Dear Joeli,
if you don't explicitly set the entity-id in
config/saml/application.yml, an ID is automatically initialized with the default value that is
{baseUrl}/saml2/service-provider-metadata/st_samlThis is also the endpoint for generating the Service Provider metadata.
I confirm that you can explicitly set the entity ID using the property
spring.security.saml2.relyingparty.registration.st_saml.entity-id
However, in most cases it's not required, you can leave the default value.
So, you can generate Semantic Turkey metadata by accessing the URL
{baseUrl}/saml2/service-provider-metadata/st_saml
and you'll notice that the entityID set in the generated XML metadata is the same as the default one.
As for:
{baseUrl}/semanticturkey/saml2/login/sso/st_saml
this URL represents the location of the Assertion Consumer Service, namely the endpoint where the SAML Response is posted after authentication.
It is not meant to be accessed manually, it is just part of the automatic SAML flow and it's designed to accept POST requests containing the SAML Response from the IdP.
So, I guess it's normal that you receive and error trying to access it.
About the optional fields on the IdP side, you don't need to configure them, so you can leave them empty.
Best regards,
Tiziano