openssl is the standard open-source, command-line tool for manipulating SSL/TLS certificates on Linux, MacOS, and other UNIX-like systems. I recently ran into an interesting problem using openssl to convert a private key obtained from GoDaddy. Someone else used GoDaddy’s “wizard” interface to generate a certificate signing request (CSR) and private key, and saved the files on their Windows workstation. They purchased an SSL cert from GoDaddy, and shared all the files with me for installation on servers. GoDaddy saved the private key in the newer PKCS #8 format (pkcs8), and one system required the key in the older PKCS #1 (pkcs1) format. It’s easy to tell the difference.