PicoCTF_2017: Keyz

Category: Cryptography Points: 20 Description:

While webshells are nice, it'd be nice to be able to login directly. To do so, please add your own public key to ~/.ssh/authorized_keys, using the webshell. Make sure to copy it correctly! The key is in the ssh banner, displayed when you login remotely with ssh, to shell2017.picoctf.com

Hint:

There are plenty of tutorials out there. This one worked for me: https://www.digitalocean.com/community/tutorials/how-to-set-up-ssh-keys--2

Write-up

Really simple, this one. Firstly, generate an id_rsa identity. Might as well go for the 4096-bit key while you are at it.

$ ssh-keygen -b 4096
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): 
Created directory '/root/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:dc9Jgcpa7Ieh+HOc0YltffBabS66IupucLeUJwfgvao root@23ee096bfcb4
The key's randomart image is:
+---[RSA 4096]----+
|                 |
|  .     .  .     |
| . o  .   ooo .  |
|  . o    o+.+= . |
|     +  S  =  +  |
|. . * o.+.* .    |
| o  .=o.+  .     |
|  o oo *=.       |
|E=+. .+++.       |
+----[SHA256]-----+
# No, none of that is real duh

Then, copy the contents of ~/.ssh/id_rsa.pub into the clipboard. Next, open up PicoCTF web shell and create the folder .ssh. Then, run this command.

$ echo "{PASTE_HERE}" >> ~/.ssh/authorized_keys

Now try SSH-ing into the box at shell2017.picoctf.com from your computer!

$ ssh lflare@shell2017.picoctf.com
The authenticity of host 'shell2017.picoctf.com (34.206.4.227)' can't be established.
ECDSA key fingerprint is SHA256:ZIqVNC9hm15Z6mdDFCWC/H0+5MzSzXEhW3a+iHP1HM4.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'shell2017.picoctf.com,34.206.4.227' (ECDSA) to the list of known hosts.
Congratulations on setting up SSH key authentication!
Here is your flag: who_needs_pwords_anyways

Therefore, the flag is who_needs_pwords_anyways.

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