With John, we can crack not only simple password hashes but also SSH Keys. It is pretty simple, so let's get started.
#copying the SSH Key that we have to crack cp ~/.ssh/id_rsa id_rsa
We will need a script, ssh2john.py. It comes along with Kali so, you don't really need to download it.
#finding the file updatedb locate ssh2john.py cp $(locate ssh2john.py) . #now, we will create a hash using it python ssh2john.py id_rsa > id_rsa.hash
Now, let's find and copy rockyou.txt.gz, our wordlist.
locate rockyou.txt cp $(locate rockyou.txt) . #now, unzip the file gunzip rockyou.txt.gz
Finally, let's use john and rockyou.txt to try and crack the SSH Key.
john id_rsa.hash -wordlist=rockyou.txt
Congratulations! Have fun SSHing!