CTF - Problem with private key authentication

Marc Neugebauer

Hi,

I found an openssh private key whilst doing a capture the flag challange and saved it as key.txt to my local machine.

Then I wanted to ssh into the machine to get root.
ssh johndoe@example.com -i key.txt

But I always get the following error message:
johndoe@example.com: Permission denied (publickey).

Retried to ssh into the machine and the error message looks now like this
Load key "id_rsa": invalid format
johndoe@example.com: Permission denied (publickey).

The format looks like this in the file:

-----BEGIN OPENSSH PRIVATE KEY-----
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam iaculis dui sed dolor scelerisque, nec bibendum risus congue. Proin placerat at elit id vulputate. Fusce ac lacus mi. Aliquam at ex eu felis commodo dictum congue quis est. Nulla tincidunt metus ante, eu vulputate ipsum elementum sit amet. Nullam sed nisi posuere, tincidunt magna non, vehicula ante. Mauris tincidunt eros porta, elementum dolor in, mollis sapien. Nunc nec fermentum tortor. Nunc non pellentesque est. Duis imperdiet, tortor gravida aliquet lacinia, tellus lacus dignissim sem, in mattis nisi odio eget libero. Suspendisse feugiat mollis ex, sed placerat orci aliquam at. Integer malesuada rutrum urna, in rhoncus dui tempor sit amet. Ut non cursus enim. Aenean ultrices est non ante ultrices, ac convallis mauris elementum.
-----END OPENSSH PRIVATE KEY-----

Can somebody let me know how to troubleshoot this issue?

Marc Neugebauer

This post is deleted!

daniel-lowrie87

Greetings, @Marc-Neugebauer

The problem you're running into is that the key you found doesn't contain normal key info (base64 encoded data string).

Instead someone put in Lorem Ipsum data, which is typically used as a 'filler'. By filler I mean, like if you want to create a website quickly and instead of it being a bunch of blank pages you can fill it with Lorem Ipsum to give it some subtance.

CTFs are usually meant to be puzzles and like many puzzles there may be 'rabbit trails' that are meant to lead you down the wrong path and this very well may be that, or maybe they want you to use RSA encryption to encrypt the Lorem Ipsum data in the key file and then take that output and add use it at the key, or some other option is the way to go. This is the fun of CTFs :)

CTFs are great for learning real-world tactics, but they usually aren't very real-world in many ways either.

I hope that helps,

Daniel

Marc Neugebauer

Hi Daniel,

You are right I have put in the Lorem Ipsum as filler to obscure the private key ;-)

The problem still persists. I am not able to ssh into the system with the private key.

Guess in a real world scenario it could be helpful to know why there are problems with the private key.

Here is the original key:

daniel-lowrie87

This post is deleted!

daniel-lowrie87

I noticed that there are a bunch of newlines in the key. Is that from you pasting it into the forum post, or are they original to your key?

Marc Neugebauer

It's from pasting in the forum. In my key file are no newlines.

After some contemplation of this issue I found the following article
how-to-convert-openssh-to-ssh2

Might be that I have to convert it somehow to a standard ssh format in order to log into the machine.

ssh-keygen -e -f path/to/opensshprivate.key > path/to/ssh2privatekey/ssh2privatekey

So I tried to use the formula mentioned in the article to no avail.

ssh-keygen -e-f /home/user/id_rsa > /home/user/standardsshkey.txt
Load key "/home/user/id_rsa": invalid format

Guess the Meta machine at HTB keeps the user flag heavily guarded...