Light-client Authority Set Handoffs

[gavofyork]

No description provided.

[rphmeier]

This is desired for light-client-friendly tracking of authority set changes.

When the authority set changes from A to A' at a block B, The header of B will contain an NewAuthorities(A') event in the digest.

When an authority imports such a block and believes the change to be the result of a valid block execution, he generates a signature on a HandoffAttestation(Hash(B), NewAuthorities(A')) message and circulates it to other authorities. Upon witnessing such signatures from a majority of authorities, he accumulates them into a full Justification(B, NewAuthorities(A')) message.

When a full or light node imports a block with this kind of digest event, it waits to fetch a justification for the change before continuing to sync on top of that chain. Full nodes will save the justification permanently in the database.

Optimization: automatically provide the justification with any relevant blocks.

This can also be used for a fast warp sync with the caveat that it is vulnerable to long range attacks.

[gavofyork]

Might be a good one for @svyatonik ?

[gavofyork]

Done in #687

[svyatonik]

Not actually - we only have a DigestItem trait now - everything else is not ready yet.

[svyatonik]

Current state:
#687 : DigestItem::AuthoritiesChange
#726 : DigestItem::AuthoritiesChange is emitted when authorities set changes
1d0f52f : (probably we don't need this at all) 'pausable' synchronization

TODO : generate && broadcast justification by authorities. Since (as @rphmeier said) this is required for non-instant finality, I'll put this away for now

[gavofyork]

@rphmeier @svyatonik is there still anything to do here for a functional & secure MVP of AfG/GRANDPA/SHAFT?

[rphmeier]

I'm not convinced we can get a light-client running for beta but perhaps for 1.0.

@svyatonik The last thing we need is a handoff-proof generating protocol. And a way to sync the handoff-proofs. The hard part (similar to #1069) is having a heuristic for syncing clients to decide if the handoff-proof has been produced already and we need to request it from a peer or if the proof has not been generated and we should be watching for it).