Public and private key pairs are used for publickey authentication method in SSH. When creating the SSH key pair for publickey authentication, you can assign a passphrase to the …
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You can easily create an SSH key pair by using ssh-keygen as in the following example.

$ ssh-keygen 
Generating public/private rsa key pair.
Enter file in which to save the key (/home/user/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/user/.ssh/id_rsa.
Your public key has been saved in /home/user/.ssh/id_rsa.pub.
The key fingerprint is:
1a:6c:a4:94:df:e0:19:ec:85:b0:3f:27:e4:a1:de:65 user@host
The key's randomart image is:
+--[ RSA 2048]----+
|                 |
|   .             |
|    o E          |
|     O o         |
|    c @ S        |
|   o A @         |
|    . E .        |
|   . =           |
|    .            |
+-----------------+

There are a few things that you might want need to take note;

1. A 2048 bit RSA key pair will be generated by default. You can use different key type by using -t option and specify any of the supported key types

   dsa ecdsa ed25519 rsa rsa1

2. The private key will be stored in ~/.ssh/id_rsa while the public key will be stored in ~/.ssh/id_rsa.pub. You can specify other location when prompted during the key generation process if you already have a key pair at the default location
3. You can specify passphrase for your key pair when prompted but you’ll have to not set the passphrase to use passwordless SSH login

   How to configure passwordless SSH login

   How to add or remove passphrase on SSH key