It’s also enormous, I assume from the attempt to have nominally composable parts that could be reused for other flows.

It’s three entire specs bundled as one. One for the XML components, another for the documents you build from them, and another for the authentication flows built on top.

The root problem with SAML is there’s a million and one permutations to do the same thing.

Signed assertions. Signed messages. Encrypted messages. Encrypted assertions. Sign after normalization. Sign before normalization. Encrypt then sign. Sign then encrypt.

There’s too many ways to do too many things.