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SSH Authentication

Authentication

Authentication is the process of verfying the identity of a person or a service which is trying to access a computing resource. It’s the process of confirming if the person is the one whom he/she claims to be. This is typicall acheived with a combination of username and password.

Public Key Authentication

While username/password combincation is the most commonly used system of authentication, it’s not the msot secure way. Moreover, it’s not always feasible to use username/password combination, particularly when computing resources/persons try to connect with another resources in a non-interactive way (automation scripts etc). Public Key authentication comes to rescue in such scenarios. Public Key authentication is the process of using a pair of keys genreated by the SSH program

How it works?

‘ssh-keygen’ generates a pair of keys known as “Public key” and “Private key”. The “Public key” is updated to the resource to which you want to authenticate. Everytime someone tries to authenticate to that resource, ssh application verifies if the person or resource trying to access the other resource has the corresponding public key. (SSH program is installed by default on all *nix machines, in Windows Open-SSH needs to be installed).

Here’s how a user: “admin” can authenticate to a server: “vm123”

  1. User Generates the keypair (public key and private key) with the “ssh-keygen” command on his machine #
  2. User copies the contents of the public key and pastes to the “authorized_keys” file located in ~/.ssh/ on the server “vm123”
  3. Enable “PubKeyAuthentication” in the “/etc/ssh/sshd_config” file. Restart ssh service with “systemctl restart ssh”
  4. Now the user can login to “vm123” with ssh command. There would be no prompt for password.

Notes:

  • SSH program is installed by default on all *nix machines, in Windows Open-SSH needs to be installed.
  • The default location where ssh keypair is stored is “~/.ssh/”
  • Typically the name of the private key file is “id_rsa” and the public file is “id_rsa.pub”