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Slashing rules

Unlike Proof of Work (PoW) where off-chain capital expenses are already deployed at the time of block construction/voting, PoS systems require capital-at-risk to prevent a logical/optimal strategy of multiple chain voting. We intend to implement slashing rules which, if broken, result some amount of the offending validator's deposited stake to be removed from circulation. Given the ordering properties of the PoH data structure, we believe we can simplify our slashing rules to the level of a voting lockout time assigned per vote.

I.e. Each vote has an associated lockout time (PoH duration) that represents a duration by any additional vote from that validator must be in a PoH that contains the original vote, or a portion of that validator's stake is slashable. This duration time is a function of the initial vote PoH count and all additional vote PoH counts. It will likely take the form:

Lockouti\(PoHi, PoHj\) = PoHj + K \* exp\(\(PoHj - PoHi\) / K\)

Where PoHi is the height of the vote that the lockout is to be applied to and PoHj is the height of the current vote on the same fork. If the validator submits a vote on a different PoH fork on any PoHk where k > j > i and PoHk < Lockout(PoHi, PoHj), then a portion of that validator's stake is at risk of being slashed.

In addition to the functional form lockout described above, early implementation may be a numerical approximation based on a First In, First Out (FIFO) data structure and the following logic:

  • FIFO queue holding 32 votes per active validator
  • new votes are pushed on top of queue (push_front)
  • expired votes are popped off top (pop_front)
  • as votes are pushed into the queue, the lockout of each queued vote doubles
  • votes are removed from back of queue if queue.len() > 32
  • the earliest and latest height that has been removed from the back of the queue should be stored

It is likely that a reward will be offered as a % of the slashed amount to any node that submits proof of this slashing condition being violated to the PoH.

Partial Slashing

In the schema described so far, when a validator votes on a given PoH stream, they are committing themselves to that fork for a time determined by the vote lockout. An open question is whether validators will be hesitant to begin voting on an available fork if the penalties are perceived too harsh for an honest mistake or flipped bit.

One way to address this concern would be a partial slashing design that results in a slashable amount as a function of either:

  1. the fraction of validators, out of the total validator pool, that were also slashed during the same time period (ala Casper)
  2. the amount of time since the vote was cast (e.g. a linearly increasing % of total deposited as slashable amount over time), or both.

This is an area currently under exploration.