Really Secure Algorithm Writeup (Dawg CTF 2021)

panda1729

Type: Crypto

Difficulty: Easy

Prompt: I like my e's like I like my trucks: big and obnoxious

Solution

Just a basic RSA challenge with a very big n. But here’s the catch - n is a square! The totient function in this case is not (p-1)² but p(p-1)

Use factordb.com or factordb-pycli (or simply take the squareroot) to factorise the huge n and note the primes (‘prime’ actually since its just one)

Download the file here

Fire up your python console and assign the respective values to n, p, e and c

>>> phi = p*(p-1)
>>> d = pow(e, -1, phi)
>>> m = pow(c, d, n)
>>> bytes.fromhex(hex(m)[2:]).decode('utf-8')
'DawgCTF{sm@ll_d_b1g_dr3am5}'