OpenSSL is an open source implementation of the SSL and TLS protocols. Versions are available for most Unix-like operating systems (including Solaris, Linux, Mac OS X and the four open source BSD operating systems), OpenVMS and Microsoft Windows. The OpenSSL project is managed by a worldwide community of volunteers.
OpenSSL is commonly used to create the CSR and private key for many different platforms, including Apache. However, it also has hundreds of different functions that allow you to view the details of a CSR or certificate, compare an MD5 hash of the certificate and private key (to make sure they match), verify that a certificate is installed properly on any website, and convert the certificate to a different format.
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Most used OpenSSL commands: