Key-based authentication in Linux CLI - Step-by-Step Execution

Concept Flow - Key-based authentication

Generate SSH key pair

↓

Copy public key to remote server

↓

Attempt SSH login

↓

Server checks public key

↓

Match

↓

Login granted

↓

Session starts

This flow shows how a user generates a key pair, copies the public key to the server, and then logs in using the private key. The server checks the key and grants or denies access.

Execution Sample

Linux CLI

ssh-keygen -t rsa -b 2048
ssh-copy-id user@server
ssh user@server

Generate SSH keys, copy the public key to the server, then log in using key-based authentication.

Execution Table

Step	Command	Action	Result	Output
1	ssh-keygen -t rsa -b 2048	Generate RSA key pair	Keys created	Your identification has been saved in ~/.ssh/id_rsa Your public key has been saved in ~/.ssh/id_rsa.pub
2	ssh-copy-id user@server	Copy public key to server	Public key added	Number of key(s) added: 1 Now try logging into the machine, with: "ssh user@server"
3	ssh user@server	Attempt SSH login	Server checks key	Welcome to server! [user@server ~]$
4	ssh user@server	If key missing or wrong	Login denied	Permission denied (publickey).
5	exit	End session	Disconnected	Connection to server closed.

💡 Login ends when session is closed or permission denied.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Private Key (~/.ssh/id_rsa)	None	Created	Exists	Used for login	Exists
Public Key (~/.ssh/id_rsa.pub)	None	Created	Copied to server	Checked by server	Exists on server
SSH Session	None	None	None	Active if key accepted	Closed after exit

Key Moments - 3 Insights

Why do we need to copy the public key to the server?

The server uses the public key to verify your private key during login. Without copying it (see step 2 in execution_table), the server cannot authenticate you.

What happens if the private key is missing or wrong?

The server denies access with 'Permission denied (publickey).' as shown in step 4 of execution_table, because it cannot match your key.

Why is the private key never copied to the server?

The private key stays on your local machine for security. Only the public key is shared with the server to verify your identity.

Visual Quiz - 3 Questions

Look at the execution_table, what output appears after copying the public key to the server?

A Your identification has been saved in ~/.ssh/id_rsa

B Permission denied (publickey).

C Number of key(s) added: 1

D Connection to server closed.

💡 Hint

Check the output column in row 2 of execution_table.

At which step does the server check the public key during login?

A Step 2

B Step 3

C Step 1

D Step 5

💡 Hint

Look at the Action column for 'Server checks key' in execution_table.

If the private key is deleted locally after step 1, what will happen at step 3?

A Permission denied (publickey).

B Login granted

C Public key copied again

D Session starts normally

💡 Hint

Refer to key_moments about what happens if private key is missing.

Concept Snapshot

Key-based authentication uses a pair of keys: private (kept secret) and public (shared with server).
Generate keys with ssh-keygen.
Copy public key to server with ssh-copy-id.
Login with ssh uses private key to prove identity.
Server grants access if keys match.
No password needed, more secure and convenient.

Full Transcript

Key-based authentication is a way to log into a remote server without typing a password each time. First, you create a pair of keys on your local computer: a private key and a public key. The private key stays on your computer and should be kept secret. The public key is copied to the server you want to access. When you try to log in, your computer uses the private key to prove your identity. The server checks if this matches the public key it has. If they match, you get access. This method is safer and faster than passwords. The main steps are: generate keys with ssh-keygen, copy the public key with ssh-copy-id, then log in with ssh. If the keys don't match or the private key is missing, the server denies access. This process helps keep your connections secure and easy to use.