The openssl program is a command line tool for using the various cryptography functions of OpenSSL's crypto library from the shell.
LibreSSL is a version of the TLS/crypto stack forked from OpenSSL in 2014, with goals of modernizing the codebase, improving security, and applying best practice development processes.
Zip archive contains 64-bit binaries of the openssl tool from LibreSSL, Cygwin and required DLLs plus man pages which are also available here: openssl openssl.cnf
One liner for creation of a self-signed SSL certificate:
openssl req -x509 -nodes -days 365 -newkey rsa:2048 -keyout selfsigned.key -out selfsigned.crt