Type for SSKR config parameters

[simonratner]

To help with guided recovery on clients, and metadata for collaborative recovery services (BlockchainCommons/Community#149), it is useful to have a type encoding just the config parameters of a unique SSKR split, without the share values.

The following parameters from https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2020-011-sskr.md would be useful as part of this type:

• identifier (id)
• group-threshold (Gt)
• group-count (g)
• member-threshold (t) per group

[simonratner]

Quick sketch if we want to keep a similar compact format (and re-use some parsing logic):

crypto-sskr-params = bytes

Bit fields of data field:

pack the id, group and member data into (3+g) bytes:
76543210        76543210        76543210
        76543210        76543210
----------------====----====----====----
identifier: 16
                group-threshold: 4
                    group-count: 4
                        repeat (× g):
                        group-index: 4
                            member-threshold: 4
                                ....
                                    ....

Alternatively, this can be a proper CBOR struct since it probably doesn't have the same pressure to optimize for size.

[simonratner]

If there is a plan to use the reserved bits of crypto-sskr for versioning, that should probably be included too.

[wolfmcnally]

I expect that when we do choose to update the SSKR format, we will indicate this using one of the reserved bits.