HTTP-POST binding - Does SP need to make an HTTP POST?

[Carl Waldbieser]

First, let me say I only know enough SAML to operate our IdP.
In the past, I've performed SSO integrations with various Slack instances
for different departments.  I was pretty sure they all used the
HTTP-Redirect binding.

I was trying to do one today, and was running into trouble.  The support
person I got a hold of pointed out I need to use HTTP-POST.  Which I did
try, but Shib then errored out with
"org.opensaml.messaging.decoder.MessageDecodingException: This message
decoder only supports the HTTP POST method".

I think Shib is trying to tell me that if the SP wants to use the HTTP-POST
binding, it needs to make an actual HTTP POST.  but the request is made via
an HTTP GET.  Since I am no SAML expert, I want to know what the error
actually means, instead of just guessing.

Thanks,
Carl Waldbieser
ITS
Lafayette College
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