How to generate a private key for the existing .crt file on Apache?

Problem

I was given a certificate but I wasn't given a key. Can I generate a key for an existing certificate?

Solution

Unfortunately, this is not possible. You cannot generate a private key out of an existing certificate. If it would be possible, you would be able to impersonate virtually any HTTPS webserver.

How should generating a certification look like

The process starts when you generate a public/private key and create CSR (Certificate Signing Request) which contains the public key, domain, and additional information. Then the CST needs to be sent to the CA (Certificate Authority) for signing. CA will send back the certificate based on the content of the CSR.

What it means for you is that you probably have the private key already. All you need to do is find it.

How to find my private key

Traditionally, private keys on Linux-based operating systems (Ubuntu, Debian, CentOS, RedHat, etc.) are openssl generated keys with the crypto toolkit and saved into files with the .key or .pem extension.

To search for the private key, use the following command:

sudo find [search_start_folder] -type f -iname 'private.key'

Replace the [search_start_folder] part with the directory where you want to start searching. You can use / to start from the root directory. Keep in mind that this may take a while.

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SSL certificate

How to generate a private key for the existing .crt file on Nginx?

Unfortunately, this is not possible. You cannot generate a private key out of an existing certificate. If it would be possible, you would be able to impersonate virtually any HTTPS webserver.

Questions

Does each subdomain need its own SSL certificate?

This depends purely on what type of certificate you have. You can have a standard single domain certificate. Aside from that, you can also get two following types of certificates.

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