Writeup from SEC-T CTF’18

SEC-T CTF 2018: Marty0ska1

Category: Crypto
Challenge Points: 51
Solves: 60+

Service: nc crypto.sect.ctf.rocks 2222

Really bad they took down the website and services soon after the CTF, I coudn’t complete my write up. So as you read, don’t expect the flag, though the method remains the same.

nc crypto.sect.ctf.rocks 2222

As you are connected to the service, you are given three numbers: p, g, and g^x. They demand x.

Discrete Logarithmic Problem

A stright DLP challenge. Things to notice, g=2, and p is factorizable. Tada!! it is Pohlig-Hellman. Use a sage script to solve the challenge.
Let y=g^x

R = IntegerModRing(p)
x = discrete_log(R(y), R(g))
print x

This will give the flag in less than two seconds. And that it.Submit x, to get the flag.

Flag

**SECT{Ru$$ian_D0LLZ_h0lDs_TH3_S3cR3T}

Further Reading

Tum CTF 2016 tacos