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Abstract

x/evidence is an implementation of a Cosmos SDK module, per ADR 009, that allows for the submission and handling of arbitrary evidence of misbehavior such as equivocation and counterfactual signing. The evidence module differs from standard evidence handling which typically expects the underlying consensus engine, e.g. CometBFT, to automatically submit evidence when it is discovered by allowing clients and foreign chains to submit more complex evidence directly. All concrete evidence types must implement the Evidence interface contract. Submitted Evidence is first routed through the evidence module’s Router in which it attempts to find a corresponding registered Handler for that specific Evidence type. Each Evidence type must have a Handler registered with the evidence module’s keeper in order for it to be successfully routed and executed. Each corresponding handler must also fulfill the Handler interface contract. The Handler for a given Evidence type can perform any arbitrary state transitions such as slashing, jailing, and tombstoning.

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Concepts

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Evidence

Any concrete type of evidence submitted to the x/evidence module must fulfill the Evidence contract outlined below. Not all concrete types of evidence will fulfill this contract in the same way and some data may be entirely irrelevant to certain types of evidence. An additional ValidatorEvidence, which extends Evidence, has also been created to define a contract for evidence against malicious validators. 
// Evidence defines the contract which concrete evidence types of misbehavior
// must implement.
type Evidence interface {
    proto.Message

	Route()

string
	String()

string
	Hash() []byte
	ValidateBasic()

error

	// Height at which the infraction occurred
	GetHeight()

int64
}

// ValidatorEvidence extends Evidence interface to define contract
// for evidence against malicious validators
type ValidatorEvidence interface {
    Evidence

	// The consensus address of the malicious validator at time of infraction
	GetConsensusAddress()

sdk.ConsAddress

	// The total power of the malicious validator at time of infraction
	GetValidatorPower()

int64

	// The total validator set power at time of infraction
	GetTotalPower()

int64
}

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Registration & Handling

The x/evidence module must first know about all types of evidence it is expected to handle. This is accomplished by registering the Route method in the Evidence contract with what is known as a Router (defined below). The Router accepts Evidence and attempts to find the corresponding Handler for the Evidence via the Route method. 
type Router interface {
    AddRoute(r string, h Handler)

Router
  HasRoute(r string)

bool
  GetRoute(path string)

Handler
  Seal()

Sealed()

bool
}
The Handler (defined below) is responsible for executing the entirety of the business logic for handling Evidence. This typically includes validating the evidence, both stateless checks via ValidateBasic and stateful checks via any keepers provided to the Handler. In addition, the Handler may also perform capabilities such as slashing and jailing a validator. All Evidence handled by the Handler should be persisted. 
// Handler defines an agnostic Evidence handler. The handler is responsible
// for executing all corresponding business logic necessary for verifying the
// evidence as valid. In addition, the Handler may execute any necessary
// slashing and potential jailing.
type Handler func(context.Context, Evidence)

error

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State

Currently the x/evidence module only stores valid submitted Evidence in state. The evidence state is also stored and exported in the x/evidence module’s GenesisState. 
// GenesisState defines the evidence module's genesis state.
message GenesisState {
  // evidence defines all the evidence at genesis.
  repeated google.protobuf.Any evidence = 1;
}
All Evidence is retrieved and stored via a prefix KVStore using prefix 0x00 (KeyPrefixEvidence).

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Messages

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MsgSubmitEvidence

Evidence is submitted through a MsgSubmitEvidence message: 
// MsgSubmitEvidence represents a message that supports submitting arbitrary
// Evidence of misbehavior such as equivocation or counterfactual signing.
message MsgSubmitEvidence {
  string              submitter = 1;
  google.protobuf.Any evidence  = 2;
}
Note, the Evidence of a MsgSubmitEvidence message must have a corresponding Handler registered with the x/evidence module’s Router in order to be processed and routed correctly. Given the Evidence is registered with a corresponding Handler, it is processed as follows: 
func SubmitEvidence(ctx Context, evidence Evidence)

error {
    if _, err := GetEvidence(ctx, evidence.Hash()); err == nil {
    return errorsmod.Wrap(types.ErrEvidenceExists, strings.ToUpper(hex.EncodeToString(evidence.Hash())))
}
    if !router.HasRoute(evidence.Route()) {
    return errorsmod.Wrap(types.ErrNoEvidenceHandlerExists, evidence.Route())
}
    handler := router.GetRoute(evidence.Route())
    if err := handler(ctx, evidence); err != nil {
    return errorsmod.Wrap(types.ErrInvalidEvidence, err.Error())
}

ctx.EventManager().EmitEvent(
		sdk.NewEvent(
			types.EventTypeSubmitEvidence,
			sdk.NewAttribute(types.AttributeKeyEvidenceHash, strings.ToUpper(hex.EncodeToString(evidence.Hash()))),
		),
	)

SetEvidence(ctx, evidence)

return nil
}
First, there must not already exist valid submitted Evidence of the exact same type. Secondly, the Evidence is routed to the Handler and executed. Finally, if there is no error in handling the Evidence, an event is emitted and it is persisted to state.

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Events

The x/evidence module emits the following events:

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Handlers

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MsgSubmitEvidence

TypeAttribute KeyAttribute Value
submit_evidenceevidence_hash{evidenceHash}
messagemoduleevidence
messagesender{senderAddress}
messageactionsubmit_evidence

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Parameters

The evidence module does not contain any parameters.

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BeginBlock

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Evidence Handling

CometBFT blocks can include Evidence that indicates if a validator committed malicious behavior. The relevant information is forwarded to the application as ABCI Evidence in abci.RequestBeginBlock so that the validator can be punished accordingly.

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Equivocation

The Cosmos SDK handles two types of evidence inside the ABCI BeginBlock:
  • DuplicateVoteEvidence,
  • LightClientAttackEvidence.
The evidence module handles these two evidence types the same way. First, the Cosmos SDK converts the CometBFT concrete evidence type to an SDK Evidence interface using Equivocation as the concrete type. 
// Reference: https://github.com/cosmos/cosmos-sdk/tree/release/v0.50.x/proto/cosmos/evidence/v1beta1/evidence.proto#L12-L32
For some Equivocation submitted in block to be valid, it must satisfy: Evidence.Timestamp >= block.Timestamp - MaxEvidenceAge Where:
  • Evidence.Timestamp is the timestamp in the block at height Evidence.Height
  • block.Timestamp is the current block timestamp.
If valid Equivocation evidence is included in a block, the validator’s stake is reduced (slashed) by SlashFractionDoubleSign as defined by the x/slashing module of what their stake was when the infraction occurred, rather than when the evidence was discovered. We want to “follow the stake”, i.e., the stake that contributed to the infraction should be slashed, even if it has since been redelegated or started unbonding. In addition, the validator is permanently jailed and tombstoned to make it impossible for that validator to ever re-enter the validator set. The Equivocation evidence is handled as follows: 
// Reference: https://github.com/cosmos/cosmos-sdk/tree/release/v0.50.x/x/evidence/keeper/infraction.go#L26-L140
Note: The slashing, jailing, and tombstoning calls are delegated through the x/slashing module that emits informative events and finally delegates calls to the x/staking module. See documentation on slashing and jailing in State Transitions.

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Client

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CLI

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

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Query

The query commands allows users to query evidence state. 
simd query evidence --help

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evidence

The evidence command allows users to list all evidence or evidence by hash. Usage: 
simd query evidence evidence [flags]
To query evidence by hash Example: 
simd query evidence evidence "DF0C23E8634E480F84B9D5674A7CDC9816466DEC28A3358F73260F68D28D7660"
Example Output: 
evidence:
  consensus_address: cosmosvalcons1ntk8eualewuprz0gamh8hnvcem2nrcdsgz563h
  height: 11
  power: 100
  time: "2021-10-20T16:08:38.194017624Z"
To get all evidence Example: 
simd query evidence list
Example Output: 
evidence:
  consensus_address: cosmosvalcons1ntk8eualewuprz0gamh8hnvcem2nrcdsgz563h
  height: 11
  power: 100
  time: "2021-10-20T16:08:38.194017624Z"
pagination:
  next_key: null
  total: "1"

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REST

A user can query the evidence module using REST endpoints.

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Evidence

Get evidence by hash 
/cosmos/evidence/v1beta1/evidence/{hash}
Example: 
curl -X GET "http://localhost:1317/cosmos/evidence/v1beta1/evidence/DF0C23E8634E480F84B9D5674A7CDC9816466DEC28A3358F73260F68D28D7660"
Example Output: 
{
  "evidence": {
    "consensus_address": "cosmosvalcons1ntk8eualewuprz0gamh8hnvcem2nrcdsgz563h",
    "height": "11",
    "power": "100",
    "time": "2021-10-20T16:08:38.194017624Z"
  }
}

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All evidence

Get all evidence 
/cosmos/evidence/v1beta1/evidence
Example: 
curl -X GET "http://localhost:1317/cosmos/evidence/v1beta1/evidence"
Example Output: 
{
  "evidence": [
    {
      "consensus_address": "cosmosvalcons1ntk8eualewuprz0gamh8hnvcem2nrcdsgz563h",
      "height": "11",
      "power": "100",
      "time": "2021-10-20T16:08:38.194017624Z"
    }
  ],
  "pagination": {
    "total": "1"
  }
}

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gRPC

A user can query the evidence module using gRPC endpoints.

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Evidence

Get evidence by hash 
cosmos.evidence.v1beta1.Query/Evidence
Example: 
grpcurl -plaintext -d '{"evidence_hash":"DF0C23E8634E480F84B9D5674A7CDC9816466DEC28A3358F73260F68D28D7660"}' localhost:9090 cosmos.evidence.v1beta1.Query/Evidence
Example Output: 
{
  "evidence": {
    "consensus_address": "cosmosvalcons1ntk8eualewuprz0gamh8hnvcem2nrcdsgz563h",
    "height": "11",
    "power": "100",
    "time": "2021-10-20T16:08:38.194017624Z"
  }
}

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All evidence

Get all evidence 
cosmos.evidence.v1beta1.Query/AllEvidence
Example: 
grpcurl -plaintext localhost:9090 cosmos.evidence.v1beta1.Query/AllEvidence
Example Output: 
{
  "evidence": [
    {
      "consensus_address": "cosmosvalcons1ntk8eualewuprz0gamh8hnvcem2nrcdsgz563h",
      "height": "11",
      "power": "100",
      "time": "2021-10-20T16:08:38.194017624Z"
    }
  ],
  "pagination": {
    "total": "1"
  }
}