I've been tasked with authenticating in an existing application via SAML 2.0 I've got the whole SAML 2.0 specification printed and sitting on my desk right now. It sure doesn't look trivial.
When people say "Don't roll your own encryption" or "Don't implement $ALGORITHM, just use libsodium", I think that also applies to web authentication (be it OAuth or SAML).
If you are the 'client' that wants to authenticate an incoming user, then I'd point you to an existing SP like Shibboleth or SimpleSAMLPHP or even CILogon (which bridges SAML and OAuth). If you are handing out authentications, then you are an IdP and I'd point you to something like Shibboleth or ADFS or Okta.
It is good to have the relevant RFCs to hand, but I would not make an implementation from scratch, unless there was a very good reason for it.
I'm trying to build an "SP" or "relying party" as I've learned in the jargon of the domain. Building an implementation from scratch is the last thing I want to do. I feel like I'd rather pull out my own teeth. But a difficulty of finding any information about it has lead me to start reading all the official specs as a kind of last resort.
In my initial research, I did discover Shibboleth, but was under the impression that it was an IDP only. I will check out the SP component, because I would love to not implement it from scratch.
It installs as an apache module and I am building a flow from my load balancers to hit the apache/SP and proxy some traffic to the application after authentication success so we can create our login session.
I have a PoC going now, but the documentation is not terribly clear to me and it is really confusing me about some options I need to tune.
For my initial testing, I built the SP and used https://samltest.id/ to test against an IDP.
If your language/environment supports using Apache HTTPD as a proxy, then you can use mod_auth_mellon to secure your web application. That's just one option, there are lots of others.
I have heard about these options, but the problem for me is identifying any of them and figuring out how to use them. My web server probably has to be IIS or Kestrel. I've been trying to figure this out for a few weeks now. And I've spent about a day trying to figure out how to do anything with the Shibboleth SP. I think I got it installed, but I can't really tell if it's doing anything or how to use it. And even then, there doesn't seem to be any information on how to actually use it for authentication in my application.
From my perspective as an application developer, writing an application that authenticates via SAML 2.0 is a nightmare, despite the ubiquitous claims of how simple it is.
If your app is .NET, look at Sustainsys (https://saml2.sustainsys.com/en/2.0/) or ITFoxtec (https://www.itfoxtec.com/IdentitySaml2) libs. Unfortunately there isn't a clear architect/dev-level guidance at the protocol level on key decisions that need to be made when implementing SP that tightly integrates with your app. Give one of these a shot and post your questions on StackOverflow.
If you're willing to use PHP, there's also "the award-winning" SimpleSAMLphp (https://simplesamlphp.org/), which can operate as IdP or SP. I can tell you that many big names in the TV Everywhere space are using it.
You're reading the entire spec, but the flow is what's trivial. 2 systems are exchanging info, they use cryptograhpy to assert that message is coming from a valid, registered resource and the message carrier format is XML. You ask for info, you get XML back, verify the sender and if correct trust the info is valid. That's the gist of the protocol and the tedious part is parsing the XML you receive.
The flow/concept is simple indeed. But with saml the devil is really in the details and they created way too many details by offering way to many options in the spec.
If you are the 'client' that wants to authenticate an incoming user, then I'd point you to an existing SP like Shibboleth or SimpleSAMLPHP or even CILogon (which bridges SAML and OAuth). If you are handing out authentications, then you are an IdP and I'd point you to something like Shibboleth or ADFS or Okta.
It is good to have the relevant RFCs to hand, but I would not make an implementation from scratch, unless there was a very good reason for it.