SSH is a secure protocol used as primary means to connect to linux servers remotely
Article Link: https://www.digitalocean.com/community/tutorials/ssh-essentials-working-with-ssh-servers-clients-and-keys
Authentication
You can authenticate using passwords (less secure) or SSH keys (very secure)
SSH Keys are a matching set of cryptographic keys which can be used for authentication. A set contains a public and private key
Public Key can be shared to anyone without concern
Private Key must be guarded and never exposed to anyone
Directly from digital ocean:
When a client connects to the host, wishing to use SSH key authentication, it will inform the server of this intent and will tell the server which public key to use. The server then checks its authorized_keys file for the public key, generates a random string, and encrypts it using the public key. This encrypted message can only be decrypted with the associated private key. The server will send this encrypted message to the client to test whether they actually have the associated private key.
Upon receipt of this message, the client will decrypt it using the private key and combine the random string that is revealed with a previously negotiated session ID. It then generates an MD5 hash of this value and transmits it back to the server. The server already had the original message and the session ID, so it can compare an MD5 hash generated by those values and determine that the client must have the private key.