SSO with external site using OpenSSL and/or xml encryption

We have a Drupal multisite setup, and need to implement a single sign on system with an external (non-PHP) site. Our tech partners at the external site suggested we use the following library: [login to view URL] The external site offers a Facebook login, but we cannot offer this directly because we need to cross reference FB accounts with the complete organization directory, which is not directly available to us (it is available through the SSO with the other site.) We cannot use the Drupal LDAP module because the LDAP server of the other app is behind a firewall and will not be made available. Please send a private message for more details.