I have previously talked about installing SimpleSAMLphp using composer, so the next step is setting up the system to actually provide authentication against a SimpleSAMLphp system. As I spend a lot of time using Drupal I wanted to set up the authentication with Drupal and SimpleSAMLphp in order to see how things worked.
First, let's define a couple of terms that are important to this setup.
SP - Service Provider - This is the system that users are trying to log into, which in this setup is Drupal. The Drupal site is providing a 'service' and as such users want to authenticate with it. Service providers will generally create a local user to track the user within the site, although that's not always the case.
IdP - Identity Provider - The SimpleSAMLphp system holds the information about the users and is therefore called an identity provider as it provides the identity of the user. This is used by the Service Provider to authenticate the user.
Read more here: https://www.hashbangcode.com/article/drupal-9-configuring-drupal-authenticate-against-remote-simplesamlphp-identity-provider