[MLS] Move / Add-in-place / Compounding

[Richard Barnes <[e-mail]>]

Hey MLS folks,

tl;dr:
- Would there be interest in a Move operation?
- How about an operation to Add a new member in an open slot instead of the
right edge?

In looking at the "partial tree" stuff (#67), it occurred to me that once
you have partial trees with blank leaves, you can in principle do two new
operations: An existing member can move from one leaf in the tree to
another; and an existing member can add a new member at a leaf that is
currently blank.  These operations would work roughly as follows:

- Move:
    - Blank out old dirpath
    - Move credential from old slot to new slow
    - Send update along new dirpath
- Add-in-place:
    - Blank out dirpath for slot
    - Install new member's leaf key in slot
    - Install new member's credential in slot

For Move, you would also want to have people step down the size of the
tree.  For example, if you go from having 10 leaves (X _ X X _ _ _ _ _ X)
to four leaves (X X X X) as the result of a move, then the resulting tree
should only have height 2, not 4 (as is required for 10 leaves).
Fortunately, this is trivial with the tree math defined in the document;
you just truncate the node vector, and the calculated root gets
correspondingly smaller.

I've prototyped this out in my JS stack [1][2].  Once you've built a tree,
and removed a couple of members, you can click one of the "Move" buttons to
move that node to the lowest open slot.  As you can see, it's not a ton of
code.

In any case, this seems to me to be potentially useful, since it addresses
the issue that was raised in the BoF that the size of the resources for a
group increases monotically.  Move allows you to reclaim resources that are
unused (assuming some orchestration outside of MLS), and Add-in-place
allows us to be conservative with resources that are already allocated (cf.
realloc).

If folks think these sound like useful tools, I'll write up a PR and we can
discuss in BKK.

--Richard

[1] https://ipv.sx/treekem/
[2]
https://github.com/bifurcation/treekem/commit/7f470422ba61ce668c3ab1d6b2d18edd9d6e06fd