Convert a .crt certificate with a separate private key to a combined .pfx

This is just a quick follow-up to yesterdays post concerning certificate signing.

As briefly mentioned yesterday, the process shown would result in a signed certificate (filename.crt) for each certificate request (filename.pem), and a corresponding key-file (filename.key). If you need to use these (for instance in an Azure key vault, as was my purpose here), you might need to combine the .crt and the .key into a single file which contains both. This can be either a .pfx file or a .pem. If my understanding is correct, .pfx is really just a different file extension, typically used on Windows. Both are essentially .pem files – that is, a certificate which can contain both public and private keys in the same file. This is in contrast with the .crt files we generated, which only contain the public certificate by itself.

So how do we do combine these? Once again, let’s use OpenSSL:

openssl pkcs12 -export -out certfile.pfx -inkey keyfile.key -in certificate.crt

You’ll be requested for a password, which will be used to secure the file while storing and transferring it. You should now have a file named certfile.pfx. Later, you’ll need to provide the password again in order to import the keys from this file into e.g. Azure, or wherever you want to use them.

Create a chain of self signed certificates

Note, update: This has a follow up post for the case where you want to keep any Subject Alternativ Name (SAN) fields in the certificate to sign.

On occasion, I’ve needed to create my own self-signed SSL-certificates for various testing purposes. At work today I needed a certificate that was signed by another certificate, i.e. I needed a chain of trusted certificates for testing, where the cert at the top of the chain is used as a trusted root certificate. The premise is is simple: If you trust the root certificate, you should also trust the certificates further down in the chain, since they are signed using the trusted certificate.

I generally work on a Windows system, but for certain tasks such as this, I often find Unix-style tools preferable. Luckily, if you used the Git Bash command line for windows and have OpenSSL included, you’ll have everything you need right there. This post provides a few quick steps needed to create such a chain of trusted certificates using OpenSSL.

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