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x/slashing

Abstract​

This section specifies the slashing module of the Cosmos SDK, which implements functionality first outlined in the Cosmos Whitepaper in June 2016.

The slashing module enables Cosmos SDK-based blockchains to disincentivize any attributable action by a protocol-recognized actor with value at stake by penalizing them ("slashing").

Penalties may include, but are not limited to:

  • Burning some amount of their stake
  • Removing their ability to vote on future blocks for a period of time.

This module will be used by the Cosmos Hub, the first hub in the Cosmos ecosystem.

Contents​

Concepts​

States​

At any given time, there are any number of validators registered in the state machine. Each block, the top MaxValidators (defined by x/staking) validators who are not jailed become bonded, meaning that they may propose and vote on blocks. Validators who are bonded are at stake, meaning that part or all of their stake and their delegators' stake is at risk if they commit a protocol fault.

For each of these validators we keep a ValidatorSigningInfo record that contains information pertaining to validator's liveness and other infraction related attributes.

Tombstone Caps​

In order to mitigate the impact of initially likely categories of non-malicious protocol faults, the Cosmos Hub implements for each validator a tombstone cap, which only allows a validator to be slashed once for a double sign fault. For example, if you misconfigure your HSM and double-sign a bunch of old blocks, you'll only be punished for the first double-sign (and then immediately tombstombed). This will still be quite expensive and desirable to avoid, but tombstone caps somewhat blunt the economic impact of unintentional misconfiguration.

Liveness faults do not have caps, as they can't stack upon each other. Liveness bugs are "detected" as soon as the infraction occurs, and the validators are immediately put in jail, so it is not possible for them to commit multiple liveness faults without unjailing in between.

Infraction Timelines​

To illustrate how the x/slashing module handles submitted evidence through CometBFT consensus, consider the following examples:

Definitions:

[ : timeline start
] : timeline end
Cn : infraction n committed
Dn : infraction n discovered
Vb : validator bonded
Vu : validator unbonded

Single Double Sign Infraction​

[----------C1----D1,Vu-----]

A single infraction is committed then later discovered, at which point the validator is unbonded and slashed at the full amount for the infraction.

Multiple Double Sign Infractions​

[----------C1--C2---C3---D1,D2,D3Vu-----]

Multiple infractions are committed and then later discovered, at which point the validator is jailed and slashed for only one infraction. Because the validator is also tombstoned, they can not rejoin the validator set.

State​

Signing Info (Liveness)​

Every block includes a set of precommits by the validators for the previous block, known as the LastCommitInfo provided by CometBFT. A LastCommitInfo is valid so long as it contains precommits from +2/3 of total voting power.

Proposers are incentivized to include precommits from all validators in the CometBFT LastCommitInfo by receiving additional fees proportional to the difference between the voting power included in the LastCommitInfo and +2/3 (see fee distribution).

type LastCommitInfo struct {
    Round int32
    Votes []VoteInfo
}

Validators are penalized for failing to be included in the LastCommitInfo for some number of blocks by being automatically jailed, potentially slashed, and unbonded.

Information about validator's liveness activity is tracked through ValidatorSigningInfo. It is indexed in the store as follows:

  • ValidatorSigningInfo: 0x01 | ConsAddrLen (1 byte) | ConsAddress -> ProtocolBuffer(ValSigningInfo)
  • MissedBlocksBitArray: 0x02 | ConsAddrLen (1 byte) | ConsAddress | LittleEndianUint64(signArrayIndex) -> VarInt(didMiss) (varint is a number encoding format)

The first mapping allows us to easily lookup the recent signing info for a validator based on the validator's consensus address.

The second mapping (MissedBlocksBitArray) acts as a bit-array of size SignedBlocksWindow that tells us if the validator missed the block for a given index in the bit-array. The index in the bit-array is given as little endian uint64. The result is a varint that takes on 0 or 1, where 0 indicates the validator did not miss (did sign) the corresponding block, and 1 indicates they missed the block (did not sign).

Note that the MissedBlocksBitArray is not explicitly initialized up-front. Keys are added as we progress through the first SignedBlocksWindow blocks for a newly bonded validator. The SignedBlocksWindow parameter defines the size (number of blocks) of the sliding window used to track validator liveness.

The information stored for tracking validator liveness is as follows:

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.1/x/slashing/proto/cosmos/slashing/v1beta1/slashing.proto#L13-L35

Params​

The slashing module stores its params in state with the prefix of 0x00, it can be updated with governance or the address with authority.

  • Params: 0x00 | ProtocolBuffer(Params)
https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.1/x/slashing/proto/cosmos/slashing/v1beta1/slashing.proto#L37-L62

Messages​

In this section we describe the processing of messages for the slashing module.

Unjail​

If a validator was automatically unbonded due to downtime and wishes to come back online & possibly rejoin the bonded set, it must send MsgUnjail:

// MsgUnjail is an sdk.Msg used for unjailing a jailed validator, thus returning
// them into the bonded validator set, so they can begin receiving provisions
// and rewards again.
message MsgUnjail {
  string validator_addr = 1;
}

Below is a pseudocode of the MsgSrv/Unjail RPC:

unjail(tx MsgUnjail)
    validator = getValidator(tx.ValidatorAddr)
    if validator == nil
      fail with "No validator found"

    if getSelfDelegation(validator) == 0
      fail with "validator must self delegate before unjailing"

    if !validator.Jailed
      fail with "Validator not jailed, cannot unjail"

    info = GetValidatorSigningInfo(operator)
    if info.Tombstoned
      fail with "Tombstoned validator cannot be unjailed"
    if block time < info.JailedUntil
      fail with "Validator still jailed, cannot unjail until period has expired"

    validator.Jailed = false
    setValidator(validator)

    return

If the validator has enough stake to be in the top n = MaximumBondedValidators, it will be automatically rebonded, and all delegators still delegated to the validator will be rebonded and begin to again collect provisions and rewards.

BeginBlock​

Liveness Tracking​

At the beginning of each block, we update the ValidatorSigningInfo for each validator and check if they've crossed below the liveness threshold over a sliding window. This sliding window is defined by SignedBlocksWindow and the index in this window is determined by (height - SignInfo.StartHeight). Notice that the position in the sliding window is incremented every block, independent of whether the validator signed or not. Once the index is determined, the MissedBlocksBitArray and MissedBlocksCounter are updated accordingly.

Finally, in order to determine if a validator crosses below the liveness threshold, we fetch the maximum number of blocks missed, maxMissed, which is SignedBlocksWindow - (MinSignedPerWindow * SignedBlocksWindow) and the minimum height at which we can determine liveness, minHeight. If the current block is greater than minHeight and the validator's MissedBlocksCounter is greater than maxMissed, they will be slashed by SlashFractionDowntime, will be jailed for DowntimeJailDuration, and have the following values reset: MissedBlocksBitArray, MissedBlocksCounter, and IndexOffset.

Note: Liveness slashes do NOT lead to a tombstombing.

height := block.Height

for vote in block.LastCommitInfo.Votes {
  signInfo := GetValidatorSigningInfo(vote.Validator.Address)

  // This is a relative index, so we counts blocks the validator SHOULD have
  // signed. We use the 0-value default signing info if not present.
  index := (height - signInfo.StartHeight) % SignedBlocksWindow()

  // Update MissedBlocksBitArray and MissedBlocksCounter. The MissedBlocksCounter
  // just tracks the sum of MissedBlocksBitArray. That way we avoid needing to
  // read/write the whole array each time.
  missedPrevious := GetValidatorMissedBlockBitArray(vote.Validator.Address, index)
  missed := !signed

  switch {
  case !missedPrevious && missed:
    // array index has changed from not missed to missed, increment counter
    SetValidatorMissedBlockBitArray(vote.Validator.Address, index, true)
    signInfo.MissedBlocksCounter++

  case missedPrevious && !missed:
    // array index has changed from missed to not missed, decrement counter
    SetValidatorMissedBlockBitArray(vote.Validator.Address, index, false)
    signInfo.MissedBlocksCounter--

  default:
    // array index at this index has not changed; no need to update counter
  }

  if missed {
    // emit events...
  }

  minHeight := signInfo.StartHeight + SignedBlocksWindow()
  maxMissed := SignedBlocksWindow() - MinSignedPerWindow()

  // If we are past the minimum height and the validator has missed too many
  // jail and slash them.
  if height > minHeight && signInfo.MissedBlocksCounter > maxMissed {
    validator := ValidatorByConsAddr(vote.Validator.Address)

    // emit events...

    // We need to retrieve the stake distribution which signed the block, so we
    // subtract ValidatorUpdateDelay from the block height, and subtract an
    // additional 1 since this is the LastCommit.
    //
    // Note, that this CAN result in a negative "distributionHeight" up to
    // -ValidatorUpdateDelay-1, i.e. at the end of the pre-genesis block (none) = at the beginning of the genesis block.
    // That's fine since this is just used to filter unbonding delegations & redelegations.
    distributionHeight := height - sdk.ValidatorUpdateDelay - 1

    SlashWithInfractionReason(vote.Validator.Address, distributionHeight, vote.Validator.Power, SlashFractionDowntime(), stakingtypes.Downtime)
    Jail(vote.Validator.Address)

    signInfo.JailedUntil = block.Time.Add(DowntimeJailDuration())

    // We need to reset the counter & array so that the validator won't be
    // immediately slashed for downtime upon rebonding.
    signInfo.MissedBlocksCounter = 0
    signInfo.IndexOffset = 0
    ClearValidatorMissedBlockBitArray(vote.Validator.Address)
  }

  SetValidatorSigningInfo(vote.Validator.Address, signInfo)
}

Hooks​

This section contains a description of the module's hooks. Hooks are operations that are executed automatically when events are raised.

Staking hooks​

The slashing module implements the StakingHooks defined in x/staking and are used as record-keeping of validators information. During the app initialization, these hooks should be registered in the staking module struct.

The following hooks impact the slashing state:

  • AfterValidatorBonded creates a ValidatorSigningInfo instance as described in the following section.
  • AfterValidatorCreated stores a validator's consensus key.
  • AfterValidatorRemoved removes a validator's consensus key.
  • AfterConsensusPubKeyUpdate handles the rotation of signing info and updates the address-pubkey relation after a consensus key update.

Validator Bonded​

Upon successful first-time bonding of a new validator, we create a new ValidatorSigningInfo structure for the now-bonded validator, which StartHeight of the current block.

If the validator was out of the validator set and gets bonded again, its new bonded height is set.

onValidatorBonded(address sdk.ValAddress)

  signingInfo, found = GetValidatorSigningInfo(address)
  if !found {
    signingInfo = ValidatorSigningInfo {
      StartHeight         : CurrentHeight,
      IndexOffset         : 0,
      JailedUntil         : time.Unix(0, 0),
      Tombstone           : false,
      MissedBloskCounter  : 0
    } else {
      signingInfo.StartHeight = CurrentHeight
    }

    setValidatorSigningInfo(signingInfo)
  }

  return

Events​

The slashing module emits the following events:

MsgServer​

MsgUnjail​

TypeAttribute KeyAttribute Value
messagemoduleslashing
messagesender{validatorAddress}

Keeper​

BeginBlocker: HandleValidatorSignature​

TypeAttribute KeyAttribute Value
slashaddress{validatorConsensusAddress}
slashpower{validatorPower}
slashreason{slashReason}
slashjailed [0]{validatorConsensusAddress}
slashburned coins{math.Int}
  • [0] Only included if the validator is jailed.
TypeAttribute KeyAttribute Value
livenessaddress{validatorConsensusAddress}
livenessmissed_blocks{missedBlocksCounter}
livenessheight{blockHeight}

Slash​

  • same as "slash" event from HandleValidatorSignature, but without the jailed attribute.

Jail​

TypeAttribute KeyAttribute Value
slashjailed{validatorAddress}

Staking Tombstone​

Abstract​

In the current implementation of the slashing module, when the consensus engine informs the state machine of a validator's consensus fault, the validator is partially slashed, and put into a "jail period", a period of time in which they are not allowed to rejoin the validator set. However, because of the nature of consensus faults and ABCI, there can be a delay between an infraction occurring, and evidence of the infraction reaching the state machine (this is one of the primary reasons for the existence of the unbonding period).

Note: The tombstone concept, only applies to faults that have a delay between the infraction occurring and evidence reaching the state machine. For example, evidence of a validator double signing may take a while to reach the state machine due to unpredictable evidence gossip layer delays and the ability of validators to selectively reveal double-signatures (e.g. to infrequently-online light clients). Liveness slashing, on the other hand, is detected immediately as soon as the infraction occurs, and therefore no slashing period is needed. A validator is immediately put into jail period, and they cannot commit another liveness fault until they unjail. In the future, there may be other types of byzantine faults that have delays (for example, submitting evidence of an invalid proposal as a transaction). When implemented, it will have to be decided whether these future types of byzantine faults will result in a tombstoning (and if not, the slash amounts will not be capped by a slashing period).

In the current system design, once a validator is put in the jail for a consensus fault, after the JailPeriod they are allowed to send a transaction to unjail themselves, and thus rejoin the validator set.

One of the "design desires" of the slashing module is that if multiple infractions occur before evidence is executed (and a validator is put in jail), they should only be punished for single worst infraction, but not cumulatively. For example, if the sequence of events is:

  1. Validator A commits Infraction 1 (worth 30% slash)
  2. Validator A commits Infraction 2 (worth 40% slash)
  3. Validator A commits Infraction 3 (worth 35% slash)
  4. Evidence for Infraction 1 reaches state machine (and validator is put in jail)
  5. Evidence for Infraction 2 reaches state machine
  6. Evidence for Infraction 3 reaches state machine

Only Infraction 2 should have its slash take effect, as it is the highest. This is done, so that in the case of the compromise of a validator's consensus key, they will only be punished once, even if the hacker double-signs many blocks. Because, the unjailing has to be done with the validator's operator key, they have a chance to re-secure their consensus key, and then signal that they are ready using their operator key. We call this period during which we track only the max infraction, the "slashing period".

Once, a validator rejoins by unjailing themselves, we begin a new slashing period; if they commit a new infraction after unjailing, it gets slashed cumulatively on top of the worst infraction from the previous slashing period.

However, while infractions are grouped based off of the slashing periods, because evidence can be submitted up to an unbondingPeriod after the infraction, we still have to allow for evidence to be submitted for previous slashing periods. For example, if the sequence of events is:

  1. Validator A commits Infraction 1 (worth 30% slash)
  2. Validator A commits Infraction 2 (worth 40% slash)
  3. Evidence for Infraction 1 reaches state machine (and Validator A is put in jail)
  4. Validator A unjails

We are now in a new slashing period, however we still have to keep the door open for the previous infraction, as the evidence for Infraction 2 may still come in. As the number of slashing periods increase, it creates more complexity as we have to keep track of the highest infraction amount for every single slashing period.

Note: Currently, according to the slashing module spec, a new slashing period is created every time a validator is unbonded then rebonded. This should probably be changed to jailed/unjailed. See issue #3205 for further details. For the remainder of this, I will assume that we only start a new slashing period when a validator gets unjailed.

The maximum number of slashing periods is the len(UnbondingPeriod) / len(JailPeriod). The current defaults in Gaia for the UnbondingPeriod and JailPeriod are 3 weeks and 2 days, respectively. This means there could potentially be up to 11 slashing periods concurrently being tracked per validator. If we set the JailPeriod >= UnbondingPeriod, we only have to track 1 slashing period (i.e not have to track slashing periods).

Currently, in the jail period implementation, once a validator unjails, all of their delegators who are delegated to them (haven't unbonded / redelegated away), stay with them. Given that consensus safety faults are so egregious (way more so than liveness faults), it is probably prudent to have delegators not "auto-rebond" to the validator.

Proposal: infinite jail​

We propose setting the "jail time" for a validator who commits a consensus safety fault, to infinite (i.e. a tombstone state). This essentially kicks the validator out of the validator set and does not allow them to re-enter the validator set. All of their delegators (including the operator themselves) have to either unbond or redelegate away. The validator operator can create a new validator if they would like, with a new operator key and consensus key, but they have to "re-earn" their delegations back.

Implementing the tombstone system and getting rid of the slashing period tracking will make the slashing module way simpler, especially because we can remove all of the hooks defined in the slashing module consumed by the staking module (the slashing module still consumes hooks defined in staking).

Single slashing amount​

Another optimization that can be made is that if we assume that all ABCI faults for CometBFT consensus are slashed at the same level, we don't have to keep track of "max slash". Once an ABCI fault happens, we don't have to worry about comparing potential future ones to find the max.

Currently the only CometBFT ABCI fault is:

  • Unjustified precommits (double signs)

It is currently planned to include the following fault in the near future:

  • Signing a precommit when you're in unbonding phase (needed to make light client bisection safe)

Given that these faults are both attributable byzantine faults, we will likely want to slash them equally, and thus we can enact the above change.

Note: This change may make sense for current CometBFT consensus, but maybe not for a different consensus algorithm or future versions of CometBFT that may want to punish at different levels (for example, partial slashing).

Parameters​

The slashing module contains the following parameters:

KeyTypeExample
SignedBlocksWindowstring (int64)"100"
MinSignedPerWindowstring (dec)"0.500000000000000000"
DowntimeJailDurationstring (ns)"600000000000"
SlashFractionDoubleSignstring (dec)"0.050000000000000000"
SlashFractionDowntimestring (dec)"0.010000000000000000"

CLI​

A user can query and interact with the slashing module using the CLI.

Query​

The query commands allow users to query slashing state.

simd query slashing --help

params​

The params command allows users to query genesis parameters for the slashing module.

simd query slashing params [flags]

Example:

simd query slashing params

Example Output:

downtime_jail_duration: 600s
min_signed_per_window: "0.500000000000000000"
signed_blocks_window: "100"
slash_fraction_double_sign: "0.050000000000000000"
slash_fraction_downtime: "0.010000000000000000"

signing-info​

The signing-info command allows users to query signing-info of the validator using consensus public key.

simd query slashing signing-infos [flags]

Example:

simd query slashing signing-info '{"@type":"/cosmos.crypto.ed25519.PubKey","key":"Auxs3865HpB/EfssYOzfqNhEJjzys6jD5B6tPgC8="}'

Example Output:

address: cosmosvalcons1nrqsld3aw6lh6t082frdqc84uwxn0t958c
index_offset: "2068"
jailed_until: "1970-01-01T00:00:00Z"
missed_blocks_counter: "0"
start_height: "0"
tombstoned: false

signing-infos​

The signing-infos command allows users to query signing infos of all validators.

simd query slashing signing-infos [flags]

Example:

simd query slashing signing-infos

Example Output:

info:
- address: cosmosvalcons1nrqsld3aw6lh6t082frdqc84uwxn0t958c
  index_offset: "2075"
  jailed_until: "1970-01-01T00:00:00Z"
  missed_blocks_counter: "0"
  start_height: "0"
  tombstoned: false
pagination:
  next_key: null
  total: "0"

Transactions​

The tx commands allow users to interact with the slashing module.

simd tx slashing --help

unjail​

The unjail command allows users to unjail a validator previously jailed for downtime.

simd tx slashing unjail --from mykey [flags]

Example:

simd tx slashing unjail --from mykey

update-params-proposal​

The update-params-proposal command allows users to submit a governance proposal to update the slashing module parameters:

simd tx slashing update-params-proposal <params> [flags]

Example:

simd tx slashing update-params-proposal '{ "signed_blocks_window": "100" }'

gRPC​

A user can query the slashing module using gRPC endpoints.

Params​

The Params endpoint allows users to query the parameters of slashing module.

cosmos.slashing.v1beta1.Query/Params

Example:

grpcurl -plaintext localhost:9090 cosmos.slashing.v1beta1.Query/Params

Example Output:

{
  "params": {
    "signedBlocksWindow": "100",
    "minSignedPerWindow": "NTAwMDAwMDAwMDAwMDAwMDAw",
    "downtimeJailDuration": "600s",
    "slashFractionDoubleSign": "NTAwMDAwMDAwMDAwMDAwMDA=",
    "slashFractionDowntime": "MTAwMDAwMDAwMDAwMDAwMDA="
  }
}

SigningInfo​

The SigningInfo queries the signing info of given cons address.

cosmos.slashing.v1beta1.Query/SigningInfo

Example:

grpcurl -plaintext -d '{"cons_address":"cosmosvalcons1nrqsld3aw6lh6t082frdqc84uwxn0t958c"}' localhost:9090 cosmos.slashing.v1beta1.Query/SigningInfo

Example Output:

{
  "valSigningInfo": {
    "address": "cosmosvalcons1nrqsld3aw6lh6t082frdqc84uwxn0t958c",
    "indexOffset": "3493",
    "jailedUntil": "1970-01-01T00:00:00Z"
  }
}

SigningInfos​

The SigningInfos queries signing info of all validators.

cosmos.slashing.v1beta1.Query/SigningInfos

Example:

grpcurl -plaintext localhost:9090 cosmos.slashing.v1beta1.Query/SigningInfos

Example Output:

{
  "info": [
    {
      "address": "cosmosvalcons1nrqslkwd3pz096lh6t082frdqc84uwxn0t958c",
      "indexOffset": "2467",
      "jailedUntil": "1970-01-01T00:00:00Z"
    }
  ],
  "pagination": {
    "total": "1"
  }
}

REST​

A user can query the slashing module using REST endpoints.

Params​

/cosmos/slashing/v1beta1/params

Example:

curl "localhost:1317/cosmos/slashing/v1beta1/params"

Example Output:

{
  "params": {
    "signed_blocks_window": "100",
    "min_signed_per_window": "0.500000000000000000",
    "downtime_jail_duration": "600s",
    "slash_fraction_double_sign": "0.050000000000000000",
    "slash_fraction_downtime": "0.010000000000000000"
}

signing_info​

/cosmos/slashing/v1beta1/signing_infos/%s

Example:

curl "localhost:1317/cosmos/slashing/v1beta1/signing_infos/cosmosvalcons1nrqslkwd3pz096lh6t082frdqc84uwxn0t958c"

Example Output:

{
  "val_signing_info": {
    "address": "cosmosvalcons1nrqslkwd3pz096lh6t082frdqc84uwxn0t958c",
    "start_height": "0",
    "index_offset": "4184",
    "jailed_until": "1970-01-01T00:00:00Z",
    "tombstoned": false,
    "missed_blocks_counter": "0"
  }
}

signing_infos​

/cosmos/slashing/v1beta1/signing_infos

Example:

curl "localhost:1317/cosmos/slashing/v1beta1/signing_infos

Example Output:

{
  "info": [
    {
      "address": "cosmosvalcons1nrqslkwd3pz096lh6t082frdqc84uwxn0t958c",
      "start_height": "0",
      "index_offset": "4169",
      "jailed_until": "1970-01-01T00:00:00Z",
      "tombstoned": false,
      "missed_blocks_counter": "0"
    }
  ],
  "pagination": {
    "next_key": null,
    "total": "1"
  }
}