A passphrase is a sequence of words or characters that you use to unlock your private key. It is used to protect an encryption key and should have at least 15, preferably 20 characters. A passphrase is optional, but highly recommended, for your SSH key. With SSH keys, if someone gains access to your computer, the attacker can gain access to every system that uses that key. To add an extra layer of security, you can add a passphrase to your SSH key. The passphrase is used to encrypt the private key, making the key file by itself useless to an attacker. The downside to passphrases is that you need to enter it every time you create a connection using SSH. To change the passphrase for an existing private key, you can use the ssh-keygen command. To avoid entering the passphrase every time you connect, you can securely save your passphrase in the SSH agent.