[MLS] Turning mandatory extensions into fields

[Théophile Wallez <[e-mail]>]

Hello All,

The MLS spec includes some extensions that are mandatory.
Considering that this is v1 of the spec, why not just make these into fields and take them out of extensions?

In particular, in the current draft, the parent hash is only an extension, but it is mandatory when the leaf is generating an UpdatePath.
Let us make it a field in KeyPackage?
Having it as an extension hides its importance for security, and makes the draft confusing to read.
Also, it is a bit weird that parent_hash is a field in ParentNode, and it is an extension in KeyPackage.

We propose the following changes:

> New structure:
> struct {
>     opaque parent_hash_value<0..255>;
> } ParentHash;
>
> New field in KeyPackage:
> optional<ParentHash> parent_hash;
>
> This optional field must be present in the `leaf_key_package` of an UpdatePath (same constraints as the current extension).

Or:

> New field in KeyPackage:
> opaque parent_hash<0..255>;
>
> `parent_hash` must be non-empty in the `leaf_key_package` of an UpdatePath (same constraints as the current extension).

Best regards,
Théophile, Benjamin, and Karthik