consensus/clique: Proof of Authority (#3753)

This PR is a prototype implementation of plugable consensus engines and the
Clique PoA protocol ethereum/EIPs#225

6 files changed, 1465 insertions, 29 deletions

diff --git a/consensus/clique/api.go b/consensus/clique/api.go
new file mode 100644
index 000000000..0cf25abff
--- /dev/null
+++ b/consensus/clique/api.go
@@ -0,0 +1,97 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package clique
+
+import (
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/consensus"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/rpc"
+)
+
+// API is a user facing RPC API to allow controlling the signer and voting
+// mechanisms of the proof-of-authority scheme.
+type API struct {
+	chain  consensus.ChainReader
+	clique *Clique
+}
+
+// GetSnapshot retrieves the state snapshot at a given block.
+func (api *API) GetSnapshot(number *rpc.BlockNumber) (interface{}, error) {
+	// Retrieve the requested block number (or current if none requested)
+	var header *types.Header
+	if number == nil || *number == rpc.LatestBlockNumber {
+		header = api.chain.CurrentHeader()
+	} else {
+		header = api.chain.GetHeaderByNumber(uint64(number.Int64()))
+	}
+	// Ensure we have an actually valid block and return its snapshot
+	if header == nil {
+		return nil, errUnknownBlock
+	}
+	return api.clique.snapshot(api.chain, header.Number.Uint64(), header.Hash(), nil)
+}
+
+// GetSigners retrieves the list of authorized signers at the specified block.
+func (api *API) GetSigners(number *rpc.BlockNumber) ([]common.Address, error) {
+	// Retrieve the requested block number (or current if none requested)
+	var header *types.Header
+	if number == nil || *number == rpc.LatestBlockNumber {
+		header = api.chain.CurrentHeader()
+	} else {
+		header = api.chain.GetHeaderByNumber(uint64(number.Int64()))
+	}
+	// Ensure we have an actually valid block and return the signers from its snapshot
+	if header == nil {
+		return nil, errUnknownBlock
+	}
+	snap, err := api.clique.snapshot(api.chain, header.Number.Uint64(), header.Hash(), nil)
+	if err != nil {
+		return nil, err
+	}
+	return snap.signers(), nil
+}
+
+// Proposals returns the current proposals the node tries to uphold and vote on.
+func (api *API) Proposals() map[common.Address]bool {
+	api.clique.lock.RLock()
+	defer api.clique.lock.RUnlock()
+
+	proposals := make(map[common.Address]bool)
+	for address, auth := range api.clique.proposals {
+		proposals[address] = auth
+	}
+	return proposals
+}
+
+// Propose injects a new authorization proposal that the signer will attempt to
+// push through.
+func (api *API) Propose(address common.Address, auth bool) {
+	api.clique.lock.Lock()
+	defer api.clique.lock.Unlock()
+
+	api.clique.proposals[address] = auth
+}
+
+// Discard drops a currently running proposal, stopping the signer from casting
+// further votes (either for or against).
+func (api *API) Discard(address common.Address) {
+	api.clique.lock.Lock()
+	defer api.clique.lock.Unlock()
+
+	delete(api.clique.proposals, address)
+}
diff --git a/consensus/clique/clique.go b/consensus/clique/clique.go
new file mode 100644
index 000000000..e516d5057
--- /dev/null
+++ b/consensus/clique/clique.go
@@ -0,0 +1,638 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// Package clique implements the proof-of-authority consensus engine.
+package clique
+
+import (
+	"bytes"
+	"errors"
+	"math/big"
+	"math/rand"
+	"sync"
+	"time"
+
+	"github.com/ethereum/go-ethereum/accounts"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/consensus"
+	"github.com/ethereum/go-ethereum/core/state"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/ethereum/go-ethereum/crypto/sha3"
+	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/params"
+	"github.com/ethereum/go-ethereum/rlp"
+	"github.com/ethereum/go-ethereum/rpc"
+	lru "github.com/hashicorp/golang-lru"
+)
+
+const (
+	checkpointInterval = 1024 // Number of blocks after which to save the vote snapshot to the database
+	inmemorySnapshots  = 128  // Number of recent vote snapshots to keep in memory
+	inmemorySignatures = 1024 // Number of recent blocks to keep in memory
+
+	wiggleTime = 500 * time.Millisecond // Random delay (per signer) to allow concurrent signers
+)
+
+// Clique proof-of-authority protocol constants.
+var (
+	epochLength = uint64(30000) // Default number of blocks after which to checkpoint and reset the pending votes
+	blockPeriod = uint64(15)    // Default minimum difference between two consecutive block's timestamps
+
+	extraVanity = 32 // Fixed number of extra-data prefix bytes reserved for signer vanity
+	extraSeal   = 65 // Fixed number of extra-data suffix bytes reserved for signer seal
+
+	nonceAuthVote = hexutil.MustDecode("0xffffffffffffffff") // Magic nonce number to vote on adding a new signer
+	nonceDropVote = hexutil.MustDecode("0x0000000000000000") // Magic nonce number to vote on removing a signer.
+
+	uncleHash = types.CalcUncleHash(nil) // Always Keccak256(RLP([])) as uncles are meaningless outside of PoW.
+
+	diffInTurn = big.NewInt(2) // Block difficulty for in-turn signatures
+	diffNoTurn = big.NewInt(1) // Block difficulty for out-of-turn signatures
+)
+
+// Various error messages to mark blocks invalid. These should be private to
+// prevent engine specific errors from being referenced in the remainder of the
+// codebase, inherently breaking if the engine is swapped out. Please put common
+// error types into the consensus package.
+var (
+	// errUnknownBlock is returned when the list of signers is requested for a block
+	// that is not part of the local blockchain.
+	errUnknownBlock = errors.New("unknown block")
+
+	// errInvalidCheckpointBeneficiary is returned if a checkpoint/epoch transition
+	// block has a beneficiary set to non zeroes.
+	errInvalidCheckpointBeneficiary = errors.New("beneficiary in checkpoint block non-zero")
+
+	// errInvalidVote is returned if a nonce value is something else that the two
+	// allowed constants of 0x00..0 or 0xff..f.
+	errInvalidVote = errors.New("vote nonce not 0x00..0 or 0xff..f")
+
+	// errInvalidCheckpointVote is returned if a checkpoint/epoch transition block
+	// has a vote nonce set to non zeroes.
+	errInvalidCheckpointVote = errors.New("vote nonce in checkpoint block non-zero")
+
+	// errMissingVanity is returned if a block's extra-data section is shorter than
+	// 32 bytes, which is required to store the signer vanity.
+	errMissingVanity = errors.New("extra-data 32 byte vanity prefix missing")
+
+	// errMissingSignature is returned if a block's extra-data section doesn't seem
+	// to contain a 65 byte secp256k1 signature.
+	errMissingSignature = errors.New("extra-data 65 byte suffix signature missing")
+
+	// errExtraSigners is returned if non-checkpoint block contain signer data in
+	// their extra-data fields.
+	errExtraSigners = errors.New("non-checkpoint block contains extra signer list")
+
+	// drrInvalidCheckpointSigners is returned if a checkpoint block contains an
+	// invalid list of signers (i.e. non divisible by 20 bytes, or not the correct
+	// ones).
+	drrInvalidCheckpointSigners = errors.New("invalid signer list on checkpoint block")
+
+	// errInvalidMixDigest is returned if a block's mix digest is non zero.
+	errInvalidMixDigest = errors.New("non-zero mix digest")
+
+	// errInvalidUncleHash is returned if a block contains an non-empty uncle list.
+	errInvalidUncleHash = errors.New("non empty uncle hash")
+
+	// errInvalidDifficulty is returned if the difficulty of a block is not either
+	// of 1 or 2, or if the value does not match the turn of the signer.
+	errInvalidDifficulty = errors.New("invalid difficulty")
+
+	// ErrInvalidTimestamp is returned if the timestamp of a block is lower than
+	// the previous block's timestamp + the minimum block period.
+	ErrInvalidTimestamp = errors.New("invalid timestamp")
+
+	// errInvalidVotingChain is returned if an authorization list is attempted to
+	// be modified via out-of-range or non-contiguous headers.
+	errInvalidVotingChain = errors.New("invalid voting chain")
+
+	// errUnauthorized is returned if a header is signed by a non authorized entity.
+	errUnauthorized = errors.New("unauthorized")
+)
+
+// SignerFn is a signer callback function to request a hash to be signed by a
+// backing account.
+type SignerFn func(accounts.Account, []byte) ([]byte, error)
+
+// sigHash returns the hash which is used as input for the proof-of-authority
+// signing. It is the hash of the entire header apart from the 65 byte signature
+// contained at the end of the extra data.
+//
+// Note, the method requires the extra data to be at least 65 bytes, otherwise it
+// panics. This is done to avoid accidentally using both forms (signature present
+// or not), which could be abused to produce different hashes for the same header.
+func sigHash(header *types.Header) (hash common.Hash) {
+	hasher := sha3.NewKeccak256()
+
+	rlp.Encode(hasher, []interface{}{
+		header.ParentHash,
+		header.UncleHash,
+		header.Coinbase,
+		header.Root,
+		header.TxHash,
+		header.ReceiptHash,
+		header.Bloom,
+		header.Difficulty,
+		header.Number,
+		header.GasLimit,
+		header.GasUsed,
+		header.Time,
+		header.Extra[:len(header.Extra)-65], // Yes, this will panic if extra is too short
+		header.MixDigest,
+		header.Nonce,
+	})
+	hasher.Sum(hash[:0])
+	return hash
+}
+
+// ecrecover extracts the Ethereum account address from a signed header.
+func ecrecover(header *types.Header) (common.Address, error) {
+	// Retrieve the signature from the header extra-data
+	if len(header.Extra) < extraSeal {
+		return common.Address{}, errMissingSignature
+	}
+	signature := header.Extra[len(header.Extra)-extraSeal:]
+
+	// Recover the public key and the Ethereum address
+	pubkey, err := crypto.Ecrecover(sigHash(header).Bytes(), signature)
+	if err != nil {
+		return common.Address{}, err
+	}
+	var signer common.Address
+	copy(signer[:], crypto.Keccak256(pubkey[1:])[12:])
+
+	return signer, nil
+}
+
+// Clique is the proof-of-authority consensus engine proposed to support the
+// Ethereum testnet following the Ropsten attacks.
+type Clique struct {
+	config *params.CliqueConfig // Consensus engine configuration parameters
+	db     ethdb.Database       // Database to store and retrieve snapshot checkpoints
+
+	recents    *lru.ARCCache // Snapshots for recent block to speed up reorgs
+	signatures *lru.ARCCache // Signatures of recent blocks to speed up mining
+
+	proposals map[common.Address]bool // Current list of proposals we are pushing
+
+	signer common.Address // Ethereum address of the signing key
+	signFn SignerFn       // Signer function to authorize hashes with
+	lock   sync.RWMutex   // Protects the signer fields
+}
+
+// New creates a Clique proof-of-authority consensus engine with the initial
+// signers set to the ones provided by the user.
+func New(config *params.CliqueConfig, db ethdb.Database) *Clique {
+	// Set any missing consensus parameters to their defaults
+	conf := *config
+	if conf.Epoch == 0 {
+		conf.Epoch = epochLength
+	}
+	if conf.Period == 0 {
+		conf.Period = blockPeriod
+	}
+	// Allocate the snapshot caches and create the engine
+	recents, _ := lru.NewARC(inmemorySnapshots)
+	signatures, _ := lru.NewARC(inmemorySignatures)
+
+	return &Clique{
+		config:     &conf,
+		db:         db,
+		recents:    recents,
+		signatures: signatures,
+		proposals:  make(map[common.Address]bool),
+	}
+}
+
+// VerifyHeader checks whether a header conforms to the consensus rules.
+func (c *Clique) VerifyHeader(chain consensus.ChainReader, header *types.Header, seal bool) error {
+	return c.verifyHeader(chain, header, nil)
+}
+
+// VerifyHeaders is similar to VerifyHeader, but verifies a batch of headers. The
+// method returns a quit channel to abort the operations and a results channel to
+// retrieve the async verifications (the order is that of the input slice).
+func (c *Clique) VerifyHeaders(chain consensus.ChainReader, headers []*types.Header, seals []bool) (chan<- struct{}, <-chan error) {
+	abort := make(chan struct{})
+	results := make(chan error, len(headers))
+
+	go func() {
+		for i, header := range headers {
+			err := c.verifyHeader(chain, header, headers[:i])
+
+			select {
+			case <-abort:
+				return
+			case results <- err:
+			}
+		}
+	}()
+	return abort, results
+}
+
+// verifyHeader checks whether a header conforms to the consensus rules.The
+// caller may optionally pass in a batch of parents (ascending order) to avoid
+// looking those up from the database. This is useful for concurrently verifying
+// a batch of new headers.
+func (c *Clique) verifyHeader(chain consensus.ChainReader, header *types.Header, parents []*types.Header) error {
+	if header.Number == nil {
+		return errUnknownBlock
+	}
+	number := header.Number.Uint64()
+
+	// Don't waste time checking blocks from the future
+	if header.Time.Cmp(big.NewInt(time.Now().Unix())) > 0 {
+		return consensus.ErrFutureBlock
+	}
+	// Checkpoint blocks need to enforce zero beneficiary
+	checkpoint := (number % c.config.Epoch) == 0
+	if checkpoint && header.Coinbase != (common.Address{}) {
+		return errInvalidCheckpointBeneficiary
+	}
+	// Nonces must be 0x00..0 or 0xff..f, zeroes enforced on checkpoints
+	if !bytes.Equal(header.Nonce[:], nonceAuthVote) && !bytes.Equal(header.Nonce[:], nonceDropVote) {
+		return errInvalidVote
+	}
+	if checkpoint && !bytes.Equal(header.Nonce[:], nonceDropVote) {
+		return errInvalidCheckpointVote
+	}
+	// Check that the extra-data contains both the vanity and signature
+	if len(header.Extra) < extraVanity {
+		return errMissingVanity
+	}
+	if len(header.Extra) < extraVanity+extraSeal {
+		return errMissingSignature
+	}
+	// Ensure that the extra-data contains a signer list on checkpoint, but none otherwise
+	signersBytes := len(header.Extra) - extraVanity - extraSeal
+	if !checkpoint && signersBytes != 0 {
+		return errExtraSigners
+	}
+	if checkpoint && signersBytes%common.AddressLength != 0 {
+		return drrInvalidCheckpointSigners
+	}
+	// Ensure that the mix digest is zero as we don't have fork protection currently
+	if header.MixDigest != (common.Hash{}) {
+		return errInvalidMixDigest
+	}
+	// Ensure that the block doesn't contain any uncles which are meaningless in PoA
+	if header.UncleHash != uncleHash {
+		return errInvalidUncleHash
+	}
+	// Ensure that the block's difficulty is meaningful (may not be correct at this point)
+	if number > 0 {
+		if header.Difficulty == nil || (header.Difficulty.Cmp(diffInTurn) != 0 && header.Difficulty.Cmp(diffNoTurn) != 0) {
+			return errInvalidDifficulty
+		}
+	}
+	// All basic checks passed, verify cascading fields
+	return c.verifyCascadingFields(chain, header, parents)
+}
+
+// verifyCascadingFields verifies all the header fields that are not standalone,
+// rather depend on a batch of previous headers. The caller may optionally pass
+// in a batch of parents (ascending order) to avoid looking those up from the
+// database. This is useful for concurrently verifying a batch of new headers.
+func (c *Clique) verifyCascadingFields(chain consensus.ChainReader, header *types.Header, parents []*types.Header) error {
+	// The genesis block is the always valid dead-end
+	number := header.Number.Uint64()
+	if number == 0 {
+		return nil
+	}
+	// Ensure that the block's timestamp isn't too close to it's parent
+	var parent *types.Header
+	if len(parents) > 0 {
+		parent = parents[len(parents)-1]
+	} else {
+		parent = chain.GetHeader(header.ParentHash, number-1)
+	}
+	if parent == nil || parent.Number.Uint64() != number-1 || parent.Hash() != header.ParentHash {
+		return consensus.ErrUnknownAncestor
+	}
+	if parent.Time.Uint64()+c.config.Period > header.Time.Uint64() {
+		return ErrInvalidTimestamp
+	}
+	// Retrieve the snapshot needed to verify this header and cache it
+	snap, err := c.snapshot(chain, number-1, header.ParentHash, parents)
+	if err != nil {
+		return err
+	}
+	// If the block is a checkpoint block, verify the signer list
+	if number%c.config.Epoch == 0 {
+		signers := make([]byte, len(snap.Signers)*common.AddressLength)
+		for i, signer := range snap.signers() {
+			copy(signers[i*common.AddressLength:], signer[:])
+		}
+		extraSuffix := len(header.Extra) - extraSeal
+		if !bytes.Equal(header.Extra[extraVanity:extraSuffix], signers) {
+			return drrInvalidCheckpointSigners
+		}
+	}
+	// All basic checks passed, verify the seal and return
+	return c.verifySeal(chain, header, parents)
+}
+
+// snapshot retrieves the authorization snapshot at a given point in time.
+func (c *Clique) snapshot(chain consensus.ChainReader, number uint64, hash common.Hash, parents []*types.Header) (*snapshot, error) {
+	// Search for a snapshot in memory or on disk for checkpoints
+	var (
+		headers []*types.Header
+		snap    *snapshot
+	)
+	for snap == nil {
+		// If an in-memory snapshot was found, use that
+		if s, ok := c.recents.Get(hash); ok {
+			snap = s.(*snapshot)
+			break
+		}
+		// If an on-disk checkpoint snapshot can be found, use that
+		if number%checkpointInterval == 0 {
+			if s, err := loadSnapshot(c.config, c.db, hash); err == nil {
+				log.Trace("Loaded voting snapshot form disk", "number", number, "hash", hash)
+				snap = s
+				break
+			}
+		}
+		// If we're at block zero, make a snapshot
+		if number == 0 {
+			genesis := chain.GetHeaderByNumber(0)
+			if err := c.VerifyHeader(chain, genesis, false); err != nil {
+				return nil, err
+			}
+			signers := make([]common.Address, (len(genesis.Extra)-extraVanity-extraSeal)/common.AddressLength)
+			for i := 0; i < len(signers); i++ {
+				copy(signers[i][:], genesis.Extra[extraVanity+i*common.AddressLength:])
+			}
+			snap = newSnapshot(c.config, 0, genesis.Hash(), signers)
+			if err := snap.store(c.db); err != nil {
+				return nil, err
+			}
+			log.Trace("Stored genesis voting snapshot to disk")
+			break
+		}
+		// No snapshot for this header, gather the header and move backward
+		var header *types.Header
+		if len(parents) > 0 {
+			// If we have explicit parents, pick from there (enforced)
+			header = parents[len(parents)-1]
+			if header.Hash() != hash || header.Number.Uint64() != number {
+				return nil, consensus.ErrUnknownAncestor
+			}
+			parents = parents[:len(parents)-1]
+		} else {
+			// No explicit parents (or no more left), reach out to the database
+			header = chain.GetHeader(hash, number)
+			if header == nil {
+				return nil, consensus.ErrUnknownAncestor
+			}
+		}
+		headers = append(headers, header)
+		number, hash = number-1, header.ParentHash
+	}
+	// Previous snapshot found, apply any pending headers on top of it
+	for i := 0; i < len(headers)/2; i++ {
+		headers[i], headers[len(headers)-1-i] = headers[len(headers)-1-i], headers[i]
+	}
+	snap, err := snap.apply(headers)
+	if err != nil {
+		return nil, err
+	}
+	c.recents.Add(snap.Hash, snap)
+
+	// If we've generated a new checkpoint snapshot, save to disk
+	if snap.Number%checkpointInterval == 0 && len(headers) > 0 {
+		if err = snap.store(c.db); err != nil {
+			return nil, err
+		}
+		log.Trace("Stored voting snapshot to disk", "number", snap.Number, "hash", snap.Hash)
+	}
+	return snap, err
+}
+
+// VerifyUncles implements consensus.Engine, always returning an error for any
+// uncles as this consensus mechanism doesn't permit uncles.
+func (c *Clique) VerifyUncles(chain consensus.ChainReader, block *types.Block) error {
+	if len(block.Uncles()) > 0 {
+		return errors.New("uncles not allowed")
+	}
+	return nil
+}
+
+// VerifySeal implements consensus.Engine, checking whether the signature contained
+// in the header satisfies the consensus protocol requirements.
+func (c *Clique) VerifySeal(chain consensus.ChainReader, header *types.Header) error {
+	return c.verifySeal(chain, header, nil)
+}
+
+// verifySeal checks whether the signature contained in the header satisfies the
+// consensus protocol requirements. The method accepts an optional list of parent
+// headers that aren't yet part of the local blockchain to generate the snapshots
+// from.
+func (c *Clique) verifySeal(chain consensus.ChainReader, header *types.Header, parents []*types.Header) error {
+	// Verifying the genesis block is not supported
+	number := header.Number.Uint64()
+	if number == 0 {
+		return errUnknownBlock
+	}
+	// Retrieve the snapshot needed to verify this header and cache it
+	snap, err := c.snapshot(chain, number-1, header.ParentHash, parents)
+	if err != nil {
+		return err
+	}
+	c.recents.Add(snap.Hash, snap)
+
+	// Resolve the authorization key and check against signers
+	signer, err := ecrecover(header)
+	if err != nil {
+		return err
+	}
+	if _, ok := snap.Signers[signer]; !ok {
+		return errUnauthorized
+	}
+	for seen, recent := range snap.Recents {
+		if recent == signer {
+			// Signer is among recents, only fail if the current block doens't shift it out
+			if limit := uint64(len(snap.Signers)/2 + 1); seen > number-limit {
+				return errUnauthorized
+			}
+		}
+	}
+	// Ensure that the difficulty corresponts to the turn-ness of the signer
+	inturn := snap.inturn(header.Number.Uint64(), signer)
+	if inturn && header.Difficulty.Cmp(diffInTurn) != 0 {
+		return errInvalidDifficulty
+	}
+	if !inturn && header.Difficulty.Cmp(diffNoTurn) != 0 {
+		return errInvalidDifficulty
+	}
+	return nil
+}
+
+// Prepare implements consensus.Engine, preparing all the consensus fields of the
+// header for running the transactions on top.
+func (c *Clique) Prepare(chain consensus.ChainReader, header *types.Header) error {
+	// If the block isn't a checkpoint, cast a random vote (good enough fror now)
+	header.Coinbase = common.Address{}
+	header.Nonce = types.BlockNonce{}
+
+	number := header.Number.Uint64()
+	if number%c.config.Epoch != 0 {
+		c.lock.RLock()
+		if len(c.proposals) > 0 {
+			addresses := make([]common.Address, 0, len(c.proposals))
+			for address := range c.proposals {
+				addresses = append(addresses, address)
+			}
+			header.Coinbase = addresses[rand.Intn(len(addresses))]
+			if c.proposals[header.Coinbase] {
+				copy(header.Nonce[:], nonceAuthVote)
+			} else {
+				copy(header.Nonce[:], nonceDropVote)
+			}
+		}
+		c.lock.RUnlock()
+	}
+	// Assemble the voting snapshot and set the correct difficulty
+	snap, err := c.snapshot(chain, number-1, header.ParentHash, nil)
+	if err != nil {
+		return err
+	}
+	header.Difficulty = diffNoTurn
+	if snap.inturn(header.Number.Uint64(), c.signer) {
+		header.Difficulty = diffInTurn
+	}
+	// Ensure the extra data has all it's components
+	if len(header.Extra) < extraVanity {
+		header.Extra = append(header.Extra, bytes.Repeat([]byte{0x00}, extraVanity-len(header.Extra))...)
+	}
+	header.Extra = header.Extra[:extraVanity]
+
+	if number%c.config.Epoch == 0 {
+		for _, signer := range snap.signers() {
+			header.Extra = append(header.Extra, signer[:]...)
+		}
+	}
+	header.Extra = append(header.Extra, make([]byte, extraSeal)...)
+
+	// Mix digest is reserved for now, set to empty
+	header.MixDigest = common.Hash{}
+
+	// Ensure the timestamp has the correct delay
+	parent := chain.GetHeader(header.ParentHash, number-1)
+	if parent == nil {
+		return consensus.ErrUnknownAncestor
+	}
+	header.Time = new(big.Int).Add(parent.Time, new(big.Int).SetUint64(c.config.Period))
+	if header.Time.Int64() < time.Now().Unix() {
+		header.Time = big.NewInt(time.Now().Unix())
+	}
+	return nil
+}
+
+// Finalize implements consensus.Engine, ensuring no uncles are set, nor block
+// rewards given, and returns the final block.
+func (c *Clique) Finalize(chain consensus.ChainReader, header *types.Header, state *state.StateDB, txs []*types.Transaction, uncles []*types.Header, receipts []*types.Receipt) (*types.Block, error) {
+	// No block rewards in PoA, so the state remains as is and uncles are dropped
+	header.Root = state.IntermediateRoot(chain.Config().IsEIP158(header.Number))
+	header.UncleHash = types.CalcUncleHash(nil)
+
+	// Assemble and return the final block for sealing
+	return types.NewBlock(header, txs, nil, receipts), nil
+}
+
+// Authorize injects a private key into the consensus engine to mint new blocks
+// with.
+func (c *Clique) Authorize(signer common.Address, signFn SignerFn) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	c.signer = signer
+	c.signFn = signFn
+}
+
+// Seal implements consensus.Engine, attempting to create a sealed block using
+// the local signing credentials.
+func (c *Clique) Seal(chain consensus.ChainReader, block *types.Block, stop <-chan struct{}) (*types.Block, error) {
+	header := block.Header()
+
+	// Sealing the genesis block is not supported
+	number := header.Number.Uint64()
+	if number == 0 {
+		return nil, errUnknownBlock
+	}
+	// Don't hold the signer fields for the entire sealing procedure
+	c.lock.RLock()
+	signer, signFn := c.signer, c.signFn
+	c.lock.RUnlock()
+
+	// Bail out if we're unauthorized to sign a block
+	snap, err := c.snapshot(chain, number-1, header.ParentHash, nil)
+	if err != nil {
+		return nil, err
+	}
+	if _, authorized := snap.Signers[signer]; !authorized {
+		return nil, errUnauthorized
+	}
+	// If we're amongs the recent signers, wait for the next block
+	for seen, recent := range snap.Recents {
+		if recent == signer {
+			// Signer is among recents, only wait if the current block doens't shift it out
+			if limit := uint64(len(snap.Signers)/2 + 1); seen > number-limit {
+				log.Info("Signed recently, must wait for others")
+				<-stop
+				return nil, nil
+			}
+		}
+	}
+	// Sweet, the protocol permits us to sign the block, wait for our time
+	delay := time.Unix(header.Time.Int64(), 0).Sub(time.Now())
+	if header.Difficulty.Cmp(diffNoTurn) == 0 {
+		// It's not our turn explicitly to sign, delay it a bit
+		wiggle := time.Duration(len(snap.Signers)/2+1) * wiggleTime
+		delay += time.Duration(rand.Int63n(int64(wiggle)))
+
+		log.Trace("Out-of-turn signing requested", "wiggle", common.PrettyDuration(wiggle))
+	}
+	log.Trace("Waiting for slot to sign and propagate", "delay", common.PrettyDuration(delay))
+
+	select {
+	case <-stop:
+		return nil, nil
+	case <-time.After(delay):
+	}
+	// Sign all the things!
+	sighash, err := signFn(accounts.Account{Address: signer}, sigHash(header).Bytes())
+	if err != nil {
+		return nil, err
+	}
+	copy(header.Extra[len(header.Extra)-extraSeal:], sighash)
+
+	return block.WithSeal(header), nil
+}
+
+// APIs implements consensus.Engine, returning the user facing RPC API to allow
+// controlling the signer voting.
+func (c *Clique) APIs(chain consensus.ChainReader) []rpc.API {
+	return []rpc.API{{
+		Namespace: "clique",
+		Version:   "1.0",
+		Service:   &API{chain: chain, clique: c},
+		Public:    false,
+	}}
+}
diff --git a/consensus/clique/snapshot.go b/consensus/clique/snapshot.go
new file mode 100644
index 000000000..f95e1e920
--- /dev/null
+++ b/consensus/clique/snapshot.go
@@ -0,0 +1,299 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package clique
+
+import (
+	"bytes"
+	"encoding/json"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/params"
+)
+
+// vote represents a single vote that an authorized signer made to modify the
+// list of authorizations.
+type vote struct {
+	Signer    common.Address `json:"signer"`    // Authorized signer that cast this vote
+	Block     uint64         `json:"block"`     // Block number the vote was cast in (expire old votes)
+	Address   common.Address `json:"address"`   // Account being voted on to change its authorization
+	Authorize bool           `json:"authorize"` // Whether to authorize or deauthorize the voted account
+}
+
+// tally is a simple vote tally to keep the current score of votes. Votes that
+// go against the proposal aren't counted since it's equivalent to not voting.
+type tally struct {
+	Authorize bool `json:"authorize"` // Whether the vote it about authorizing or kicking someone
+	Votes     int  `json:"votes"`     // Number of votes until now wanting to pass the proposal
+}
+
+// snapshot is the state of the authorization voting at a given point in time.
+type snapshot struct {
+	config *params.CliqueConfig // Consensus engine parameters to fine tune behavior
+
+	Number  uint64                      `json:"number"`  // Block number where the snapshot was created
+	Hash    common.Hash                 `json:"hash"`    // Block hash where the snapshot was created
+	Signers map[common.Address]struct{} `json:"signers"` // Set of authorized signers at this moment
+	Recents map[uint64]common.Address   `json:"recents"` // Set of recent signers for spam protections
+	Votes   []*vote                     `json:"votes"`   // List of votes cast in chronological order
+	Tally   map[common.Address]tally    `json:"tally"`   // Current vote tally to avoid recalculating
+}
+
+// newSnapshot create a new snapshot with the specified startup parameters. This
+// method does not initialize the set of recent signers, so only ever use if for
+// the genesis block.
+func newSnapshot(config *params.CliqueConfig, number uint64, hash common.Hash, signers []common.Address) *snapshot {
+	snap := &snapshot{
+		config:  config,
+		Number:  number,
+		Hash:    hash,
+		Signers: make(map[common.Address]struct{}),
+		Recents: make(map[uint64]common.Address),
+		Tally:   make(map[common.Address]tally),
+	}
+	for _, signer := range signers {
+		snap.Signers[signer] = struct{}{}
+	}
+	return snap
+}
+
+// loadSnapshot loads an existing snapshot from the database.
+func loadSnapshot(config *params.CliqueConfig, db ethdb.Database, hash common.Hash) (*snapshot, error) {
+	blob, err := db.Get(append([]byte("clique-"), hash[:]...))
+	if err != nil {
+		return nil, err
+	}
+	snap := new(snapshot)
+	if err := json.Unmarshal(blob, snap); err != nil {
+		return nil, err
+	}
+	snap.config = config
+
+	return snap, nil
+}
+
+// store inserts the snapshot into the database.
+func (s *snapshot) store(db ethdb.Database) error {
+	blob, err := json.Marshal(s)
+	if err != nil {
+		return err
+	}
+	return db.Put(append([]byte("clique-"), s.Hash[:]...), blob)
+}
+
+// copy creates a deep copy of the snapshot, though not the individual votes.
+func (s *snapshot) copy() *snapshot {
+	cpy := &snapshot{
+		config:  s.config,
+		Number:  s.Number,
+		Hash:    s.Hash,
+		Signers: make(map[common.Address]struct{}),
+		Recents: make(map[uint64]common.Address),
+		Votes:   make([]*vote, len(s.Votes)),
+		Tally:   make(map[common.Address]tally),
+	}
+	for signer := range s.Signers {
+		cpy.Signers[signer] = struct{}{}
+	}
+	for block, signer := range s.Recents {
+		cpy.Recents[block] = signer
+	}
+	for address, tally := range s.Tally {
+		cpy.Tally[address] = tally
+	}
+	copy(cpy.Votes, s.Votes)
+
+	return cpy
+}
+
+// cast adds a new vote into the tally.
+func (s *snapshot) cast(address common.Address, authorize bool) bool {
+	// Ensure the vote is meaningful
+	_, signer := s.Signers[address]
+	if (signer && authorize) || (!signer && !authorize) {
+		return false
+	}
+	// Cast the vote into an existing or new tally
+	if old, ok := s.Tally[address]; ok {
+		old.Votes++
+		s.Tally[address] = old
+	} else {
+		s.Tally[address] = tally{Authorize: authorize, Votes: 1}
+	}
+	return true
+}
+
+// uncast removes a previously cast vote from the tally.
+func (s *snapshot) uncast(address common.Address, authorize bool) bool {
+	// If there's no tally, it's a dangling vote, just drop
+	tally, ok := s.Tally[address]
+	if !ok {
+		return false
+	}
+	// Ensure we only revert counted votes
+	if tally.Authorize != authorize {
+		return false
+	}
+	// Otherwise revert the vote
+	if tally.Votes > 1 {
+		tally.Votes--
+		s.Tally[address] = tally
+	} else {
+		delete(s.Tally, address)
+	}
+	return true
+}
+
+// apply creates a new authorization snapshot by applying the given headers to
+// the original one.
+func (s *snapshot) apply(headers []*types.Header) (*snapshot, error) {
+	// Allow passing in no headers for cleaner code
+	if len(headers) == 0 {
+		return s, nil
+	}
+	// Sanity check that the headers can be applied
+	for i := 0; i < len(headers)-1; i++ {
+		if headers[i+1].Number.Uint64() != headers[i].Number.Uint64()+1 {
+			return nil, errInvalidVotingChain
+		}
+	}
+	if headers[0].Number.Uint64() != s.Number+1 {
+		return nil, errInvalidVotingChain
+	}
+	// Iterate through the headers and create a new snapshot
+	snap := s.copy()
+
+	for _, header := range headers {
+		// Remove any votes on checkpoint blocks
+		number := header.Number.Uint64()
+		if number%s.config.Epoch == 0 {
+			snap.Votes = nil
+			snap.Tally = make(map[common.Address]tally)
+		}
+		// Delete the oldest signer from the recent list to allow it signing again
+		if limit := uint64(len(snap.Signers)/2 + 1); number >= limit {
+			delete(snap.Recents, number-limit)
+		}
+		// Resolve the authorization key and check against signers
+		signer, err := ecrecover(header)
+		if err != nil {
+			return nil, err
+		}
+		if _, ok := snap.Signers[signer]; !ok {
+			return nil, errUnauthorized
+		}
+		for _, recent := range snap.Recents {
+			if recent == signer {
+				return nil, errUnauthorized
+			}
+		}
+		snap.Recents[number] = signer
+
+		// Header authorized, discard any previous votes from the signer
+		for i, vote := range snap.Votes {
+			if vote.Signer == signer && vote.Address == header.Coinbase {
+				// Uncast the vote from the cached tally
+				snap.uncast(vote.Address, vote.Authorize)
+
+				// Uncast the vote from the chronological list
+				snap.Votes = append(snap.Votes[:i], snap.Votes[i+1:]...)
+				break // only one vote allowed
+			}
+		}
+		// Tally up the new vote from the signer
+		var authorize bool
+		switch {
+		case bytes.Compare(header.Nonce[:], nonceAuthVote) == 0:
+			authorize = true
+		case bytes.Compare(header.Nonce[:], nonceDropVote) == 0:
+			authorize = false
+		default:
+			return nil, errInvalidVote
+		}
+		if snap.cast(header.Coinbase, authorize) {
+			snap.Votes = append(snap.Votes, &vote{
+				Signer:    signer,
+				Block:     number,
+				Address:   header.Coinbase,
+				Authorize: authorize,
+			})
+		}
+		// If the vote passed, update the list of signers
+		if tally := snap.Tally[header.Coinbase]; tally.Votes > len(snap.Signers)/2 {
+			if tally.Authorize {
+				snap.Signers[header.Coinbase] = struct{}{}
+			} else {
+				delete(snap.Signers, header.Coinbase)
+
+				// Signer list shrunk, delete any leftover recent caches
+				if limit := uint64(len(snap.Signers)/2 + 1); number >= limit {
+					delete(snap.Recents, number-limit)
+				}
+				// Discard any previous votes the deauthorized signer cast
+				for i := 0; i < len(snap.Votes); i++ {
+					if snap.Votes[i].Signer == header.Coinbase {
+						// Uncast the vote from the cached tally
+						snap.uncast(snap.Votes[i].Address, snap.Votes[i].Authorize)
+
+						// Uncast the vote from the chronological list
+						snap.Votes = append(snap.Votes[:i], snap.Votes[i+1:]...)
+
+						i--
+					}
+				}
+			}
+			// Discard any previous votes around the just changed account
+			for i := 0; i < len(snap.Votes); i++ {
+				if snap.Votes[i].Address == header.Coinbase {
+					snap.Votes = append(snap.Votes[:i], snap.Votes[i+1:]...)
+					i--
+				}
+			}
+			delete(snap.Tally, header.Coinbase)
+		}
+	}
+	snap.Number += uint64(len(headers))
+	snap.Hash = headers[len(headers)-1].Hash()
+
+	return snap, nil
+}
+
+// signers retrieves the list of authorized signers in ascending order.
+func (s *snapshot) signers() []common.Address {
+	signers := make([]common.Address, 0, len(s.Signers))
+	for signer := range s.Signers {
+		signers = append(signers, signer)
+	}
+	for i := 0; i < len(signers); i++ {
+		for j := i + 1; j < len(signers); j++ {
+			if bytes.Compare(signers[i][:], signers[j][:]) > 0 {
+				signers[i], signers[j] = signers[j], signers[i]
+			}
+		}
+	}
+	return signers
+}
+
+// inturn returns if a signer at a given block height is in-turn or not.
+func (s *snapshot) inturn(number uint64, signer common.Address) bool {
+	signers, offset := s.signers(), 0
+	for offset < len(signers) && signers[offset] != signer {
+		offset++
+	}
+	return (number % uint64(len(signers))) == uint64(offset)
+}
diff --git a/consensus/clique/snapshot_test.go b/consensus/clique/snapshot_test.go
new file mode 100644
index 000000000..3ebe33bc5
--- /dev/null
+++ b/consensus/clique/snapshot_test.go
@@ -0,0 +1,404 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package clique
+
+import (
+	"bytes"
+	"crypto/ecdsa"
+	"math/big"
+	"testing"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/core"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/ethereum/go-ethereum/ethdb"
+	"github.com/ethereum/go-ethereum/params"
+)
+
+type testerVote struct {
+	signer string
+	voted  string
+	auth   bool
+}
+
+// testerAccountPool is a pool to maintain currently active tester accounts,
+// mapped from textual names used in the tests below to actual Ethereum private
+// keys capable of signing transactions.
+type testerAccountPool struct {
+	accounts map[string]*ecdsa.PrivateKey
+}
+
+func newTesterAccountPool() *testerAccountPool {
+	return &testerAccountPool{
+		accounts: make(map[string]*ecdsa.PrivateKey),
+	}
+}
+
+func (ap *testerAccountPool) sign(header *types.Header, signer string) {
+	// Ensure we have a persistent key for the signer
+	if ap.accounts[signer] == nil {
+		ap.accounts[signer], _ = crypto.GenerateKey()
+	}
+	// Sign the header and embed the signature in extra data
+	sig, _ := crypto.Sign(sigHash(header).Bytes(), ap.accounts[signer])
+	copy(header.Extra[len(header.Extra)-65:], sig)
+}
+
+func (ap *testerAccountPool) address(account string) common.Address {
+	// Ensure we have a persistent key for the account
+	if ap.accounts[account] == nil {
+		ap.accounts[account], _ = crypto.GenerateKey()
+	}
+	// Resolve and return the Ethereum address
+	return crypto.PubkeyToAddress(ap.accounts[account].PublicKey)
+}
+
+// testerChainReader implements consensus.ChainReader to access the genesis
+// block. All other methods and requests will panic.
+type testerChainReader struct {
+	db ethdb.Database
+}
+
+func (r *testerChainReader) Config() *params.ChainConfig                 { panic("not supported") }
+func (r *testerChainReader) CurrentHeader() *types.Header                { panic("not supported") }
+func (r *testerChainReader) GetHeader(common.Hash, uint64) *types.Header { panic("not supported") }
+func (r *testerChainReader) GetBlock(common.Hash, uint64) *types.Block   { panic("not supported") }
+func (r *testerChainReader) GetHeaderByNumber(number uint64) *types.Header {
+	if number == 0 {
+		return core.GetHeader(r.db, core.GetCanonicalHash(r.db, 0), 0)
+	}
+	panic("not supported")
+}
+
+// Tests that voting is evaluated correctly for various simple and complex scenarios.
+func TestVoting(t *testing.T) {
+	// Define the various voting scenarios to test
+	tests := []struct {
+		epoch   uint64
+		signers []string
+		votes   []testerVote
+		results []string
+	}{
+		{
+			// Single signer, no votes cast
+			signers: []string{"A"},
+			votes:   []testerVote{{signer: "A"}},
+			results: []string{"A"},
+		}, {
+			// Single signer, voting to add two others (only accept first, second needs 2 votes)
+			signers: []string{"A"},
+			votes: []testerVote{
+				{signer: "A", voted: "B", auth: true},
+				{signer: "B"},
+				{signer: "A", voted: "C", auth: true},
+			},
+			results: []string{"A", "B"},
+		}, {
+			// Two signers, voting to add three others (only accept first two, third needs 3 votes already)
+			signers: []string{"A", "B"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: true},
+				{signer: "B", voted: "C", auth: true},
+				{signer: "A", voted: "D", auth: true},
+				{signer: "B", voted: "D", auth: true},
+				{signer: "C"},
+				{signer: "A", voted: "E", auth: true},
+				{signer: "B", voted: "E", auth: true},
+			},
+			results: []string{"A", "B", "C", "D"},
+		}, {
+			// Single signer, dropping itself (weird, but one less cornercase by explicitly allowing this)
+			signers: []string{"A"},
+			votes: []testerVote{
+				{signer: "A", voted: "A", auth: false},
+			},
+			results: []string{},
+		}, {
+			// Two signers, actually needing mutual consent to drop either of them (not fulfilled)
+			signers: []string{"A", "B"},
+			votes: []testerVote{
+				{signer: "A", voted: "B", auth: false},
+			},
+			results: []string{"A", "B"},
+		}, {
+			// Two signers, actually needing mutual consent to drop either of them (fulfilled)
+			signers: []string{"A", "B"},
+			votes: []testerVote{
+				{signer: "A", voted: "B", auth: false},
+				{signer: "B", voted: "B", auth: false},
+			},
+			results: []string{"A"},
+		}, {
+			// Three signers, two of them deciding to drop the third
+			signers: []string{"A", "B", "C"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: false},
+				{signer: "B", voted: "C", auth: false},
+			},
+			results: []string{"A", "B"},
+		}, {
+			// Four signers, consensus of two not being enough to drop anyone
+			signers: []string{"A", "B", "C", "D"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: false},
+				{signer: "B", voted: "C", auth: false},
+			},
+			results: []string{"A", "B", "C", "D"},
+		}, {
+			// Four signers, consensus of three already being enough to drop someone
+			signers: []string{"A", "B", "C", "D"},
+			votes: []testerVote{
+				{signer: "A", voted: "D", auth: false},
+				{signer: "B", voted: "D", auth: false},
+				{signer: "C", voted: "D", auth: false},
+			},
+			results: []string{"A", "B", "C"},
+		}, {
+			// Authorizations are counted once per signer per target
+			signers: []string{"A", "B"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: true},
+				{signer: "B"},
+				{signer: "A", voted: "C", auth: true},
+				{signer: "B"},
+				{signer: "A", voted: "C", auth: true},
+			},
+			results: []string{"A", "B"},
+		}, {
+			// Authorizing multiple accounts concurrently is permitted
+			signers: []string{"A", "B"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: true},
+				{signer: "B"},
+				{signer: "A", voted: "D", auth: true},
+				{signer: "B"},
+				{signer: "A"},
+				{signer: "B", voted: "D", auth: true},
+				{signer: "A"},
+				{signer: "B", voted: "C", auth: true},
+			},
+			results: []string{"A", "B", "C", "D"},
+		}, {
+			// Deauthorizations are counted once per signer per target
+			signers: []string{"A", "B"},
+			votes: []testerVote{
+				{signer: "A", voted: "B", auth: false},
+				{signer: "B"},
+				{signer: "A", voted: "B", auth: false},
+				{signer: "B"},
+				{signer: "A", voted: "B", auth: false},
+			},
+			results: []string{"A", "B"},
+		}, {
+			// Deauthorizing multiple accounts concurrently is permitted
+			signers: []string{"A", "B", "C", "D"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: false},
+				{signer: "B"},
+				{signer: "C"},
+				{signer: "A", voted: "D", auth: false},
+				{signer: "B"},
+				{signer: "C"},
+				{signer: "A"},
+				{signer: "B", voted: "D", auth: false},
+				{signer: "C", voted: "D", auth: false},
+				{signer: "A"},
+				{signer: "B", voted: "C", auth: false},
+			},
+			results: []string{"A", "B"},
+		}, {
+			// Votes from deauthorized signers are discarded immediately (deauth votes)
+			signers: []string{"A", "B", "C"},
+			votes: []testerVote{
+				{signer: "C", voted: "B", auth: false},
+				{signer: "A", voted: "C", auth: false},
+				{signer: "B", voted: "C", auth: false},
+				{signer: "A", voted: "B", auth: false},
+			},
+			results: []string{"A", "B"},
+		}, {
+			// Votes from deauthorized signers are discarded immediately (auth votes)
+			signers: []string{"A", "B", "C"},
+			votes: []testerVote{
+				{signer: "C", voted: "B", auth: false},
+				{signer: "A", voted: "C", auth: false},
+				{signer: "B", voted: "C", auth: false},
+				{signer: "A", voted: "B", auth: false},
+			},
+			results: []string{"A", "B"},
+		}, {
+			// Cascading changes are not allowed, only the the account being voted on may change
+			signers: []string{"A", "B", "C", "D"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: false},
+				{signer: "B"},
+				{signer: "C"},
+				{signer: "A", voted: "D", auth: false},
+				{signer: "B", voted: "C", auth: false},
+				{signer: "C"},
+				{signer: "A"},
+				{signer: "B", voted: "D", auth: false},
+				{signer: "C", voted: "D", auth: false},
+			},
+			results: []string{"A", "B", "C"},
+		}, {
+			// Changes reaching consensus out of bounds (via a deauth) execute on touch
+			signers: []string{"A", "B", "C", "D"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: false},
+				{signer: "B"},
+				{signer: "C"},
+				{signer: "A", voted: "D", auth: false},
+				{signer: "B", voted: "C", auth: false},
+				{signer: "C"},
+				{signer: "A"},
+				{signer: "B", voted: "D", auth: false},
+				{signer: "C", voted: "D", auth: false},
+				{signer: "A"},
+				{signer: "C", voted: "C", auth: true},
+			},
+			results: []string{"A", "B"},
+		}, {
+			// Changes reaching consensus out of bounds (via a deauth) may go out of consensus on first touch
+			signers: []string{"A", "B", "C", "D"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: false},
+				{signer: "B"},
+				{signer: "C"},
+				{signer: "A", voted: "D", auth: false},
+				{signer: "B", voted: "C", auth: false},
+				{signer: "C"},
+				{signer: "A"},
+				{signer: "B", voted: "D", auth: false},
+				{signer: "C", voted: "D", auth: false},
+				{signer: "A"},
+				{signer: "B", voted: "C", auth: true},
+			},
+			results: []string{"A", "B", "C"},
+		}, {
+			// Ensure that pending votes don't survive authorization status changes. This
+			// corner case can only appear if a signer is quickly added, remove and then
+			// readded (or the inverse), while one of the original voters dropped. If a
+			// past vote is left cached in the system somewhere, this will interfere with
+			// the final signer outcome.
+			signers: []string{"A", "B", "C", "D", "E"},
+			votes: []testerVote{
+				{signer: "A", voted: "F", auth: true}, // Authorize F, 3 votes needed
+				{signer: "B", voted: "F", auth: true},
+				{signer: "C", voted: "F", auth: true},
+				{signer: "D", voted: "F", auth: false}, // Deauthorize F, 4 votes needed (leave A's previous vote "unchanged")
+				{signer: "E", voted: "F", auth: false},
+				{signer: "B", voted: "F", auth: false},
+				{signer: "C", voted: "F", auth: false},
+				{signer: "D", voted: "F", auth: true}, // Almost authorize F, 2/3 votes needed
+				{signer: "E", voted: "F", auth: true},
+				{signer: "B", voted: "A", auth: false}, // Deauthorize A, 3 votes needed
+				{signer: "C", voted: "A", auth: false},
+				{signer: "D", voted: "A", auth: false},
+				{signer: "B", voted: "F", auth: true}, // Finish authorizing F, 3/3 votes needed
+			},
+			results: []string{"B", "C", "D", "E", "F"},
+		}, {
+			// Epoch transitions reset all votes to allow chain checkpointing
+			epoch:   3,
+			signers: []string{"A", "B"},
+			votes: []testerVote{
+				{signer: "A", voted: "C", auth: true},
+				{signer: "B"},
+				{signer: "A"}, // Checkpoint block, (don't vote here, it's validated outside of snapshots)
+				{signer: "B", voted: "C", auth: true},
+			},
+			results: []string{"A", "B"},
+		},
+	}
+	// Run through the scenarios and test them
+	for i, tt := range tests {
+		// Create the account pool and generate the initial set of signers
+		accounts := newTesterAccountPool()
+
+		signers := make([]common.Address, len(tt.signers))
+		for j, signer := range tt.signers {
+			signers[j] = accounts.address(signer)
+		}
+		for j := 0; j < len(signers); j++ {
+			for k := j + 1; k < len(signers); k++ {
+				if bytes.Compare(signers[j][:], signers[k][:]) > 0 {
+					signers[j], signers[k] = signers[k], signers[j]
+				}
+			}
+		}
+		// Create the genesis block with the initial set of signers
+		genesis := &core.Genesis{
+			ExtraData: make([]byte, extraVanity+common.AddressLength*len(signers)+extraSeal),
+		}
+		for j, signer := range signers {
+			copy(genesis.ExtraData[extraVanity+j*common.AddressLength:], signer[:])
+		}
+		// Create a pristine blockchain with the genesis injected
+		db, _ := ethdb.NewMemDatabase()
+		genesis.Commit(db)
+
+		// Assemble a chain of headers from the cast votes
+		headers := make([]*types.Header, len(tt.votes))
+		for j, vote := range tt.votes {
+			headers[j] = &types.Header{
+				Number:   big.NewInt(int64(j) + 1),
+				Time:     big.NewInt(int64(j) * int64(blockPeriod)),
+				Coinbase: accounts.address(vote.voted),
+				Extra:    make([]byte, extraVanity+extraSeal),
+			}
+			if j > 0 {
+				headers[j].ParentHash = headers[j-1].Hash()
+			}
+			if vote.auth {
+				copy(headers[j].Nonce[:], nonceAuthVote)
+			}
+			accounts.sign(headers[j], vote.signer)
+		}
+		// Pass all the headers through clique and ensure tallying succeeds
+		head := headers[len(headers)-1]
+
+		snap, err := New(&params.CliqueConfig{Epoch: tt.epoch}, db).snapshot(&testerChainReader{db: db}, head.Number.Uint64(), head.Hash(), headers)
+		if err != nil {
+			t.Errorf("test %d: failed to create voting snapshot: %v", i, err)
+			continue
+		}
+		// Verify the final list of signers against the expected ones
+		signers = make([]common.Address, len(tt.results))
+		for j, signer := range tt.results {
+			signers[j] = accounts.address(signer)
+		}
+		for j := 0; j < len(signers); j++ {
+			for k := j + 1; k < len(signers); k++ {
+				if bytes.Compare(signers[j][:], signers[k][:]) > 0 {
+					signers[j], signers[k] = signers[k], signers[j]
+				}
+			}
+		}
+		result := snap.signers()
+		if len(result) != len(signers) {
+			t.Errorf("test %d: signers mismatch: have %x, want %x", i, result, signers)
+			continue
+		}
+		for j := 0; j < len(result); j++ {
+			if !bytes.Equal(result[j][:], signers[j][:]) {
+				t.Errorf("test %d, signer %d: signer mismatch: have %x, want %x", i, j, result[j], signers[j])
+			}
+		}
+	}
+}
diff --git a/consensus/errors.go b/consensus/errors.go
index f94bcb329..3b136dbdd 100644
--- a/consensus/errors.go
+++ b/consensus/errors.go
@@ -23,14 +23,6 @@ var (
 	// that is unknown.
 	ErrUnknownAncestor = errors.New("unknown ancestor")
 
-	// ErrLargeBlockTime is returned if the value of the timestamp is beyond
-	// any reasonable value.
-	ErrLargeBlockTime = errors.New("timestamp too big")
-
-	// ErrZeroBlockTime is returned if the block's timestamp is the same as the one
-	// its parent has.
-	ErrZeroBlockTime = errors.New("timestamp equals parent's")
-
 	// ErrFutureBlock is returned when a block's timestamp is in the future according
 	// to the current node.
 	ErrFutureBlock = errors.New("block in the future")
diff --git a/consensus/ethash/consensus.go b/consensus/ethash/consensus.go
index 4a3a74a93..f4f8f4b17 100644
--- a/consensus/ethash/consensus.go
+++ b/consensus/ethash/consensus.go
@@ -41,16 +41,22 @@ var (
 	maxUncles            = 2                 // Maximum number of uncles allowed in a single block
 )
 
+// Various error messages to mark blocks invalid. These should be private to
+// prevent engine specific errors from being referenced in the remainder of the
+// codebase, inherently breaking if the engine is swapped out. Please put common
+// error types into the consensus package.
 var (
-	ErrInvalidChain      = errors.New("invalid header chain")
-	ErrTooManyUncles     = errors.New("too many uncles")
-	ErrDuplicateUncle    = errors.New("duplicate uncle")
-	ErrUncleIsAncestor   = errors.New("uncle is ancestor")
-	ErrDanglingUncle     = errors.New("uncle's parent is not ancestor")
-	ErrNonceOutOfRange   = errors.New("nonce out of range")
-	ErrInvalidDifficulty = errors.New("non-positive difficulty")
-	ErrInvalidMixDigest  = errors.New("invalid mix digest")
-	ErrInvalidPoW        = errors.New("invalid proof-of-work")
+	errInvalidChain      = errors.New("invalid header chain")
+	errLargeBlockTime    = errors.New("timestamp too big")
+	errZeroBlockTime     = errors.New("timestamp equals parent's")
+	errTooManyUncles     = errors.New("too many uncles")
+	errDuplicateUncle    = errors.New("duplicate uncle")
+	errUncleIsAncestor   = errors.New("uncle is ancestor")
+	errDanglingUncle     = errors.New("uncle's parent is not ancestor")
+	errNonceOutOfRange   = errors.New("nonce out of range")
+	errInvalidDifficulty = errors.New("non-positive difficulty")
+	errInvalidMixDigest  = errors.New("invalid mix digest")
+	errInvalidPoW        = errors.New("invalid proof-of-work")
 )
 
 // VerifyHeader checks whether a header conforms to the consensus rules of the
@@ -104,7 +110,7 @@ func (ethash *Ethash) VerifyHeaders(chain consensus.ChainReader, headers []*type
 			for index := range inputs {
 				// If we've found a bad block already before this, stop validating
 				if bad := atomic.LoadUint64(&badblock); bad != 0 && bad <= headers[index].Number.Uint64() {
-					outputs <- result{index: index, err: ErrInvalidChain}
+					outputs <- result{index: index, err: errInvalidChain}
 					continue
 				}
 				// We need to look up the first parent
@@ -194,7 +200,7 @@ func (ethash *Ethash) VerifyUncles(chain consensus.ChainReader, block *types.Blo
 	}
 	// Verify that there are at most 2 uncles included in this block
 	if len(block.Uncles()) > maxUncles {
-		return ErrTooManyUncles
+		return errTooManyUncles
 	}
 	// Gather the set of past uncles and ancestors
 	uncles, ancestors := set.New(), make(map[common.Hash]*types.Header)
@@ -219,16 +225,16 @@ func (ethash *Ethash) VerifyUncles(chain consensus.ChainReader, block *types.Blo
 		// Make sure every uncle is rewarded only once
 		hash := uncle.Hash()
 		if uncles.Has(hash) {
-			return ErrDuplicateUncle
+			return errDuplicateUncle
 		}
 		uncles.Add(hash)
 
 		// Make sure the uncle has a valid ancestry
 		if ancestors[hash] != nil {
-			return ErrUncleIsAncestor
+			return errUncleIsAncestor
 		}
 		if ancestors[uncle.ParentHash] == nil || uncle.ParentHash == block.ParentHash() {
-			return ErrDanglingUncle
+			return errDanglingUncle
 		}
 		if err := ethash.verifyHeader(chain, uncle, ancestors[uncle.ParentHash], true, true); err != nil {
 			return err
@@ -249,7 +255,7 @@ func (ethash *Ethash) verifyHeader(chain consensus.ChainReader, header, parent *
 	// Verify the header's timestamp
 	if uncle {
 		if header.Time.Cmp(math.MaxBig256) > 0 {
-			return consensus.ErrLargeBlockTime
+			return errLargeBlockTime
 		}
 	} else {
 		if header.Time.Cmp(big.NewInt(time.Now().Unix())) > 0 {
@@ -257,7 +263,7 @@ func (ethash *Ethash) verifyHeader(chain consensus.ChainReader, header, parent *
 		}
 	}
 	if header.Time.Cmp(parent.Time) <= 0 {
-		return consensus.ErrZeroBlockTime
+		return errZeroBlockTime
 	}
 	// Verify the block's difficulty based in it's timestamp and parent's difficulty
 	expected := CalcDifficulty(chain.Config(), header.Time.Uint64(), parent.Time.Uint64(), parent.Number, parent.Difficulty)
@@ -403,7 +409,7 @@ func (ethash *Ethash) VerifySeal(chain consensus.ChainReader, header *types.Head
 	if ethash.fakeMode {
 		time.Sleep(ethash.fakeDelay)
 		if ethash.fakeFail == header.Number.Uint64() {
-			return ErrInvalidPoW
+			return errInvalidPoW
 		}
 		return nil
 	}
@@ -415,11 +421,11 @@ func (ethash *Ethash) VerifySeal(chain consensus.ChainReader, header *types.Head
 	number := header.Number.Uint64()
 	if number/epochLength >= uint64(len(cacheSizes)) {
 		// Go < 1.7 cannot calculate new cache/dataset sizes (no fast prime check)
-		return ErrNonceOutOfRange
+		return errNonceOutOfRange
 	}
 	// Ensure that we have a valid difficulty for the block
 	if header.Difficulty.Sign() <= 0 {
-		return ErrInvalidDifficulty
+		return errInvalidDifficulty
 	}
 	// Recompute the digest and PoW value and verify against the header
 	cache := ethash.cache(number)
@@ -430,11 +436,11 @@ func (ethash *Ethash) VerifySeal(chain consensus.ChainReader, header *types.Head
 	}
 	digest, result := hashimotoLight(size, cache, header.HashNoNonce().Bytes(), header.Nonce.Uint64())
 	if !bytes.Equal(header.MixDigest[:], digest) {
-		return ErrInvalidMixDigest
+		return errInvalidMixDigest
 	}
 	target := new(big.Int).Div(maxUint256, header.Difficulty)
 	if new(big.Int).SetBytes(result).Cmp(target) > 0 {
-		return ErrInvalidPoW
+		return errInvalidPoW
 	}
 	return nil
 }