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# Concepts

## 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.

```go
// Evidence defines the contract which concrete evidence types of misbehavior
// must implement.
type Evidence interface {
	proto.Message

	Route() string
	Type() string
	String() string
	Hash() tmbytes.HexBytes
	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
}
```

## 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.

```go
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.

```go
// 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(sdk.Context, Evidence) error
```
