题目来源:alexctf-2017

0x00 下载附件

1、补全后缀名

修改文件后缀名为.tar

2、解压文件

解压缩，得到base64编码的flag文件flag.b64和RSA的公钥key.pub

flag.b64：

Ni45iH4UnXSttNuf0Oy80+G5J7tm8sBJuDNN7qfTIdEKJow4siF2cpSbP/qIWDjSi+w=

key.pub：

-----BEGIN PUBLIC KEY-----
ME0wDQYJKoZIhvcNAQEBBQADPAAwOQIyUqmeJJ7nzzwMv5Y6AJZhdyvJzfbh4/v8
bkSgel4PiURXqfgcOuEyrFaD01soulwyQkMCAwEAAQ==
-----END PUBLIC KEY-----

0x01 读取RSA参数信息

1、首先获取n和e的值

python脚本：

from Crypto.PublicKey import RSAwith open("./key.pub", "rb") as file:key = file.read()print(key)
pub = RSA.importKey(key)
n = pub.n
e = pub.e
print("n = ", n)
print("e = ", e)

运行结果：

n =  833810193564967701912362955539789451139872863794534923259743419423089229206473091408403560311191545764221310666338878019
e =  65537

2、使用factordb.com对n进行分解

得到p和q：

p = 863653476616376575308866344984576466644942572246900013156919
q = 965445304326998194798282228842484732438457170595999523426901

3、计算d的值

p = 863653476616376575308866344984576466644942572246900013156919
q = 965445304326998194798282228842484732438457170595999523426901
phi_n = (p - 1) * (q - 1)
d = gmpy2.invert(e, phi_n)
print("d = ", d)

d =  521250646663056391768764366517618655312275374668692430321064634566533568373969990465313092928455546989832961905578375473

0x02 解密密文

1、生成私钥

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

2、对密文进行解密

首先是base64解码，然后是rsa解密

with open("./flag.b64", "rb") as file:cipher = file.read()
cipher = base64.b64decode(cipher)
flag = rsa.decrypt(cipher, priv).decode()
print(flag)

0x03 得到flag

ALEXCTF{SMALL_PRIMES_ARE_BAD}