How to change or update SSH key Passphrase on Linux / Unix

 1 year ago
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Let’s look at how you can update or change your SSH key Passphrase on a Linux system. SSH keys are often used to authenticate users to some kind of information systems. The SSH keys themselves are private keys; the private key is further encrypted using a symmetric encryption key derived from a passphrase. It is easy to change your SSH Key passphrase on a Linux/Unix system.

What is SSH Key Passphrase?

A passphrase is similar to a password and is used to secure your SSH private key from unauthorized access and usage. It is always recommended to set a strong Passphrase for your SSH keys, with at least 15, preferably 20 characters and be difficult to guess.

How to Change or update SSH key Passphrase on Linux / Unix

At times you may need to update your SSH key passphrase or set one if you didn’t set at the time of generating your SSH keys.

As an example, let’s generate SSH key without a passphrase:

# ssh-keygen 
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:1gSD3mPgxaD0C88YLU+TdYs2T3nBO5ttK5Jj0bvz0gs [email protected]
The key's randomart image is:
+---[RSA 2048]----+
|    . .++ ...    |
|   . +ooo= o..   |
|    =o*++ = ..   |
|     Xo++* .o    |
|    . =.S.o. =   |
|       .  . + o  |
|           oE+ . |
|          = =.o  |
|         . o.*o. |

Now use the command below to set a passphrase:

# ssh-keygen -p -f ~/.ssh/id_rsa
Enter new passphrase (empty for no passphrase): <Enter passphrase>
Enter same passphrase again:<Retype passphrase>
Your identification has been saved with the new passphrase.

If using a custom path for the private key, replace ~/.ssh/id_rsa with the path to your private key.

The same command applies when resetting the passphrase, you will be asked for the old one, and the new one to set.

# ssh-keygen -p -f ~/.ssh/id_rsa
Enter old passphrase: <Enter old passphrase>
Enter new passphrase (empty for no passphrase): <Enter new passphrase> 
Enter same passphrase again: <Retype new passphrase>
Your identification has been saved with the new passphrase.

Testing new passphrase

To test that your new passphrase is working, copy ssh public key to a remote server and try to ssh with it.

$ ssh-copy-id [email protected]
Enter passphrase for key '/home/jmutai/.ssh/id_rsa': 
Now try logging into the machine, with "ssh '[email protected]'", and check in:
to make sure we haven't added extra keys that you weren't expecting.

Save private key passphrase

With ssh, you can configure authentication agent to save passphrase so that you won’t have to re-enter your passphrase every time you use your SSH keys.

# Start agent on demand
eval $(ssh-agent) 

# Add default key

# List keys
ssh-add -l 

# Add specific key
ssh-add ~/.ssh/id_rsa 

# Add with timeout
ssh-add -t 3600 ~/.ssh/id_rsa

# Drop keys
ssh-add -D 

For a complete guide on how to use SSH, check SSH cheatsheet for Linux SysAdmins

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