prysm-pulse/beacon-chain/core/helpers/randao.go

package helpers

import (
	"github.com/pkg/errors"
	pb "github.com/prysmaticlabs/prysm/proto/beacon/p2p/v1"
	"github.com/prysmaticlabs/prysm/shared/bytesutil"
	"github.com/prysmaticlabs/prysm/shared/hashutil"
	"github.com/prysmaticlabs/prysm/shared/params"
)

// ErrInvalidStateLatestActiveIndexRoots is returned when the state active
// index root count does not match the expected EpochsPerHistoricalVector.
var ErrInvalidStateLatestActiveIndexRoots = errors.New("state does not have correct number of latest active index roots")

// Seed returns the randao seed used for shuffling of a given epoch.
//
// Spec pseudocode definition:
//  def get_seed(state: BeaconState, epoch: Epoch, domain_type: DomainType) -> Hash:
//    """
//    Return the seed at ``epoch``.
//    """
//    mix = get_randao_mix(state, Epoch(epoch + EPOCHS_PER_HISTORICAL_VECTOR - MIN_SEED_LOOKAHEAD - 1))  # Avoid underflow
//    return hash(domain_type + int_to_bytes(epoch, length=8) + mix)
func Seed(state *pb.BeaconState, epoch uint64, domain []byte) ([32]byte, error) {
	// See https://github.com/ethereum/eth2.0-specs/pull/1296 for
	// rationale on why offset has to look down by 1.
	lookAheadEpoch := epoch + params.BeaconConfig().EpochsPerHistoricalVector -
		params.BeaconConfig().MinSeedLookahead - 1

	randaoMix := RandaoMix(state, lookAheadEpoch)

	seed := append(domain, bytesutil.Bytes8(epoch)...)
	seed = append(seed, randaoMix...)

	seed32 := hashutil.Hash(seed)

	return seed32, nil
}

// RandaoMix returns the randao mix (xor'ed seed)
// of a given slot. It is used to shuffle validators.
//
// Spec pseudocode definition:
//   def get_randao_mix(state: BeaconState, epoch: Epoch) -> Hash:
//    """
//    Return the randao mix at a recent ``epoch``.
//    """
//    return state.randao_mixes[epoch % EPOCHS_PER_HISTORICAL_VECTOR]
func RandaoMix(state *pb.BeaconState, epoch uint64) []byte {
	newMixLength := len(state.RandaoMixes[epoch%params.BeaconConfig().EpochsPerHistoricalVector])
	newMix := make([]byte, newMixLength)
	copy(newMix, state.RandaoMixes[epoch%params.BeaconConfig().EpochsPerHistoricalVector])
	return newMix
}