Allow light client to verify signatures at period boundary

As the sync committee signs the previous block, the situation arises at every sync committee period boundary, that the new sync committee signs a block in the previous sync committee period. The light client cannot reliably detect this condition (e.g., assume that this is the case when it is currently on the last slot of a sync committee period), because the last couple slots of a sync committee period may not have a block.

For example, when receiving a FinalizedHeaderUpdate that is constructed as in the following illustration, it is unknown whether sync_aggregate was signed by the current or next sync committee at attested_header.


        slot N           N + 1   |            N + 2   (slot not sent!)
                                 |
  +-----------------+     \ /    |     +----------------+
  | attested_header | <--- X ----|---- | sync_aggregate |
  +-----------------+     / \    |     +----------------+
                        missed   |
                                 |
                          sync committee
                          period boundary

This patch addresses this edge case by including the slot at which the sync_aggregate was created into the FinalizedHeaderUpdate object.

Note that the signature_slot cannot be trusted beyond the purpose of signature verification, as it could be manipulated to any other slot within the same sync committee period and fork version, without making the sync_aggregate invalid.

darwinia-network / darwinia-messages-sol

Allow light client to verify signatures at period boundary #173