What is "ssh-agent"

SSH is a protocol allowing secure remote login to a computer on a network using public-key cryptography. SSH client programs (such as ssh from OpenSSH) typically run for the duration of a remote login session and are configured to look for the user's private key in a file in the user's home directory (e.g., .ssh/id_rsa). For added security (for instance, against an attacker that can read any file on the local filesystem), it is common to store the private key in an encrypted form, where the encryption key is computed from a passphrase that the user has memorized. Because typing the passphrase can be tedious, many users would prefer to enter it just once per local login session. The most secure place to store the unencrypted key is in program memory, and in Unix-like operating systems, memory is normally associated with a process. A normal SSH client process cannot be used to store the unencrypted key because SSH client processes only last the duration of a remote login session. Therefore, users run a program called ssh-agent that runs the duration of a local login session, stores unencrypted keys in memory, and communicates with SSH clients using a Unix domain socket.