While looking to get this working: https://github.com/shield-9/u2f-login/, I was seeing errors in the http logs referring to OPENSSL_VERSION_TEXT. These were being generated by the following lines in the u2f.php file:
After running a few more tests, it seemed that this PHP Constant should have been defined, but was not. Goggling didn’t provide any quick answers, but I soon worked it out for myself.
In the php.ini file (under arch in /etc/php/), uncomment the line: “extension=openssl.so” and restart httpd. Once the extension is enabled, the Constant is then available.
In the end I did not use this plugin, I was unable to register my Yubikeys. The plugin seems to have been superseded by this one: http://github.com/georgestephanis/two-factor/. In order to have the ability to use U2F login using this plugin, the openssl.so extension has to be enabled; otherwise, the U2F login option is not available.