John the Ripper is a popular password cracking tool that uses a variety of techniques to guess passwords. When used to crack an SSH key, John can attempt to guess the passphrase used to encrypt the private key, allowing an attacker to gain unauthorized access to the remote server.
Command Reference:
Wordlist: /opt/wordlists/rockyou.txt
Private SSH key in John format: sshkey.john
john --wordlist=/opt/wordlists/rockyou.txt sshkey.john
sshng2john.py id_rsa