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 donât 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.
I was recently in a meeting where a person needed to generate a private and public key for RSA encryption, but they were using a PC (Windows). This is something that is easily done via a terminal using ssh-keygen on Mac and Linux, however on Windows this tool is not easily accessible to the non-technical person. The -y option will read a private SSH key file and prints an SSH public key to stdout. The public key part is redirected to the file with the same name as the private key but with the.pub file extension. If the key has a password set, the password will be required to generate the public key. To check the details of the generated public key. If you do not have a private key: Open puttygen, Select the desired key type SSH2 DSA (you may use RSA or DSA) within the Parameters section. And it is important that you leave the passphrase field blank, Press generate and follow instructions to generate (public/private) key pair.
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 â[email protected]â and we want to generate the public key and name it âauthorized_keysâ. Below is the command to do this.
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â> authorized_keysâ: instead of printing the output to the console redirect the output (public key) to a file named âauthorized_keysâ
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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 â.sshâ 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.
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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!
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