Principle
The verification to the server is based on challenge-response authentication. ssh connects to the server with a user name and the request for a key. The ssh daemon gets the request and sends back a challenge based on the public key stored in the authentication file. ssh uses the private key to construct a key response, and sends it to the waiting sshd on the other end of the connection. It does not send the private key itself. The ssh daemon validates the key response, and if valid, grants access to the system. ssh-agent simplifies this by creating a socket that listens for SSH connections. The user simply starts ssh-agent, telling it how to find their keys (if they are not in the default location), enters the passphrase for each key to be used, on a one-time basis, and then ssh-agent handles the rest every time the user connects to a remote server.
Read more about this topic: Ssh-agent
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