OpenKM has many interesting features, but requires some configuration process to show its full potential.
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I did some test a couple of years ago using JOSSO and worked fine because is implemented at JAAS layer. Didn't try CAS, son I can't help you but would be nice to receive any feedback in this sense.
I was actually able to get OpenKM to redirect to CAS and authentication, but is still unable to do the authorization. Do you have any clues or hints as to how the authentication works?