbits - high_bits)
    
    # 多项式：hint - (r_high + x) ≡ 0 mod p
    P.<x> = PolynomialRing(Zmod(N))
    f = hint - (r_high + x)
    f = f.monic()
    roots = f.small_roots(X, beta=0.495, epsilon=0.03)
    
    if roots:
        r_low = roots[0]
        r = r_high + r_low
        print(f"Found r = {r}")
        break

得到r，后面就没什么了

exp:

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r = 7166351305785506670352015492214713707534657162937963088592442157834795391917
c = "476922b694c764725338cca99d99c7471ec448d6bf60de797eb7cc6e71253221035eb577075f9658ac7f1a40747778ac261787baad21ee567256872fa9400c37"
c = bytes.fromhex(c)
leak=N*r
p = gcd(hint - r, N)
q = N // p
r_bytes = long_to_bytes(leak)
iv = r_bytes[:16] if len(r_bytes) >= 16 else r_bytes + b'\0'*(16-len(r_bytes))
key = sha256(str(p + q + r).encode()).digest()[:16] 
crypt_sm4 = CryptSM4()
crypt_sm4.set_key(key, SM4_DECRYPT) 
decrypted = crypt_sm4.crypt_cbc(iv, c)  
flag = unpad(decrypted, 16)
print("Flag:", flag.decode())