Level goal

The password for the next level is stored in /etc/bandit_pass/bandit14 and can only be read by user bandit14. For this level, you don’t get the next password, but you get a private SSH key that can be used to log into the next level. Note: localhost is a hostname that refers to the machine you are working on.

Login

ssh bandit13@bandit.labs.overthewire.org -p 2220
FO5dwFsc0cbaIiH0h8J2eUks2vdTDwAn

Doc

ssh (1)              - OpenSSH remote login client
usage: ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-B bind_interface] [-b bind_address]
           [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
           [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
           [-J destination] [-L address] [-l login_name] [-m mac_spec]
           [-O ctl_cmd] [-o option] [-P tag] [-p port] [-R address]
           [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]]
           destination [command [argument ...]]
       ssh [-Q query_option]

A bit of backstory

An SSH key is a secure access credential used in the Secure Shell (SSH) protocol. SSH key pairs use public key infrastructure (PKI) technology, the gold standard for digital identity authentication and encryption, to provide a secure and scalable method of authentication. Source

Solution

By using ls, you can see that there`s a file named sshkey.private. Since it is stated that only bandit14 has access to the flag stored in /etc/bandit_pass/bandit14 our approach should be making our request as bandit14. When sshing you can use a key as your login method instead of the text password we were used to until now by specifing the -i flag. 

bandit13@bandit:~$ ls
sshkey.private
Since bandit13, the endpoint we are also on, is hosted on bandit.labs.overthewire.org, we can simply specify localhost as the host and use ssh in pair with the key to connect and then just cat the respective file. 
bandit13@bandit:~$ ls
sshkey.private
bandit13@bandit:~$ ssh -i sshkey.private bandit14@localhost -p 2220
The authenticity of host '[localhost]:2220 ([127.0.0.1]:2220)' can't be established.
ED25519 key fingerprint is SHA256:C2ihUBV7ihnV1wUXRb4RrEcLfXC5CXlhmAAM/urerLY.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Could not create directory '/home/bandit13/.ssh' (Permission denied).
Failed to add the host to the list of known hosts (/home/bandit13/.ssh/known_hosts).
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                      This is an OverTheWire game server.
            More information on http://www.overthewire.org/wargames

!!! You are trying to log into this SSH server with a password on port 2220 from localhost.
!!! Connecting from localhost is blocked to conserve resources.
!!! Please log out and log in again.


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  www. `---` ver     '---' he       '---" ire.org


Welcome to OverTheWire!

....

  Enjoy your stay!

bandit14@bandit:~$ cat /etc/bandit_pass/bandit14
MU4VWeTyJk8ROof1qqmcBPaLh7lDCPvS