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Make Validator Set Network a first-class property

There already should only be one validator set operating per network. This
formalizes that. Then, validator sets used to be able to operate over multiple
networks. That is no longer possible.

This formalization increases validator set flexibility while also allowing the
ability to formalize the definiton of tokens (which is necessary to define a
gas asset).

2023-03-25 01:30:53 -04:00

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Validator Sets

Validator Sets are defined at the protocol level, with the following parameters:

bond (Amount): Amount of bond per key-share.
network (Network): The network this validator set operates over.
participants (Vec<SeraiAddress>): List of participants within this set.

Validator Sets are referred to by NetworkId yet have their data accessible via ValidatorSetInstance.

Participation in consensus

All Validator Sets participate in consensus. In the future, a dedicated group to order Serai is planned.

Multisig

Every Validator Set is expected to form a t-of-n multisig, where n is the amount of key shares in the Validator Set and t is n * 2 / 3 + 1, for each of its networks. This multisig is secure to hold coins up to 67% of the Validator Set's bonded value. If the coins exceed that threshold, there's more value in the multisig than in the supermajority of bond that must be put forth to control it. Accordingly, it'd be no longer financially secure, and it MUST reject newly added coins which would cross that threshold.

Multisig Creation

Multisigs are created by processors, communicating via their Coordinators. They're then confirmed on chain via the validator-sets pallet. This is done by having 100% of participants agree on the resulting group key. While this isn't fault tolerant, a malicious actor who forces a t-of-n multisig to be t-of-n-1 reduces the fault tolerance of the multisig which is a greater issue. If a node does prevent multisig creation, other validators should issue slashes for it/remove it from the Validator Set entirely.

Due to the fact multiple key generations may occur to account for faulty/malicious nodes, voting on multiple keys for a single coin is allowed, with the first key to be confirmed becoming the key for that coin.

Placing it on chain also solves the question of if the multisig was successfully created or not. Processors cannot simply ask each other if they succeeded without creating an instance of the Byzantine Generals Problem. Placing results within a Byzantine Fault Tolerant system resolves this.

Multisig Lifetime

The keys for a Validator Set remain valid until its participants change. If a Validator Set adds a new member, and then they leave, the set's historical keys are not reused.

Multisig Handoffs

Once new keys are confirmed for a given Validator Set, they become tracked and the recommended set of keys for incoming coins. The old keys are still eligible to receive coins for a provided grace period, requiring the current Validator Set to track both sets of keys. The old keys are also prioritized for handling outbound transfers, until the end of the grace period, at which point they're no longer eligible to receive coins and they forward all of their coins to the new set of keys. It is only then that validators in the previous instance of the set, yet not the current instance, may unbond their stake.

Vote (message)

coin (Coin): Coin whose key is being voted for.
key (Key): Key being voted on.

Once a key is voted on by every member, it's adopted as detailed above.

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