Damian Poddebniak

> We should fix this upstream ... Can you open an issue here in openmls to update this once it's fixed? _Originally posted by @franziskuskiefer in https://github.com/openmls/openmls/pull/1363#discussion_r1176284664_

Context: https://github.com/openmls/openmls/pull/1331#discussion_r1144842722, https://github.com/openmls/openmls/pull/1331/commits/70c3c9ec98e76c520e7861a10b2db75e3c519b62

* [ ] Generate secret tree. * [ ] Generate PSK * [ ] Generate Welcome * [x] Tree validation * [x] #1289 * [ ] Message protection * [...

* [ ] [Implement PSK validation.](https://github.com/openmls/openmls/issues/1122) * [x] Implement PSK handling. #1351

* openmls/src/group/core_group/mod.rs * openmls/src/group/core_group/new_from_external_init.rs

Context: https://github.com/openmls/openmls/pull/1351#discussion_r1170928907

Context: https://github.com/openmls/openmls/pull/1351#discussion_r1170928907

See `openmls/src/group/tests/test_proposal_validation.rs`. https://github.com/openmls/openmls/blob/31ddea5134c39a3e1e69761e542526ee2ee9d858/openmls/src/group/tests_and_kats/tests/proposal_validation.rs#L2285

This is the other half for GREASE. Context: OpenMLS should not only accept unknown values (which is issue #1337) but GREASE itself (which is this issue).

* [ ] Possibly delete `PskError::Duplicate` eventually.