xctf攻防世界 CRYPTO高手进阶区 cr4-poor-rsa

0x01. 进入环境，下载附件

题目给出的一个压缩包，解压压缩包后，发现是一个无后缀文件，放入kali中查看：

0x02. 问题分析

0x02_1. 得到套娃文件

修改文件的后缀为.tar，并进行压缩，得到两个文件：

0x02_2. 拿到公钥信息

from Crypto.PublicKey import RSA


with open('pic/key.pub', 'rb') as file:
f = file.read()

pub = RSA.importKey(f)
n = pub.n
e = pub.e

其中

n=833810193564967701912362955539789451139872863794534923259743419423089229206473091408403560311191545764221310666338878019

尝试在线进行大素数分解：​​大素数分解​​​，得到分解后的 和 ，其中：

p = 863653476616376575308866344984576466644942572246900013156919
q = 965445304326998194798282228842484732438457170595999523426901

0x02_3. 得到e

由p,q,e得到d

from gmpy2 import invert


p = 863653476616376575308866344984576466644942572246900013156919
q = 965445304326998194798282228842484732438457170595999523426901
d = int(invert(e, (p - 1) * (q - 1)))

0x02_4. 得到私钥

rsa库的PrivateKey生成私钥

import rsa

key = rsa.PrivateKey(n, e, d, p, q)

0x02_5. 对flag.b64用base64解码，再用key解密

from base64 import b64decode


with open('pic/flag.b64', 'r') as file:
f = file.read()
c = b64decode(f)
flag = rsa.decrypt(c, key).decode()
print(flag)

最终答案为：​​ALEXCTF{SMALL_PRIMES_ARE_BAD}​​

0x02_6. 总体代码

代码整理如下：

from Crypto.PublicKey import RSA
from base64 import b64decode
from gmpy2 import invert
import rsa


with open('pic/key.pub', 'rb') as file:
f = file.read()
pub = RSA.importKey(f)
n = pub.n
e = pub.e

p = 863653476616376575308866344984576466644942572246900013156919
q = 965445304326998194798282228842484732438457170595999523426901
d = int(invert(e, (p - 1) * (q - 1)))

key = rsa.PrivateKey(n, e, d, p, q)

with open('pic/flag.b64', 'r') as file:
f = file.read()
c = b64decode(f)
flag = rsa.decrypt(c, key).decode()

print(flag)

RSA还是重中之重，长时间未接触，有点忘记，对于公钥私钥的加解密需要重新整理。