additional equivocation proposal logic is required

[MSalopek]

Problem

When submitting an equivocation proposal the evidence listed in the proposal can become stale.
If the proposal with stale evidence passes the validator will not get slashed because the HandleEquivocationEvidence will exit early with no errors.

Closing criteria

Add validation that will reject the proposal if the evidence can be deemed too old at:

• proposal submission time
• proposal execution time

Problem details

When submitting evidence in the Equivocation proposal, the height must fall within the Evidence.MaxAgeNumBlocks and EvidenceMaxAgeDuration on both proposal submission and execution - otherwise the slash will not occur.

The relevant function from cosmosSDK:

func (k Keeper) HandleEquivocationEvidence(ctx sdk.Context, evidence *types.Equivocation) {
	logger := k.Logger(ctx)
	consAddr := evidence.GetConsensusAddress()

	// calculate the age of the evidence
	infractionHeight := evidence.GetHeight()
	infractionTime := evidence.GetTime()
	ageDuration := ctx.BlockHeader().Time.Sub(infractionTime)
	ageBlocks := ctx.BlockHeader().Height - infractionHeight

	// Reject evidence if the double-sign is too old. Evidence is considered stale
	// if the difference in time and number of blocks is greater than the allowed
	// parameters defined.
	cp := ctx.ConsensusParams()
	if cp != nil && cp.Evidence != nil {
                 // NOTE: this will exit early!
		if ageDuration > cp.Evidence.MaxAgeDuration && ageBlocks > cp.Evidence.MaxAgeNumBlocks {
			logger.Info(
				"ignored equivocation; evidence too old",
				"validator", consAddr,
				"infraction_height", infractionHeight,
				"max_age_num_blocks", cp.Evidence.MaxAgeNumBlocks,
				"infraction_time", infractionTime,
				"max_age_duration", cp.Evidence.MaxAgeDuration,
			)
			return
		}
	}

Possible cases:

1. Evidence is too old at submission -> prop submission tx rejected
2. Evidence is valid at submission but invalid at execution time -> prop tx accepted, proposal can pass voting, validator cannot be slashed
3. Evidence valid at submission and valid at execution time -> prop tx accepted, proposal can pass voting, validator can be slashed

TODOs

• Labels have been added for issue
• Issue has been added to the ICS project

[tbruyelle]

If nobody started to work on it, I can do it. Please assign it to me in that case.

[jtremback]

I don't think this is worth doing. Instead we should fix the proposal so that it pauses unbondings for the validator, or we should just get cryptographically automatic equivocation slashing working as we've discussed.