OpenSSH – Regenerate Public Key from Private Key

Lets say you have a private/public key pair that you use to login to your server via SSH and you lose the public key, either it was deleted or corrupt and you don’t want to have to regenerate a new pair what options do you have? In this post I will demonstrate how to regenerate a public key from the corresponding private key that you still have.

 

Generate public key and store into a file

It is a simple one liner command to generate a public key from a private key, so lets say our private key is named ‘user@myserver.key’ and we want to generate the public key and name it ‘authorized_keys’. Below is the command to do this.

Command Breakdown:

‘ssh-keygen -y’ : tells the prorgram ssh-keygen to output a public key

‘-f user@myserver.key’: references the key file user@myserver.key to generate the public key from

‘> authorized_keys’: instead of printing the output to the console redirect the output (public key) to a file named ‘authorized_keys’

 

Copy public key to your server

Now we will need to take our authorized_keys file and store it on the server, if we can currently login via SSH to our server we can use rsync over ssh to securely copy the public key to the server, Although this key is displayed to anyone who attempts to login to your server it is smart to use SSH or another secure method to copy it up to the server to ensure that the key is not tampered with while it is being transmitted.

Below is an example using rsync to copy the authorized keys file to its destination on the server and removing it from your local computer.

At this point our public key file is on our server where it is supposed to be in our users ‘.ssh’ directory.

 

Set proper permissions for public key

Lastly we will need to make sure our permissions are set properly on our public key to prevent anyone else tampering with it. We should give our key file RW, R, R permissions, below is the command to properly set permission on the key.

 

At this point we are all set to begin using our private key to login to our server again. If you have any questions about this post please leave a comment below and if this helped you at all please share it on social media!

2 Trackbacks / Pingbacks

  1. OpenSSH - Configure Key Based Authentication - Kyle Kowalczyk
  2. Generate OpenSSH Public/Private Key Pair - Kyle Kowalczyk

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