Guess password easy Google beginner's quest CTF 2026 write up

Source: Dev.to

Overview

The first CTF of the beginner’s quest series is Guess password Easy, a crypto‑category challenge with the description: “You will never guess my password, even if I give you the first 5 letters!”

The challenge provides a snippet of the server’s source code and a network service to connect to.

Challenge Details

Connecting to the service shows:

$ nc guess-password-easy.2025-bq.ctfcompetition.com 1337
== proof-of-work: disabled ==
Password is mnyko...............
Your guess:

The server prints the first five characters of a randomly generated 20‑character password and then waits for a full guess.

Source Code Analysis

Key parts of the provided source:

// Lines 14‑21
string generateRandomPassword()
{
    string res(20, '.');
    for (int i = 0; i 
#include 
#include 
#include 

std::string generateRandomPassword()
{
    std::string res(20, '.');
    for (int i = 0; i  **Note:** `srand` behaves differently across operating systems. To reproduce the exact passwords generated by the challenge, compile and run the program on a Linux system.

### Running the attack  

```bash
$ g++ main.cpp -o seed_bruteforce
$ ./seed_bruteforce
$ less output.txt   # search for the displayed prefix, e.g. /ftbrh

The search reveals the full password that matches the shown prefix.

Solving the Challenge

When the server displayed:

Password is ftbrh......
Your guess:

the brute‑force script found the corresponding full password:

ftbrhpjdhohfnmqqaddv

Submitting this guess to the service yields the flag:

$ nc guess-password-easy.2025-bq.ctfcompetition.com 1337
== proof-of-work: disabled ==
Password is ftbrh......
Your guess: ftbrhpjdhohfnmqqaddv
CTF{flag}

Conclusion

By exploiting the predictable srand(time(0)) seed, we reduced the problem to a small time‑window brute‑force search. The approach works for any similar challenge where a password is derived from a time‑seeded PRNG and a partial prefix is disclosed.