Ask HN? How would you implement SSO for Rails apps?

I've been tasked with building out SSO. It looks like my only real option is implementing a CAS server/client with a paid solution like OneLogin or rbCAS (open).

- https://www.onelogin.com/ - https://github.com/rbCAS/CASino

1. Do I have more options besides the above? If so, which? 2. Is there a better solution besides CAS (http://jasig.github.io/cas/4.0.0/index.html)?

Recommendations are welcome. Thanks in advance.