E2E Cross Signing - Spec

[lampholder]

Do the spec for #2714

[lampholder]

We don't think we'll have time to complete the cross signing spec in RW011, but also we can't reasonably break it down into sensible chunks without spending time thinking about it.

Our commitment for RW011 therefore is a timeboxed investigation into the spec - Dave will devote the time in the iteration (that is not otherwise allocated) to thinking about the spec, breaking it up if that is useful, and making as much progress as possible.

[lampholder]

We didn't make any progress on this in RW011 and have in fact adopted a different strategy to this, meaning this work item isn't really useful or representative any more.

I'm going to close it.