# x/slashing
# Abstract
This section specifies the slashing module of the Cosmos SDK, which implements functionality first outlined in the Cosmos Whitepaper (opens new window) 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 partaining 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).
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:
# Params
The slashing module stores it's params in state with the prefix of 0x00,
it can be updated with governance or the address with authority.
- Params:
0x00 | ProtocolBuffer(Params)
# 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:
Below is a pseudocode of the MsgSrv/Unjail RPC:
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 IndexOffset found in the validator's
ValidatorSigningInfo. For each block processed, the IndexOffset is incremented
regardless if 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.
# 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:
AfterValidatorBondedcreates aValidatorSigningInfoinstance as described in the following section.AfterValidatorCreatedstores a validator's consensus key.AfterValidatorRemovedremoves a validator's consensus key.
# 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.
# Events
The slashing module emits the following events:
# MsgServer
# MsgUnjail
| Type | Attribute Key | Attribute Value |
|---|---|---|
| message | module | slashing |
| message | sender |
# Keeper
# BeginBlocker: HandleValidatorSignature
| Type | Attribute Key | Attribute Value |
|---|---|---|
| slash | address | |
| slash | power | |
| slash | reason | |
| slash | jailed [0] | |
| slash | burned coins |
- [0] Only included if the validator is jailed.
| Type | Attribute Key | Attribute Value |
|---|---|---|
| liveness | address | |
| liveness | missed_blocks | |
| liveness | height |
# Slash
- same as
"slash"event fromHandleValidatorSignature, but without thejailedattribute.
# Jail
| Type | Attribute Key | Attribute Value |
|---|---|---|
| slash | jailed |
# 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:
- Validator A commits Infraction 1 (worth 30% slash)
- Validator A commits Infraction 2 (worth 40% slash)
- Validator A commits Infraction 3 (worth 35% slash)
- Evidence for Infraction 1 reaches state machine (and validator is put in jail)
- Evidence for Infraction 2 reaches state machine
- 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:
- Validator A commits Infraction 1 (worth 30% slash)
- Validator A commits Infraction 2 (worth 40% slash)
- Evidence for Infraction 1 reaches state machine (and Validator A is put in jail)
- 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
slashingmodule 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 (opens new window) 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:
| Key | Type | Example |
|---|---|---|
| SignedBlocksWindow | string (int64) | "100" |
| MinSignedPerWindow | string (dec) | "0.500000000000000000" |
| DowntimeJailDuration | string (ns) | "600000000000" |
| SlashFractionDoubleSign | string (dec) | "0.050000000000000000" |
| SlashFractionDowntime | string (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.
# params
The params command allows users to query genesis parameters for the slashing module.
Example:
Example Output:
# signing-info
The signing-info command allows users to query signing-info of the validator using consensus public key.
Example:
Example Output:
# signing-infos
The signing-infos command allows users to query signing infos of all validators.
Example:
Example Output:
# Transactions
The tx commands allow users to interact with the slashing module.
# unjail
The unjail command allows users to unjail a validator previously jailed for downtime.
Example:
# gRPC
A user can query the slashing module using gRPC endpoints.
# Params
The Params endpoint allows users to query the parameters of slashing module.
Example:
Example Output:
# SigningInfo
The SigningInfo queries the signing info of given cons address.
Example:
Example Output:
# SigningInfos
The SigningInfos queries signing info of all validators.
Example:
Example Output:
# REST
A user can query the slashing module using REST endpoints.
# Params
Example:
Example Output:
# signing_info
Example:
Example Output:
# signing_infos
Example:
Example Output: