accounts, eth, clique, signer: support for external signer API (#18079)

* accounts, eth, clique: implement external backend + move sighash calc to backend

* signer: implement account_Version on external API

* accounts/external: enable ipc, add copyright

* accounts, internal, signer: formatting

* node: go fmt

* flags: disallow --dev in combo with --externalsigner

* accounts: remove clique-specific signing method, replace with more generic

* accounts, consensus: formatting + fix error in tests

* signer/core: remove (test-) import cycle

* clique: remove unused import

* accounts: remove CliqueHash and avoid dependency on package crypto

* consensus/clique: unduplicate header encoding

17 files changed, 458 insertions, 115 deletions

diff --git a/accounts/accounts.go b/accounts/accounts.go
index cb1eae281..11232b19a 100644
--- a/accounts/accounts.go
+++ b/accounts/accounts.go
@@ -18,12 +18,14 @@
 package accounts
 
 import (
+	"fmt"
 	"math/big"
 
 	ethereum "github.com/ethereum/go-ethereum"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/event"
+	"golang.org/x/crypto/sha3"
 )
 
 // Account represents an Ethereum account located at a specific location defined
@@ -87,8 +89,20 @@ type Wallet interface {
 	// chain state reader.
 	SelfDerive(base DerivationPath, chain ethereum.ChainStateReader)
 
-	// SignHash requests the wallet to sign the given hash.
+	// SignData requests the wallet to sign the hash of the given data
+	// It looks up the account specified either solely via its address contained within,
+	// or optionally with the aid of any location metadata from the embedded URL field.
 	//
+	// If the wallet requires additional authentication to sign the request (e.g.
+	// a password to decrypt the account, or a PIN code o verify the transaction),
+	// an AuthNeededError instance will be returned, containing infos for the user
+	// about which fields or actions are needed. The user may retry by providing
+	// the needed details via SignHashWithPassphrase, or by other means (e.g. unlock
+	// the account in a keystore).
+	SignData(account Account, mimeType string, data []byte) ([]byte, error)
+
+	// Signtext requests the wallet to sign the hash of a given piece of data, prefixed
+	// by the Ethereum prefix scheme
 	// It looks up the account specified either solely via its address contained within,
 	// or optionally with the aid of any location metadata from the embedded URL field.
 	//
@@ -98,7 +112,7 @@ type Wallet interface {
 	// about which fields or actions are needed. The user may retry by providing
 	// the needed details via SignHashWithPassphrase, or by other means (e.g. unlock
 	// the account in a keystore).
-	SignHash(account Account, hash []byte) ([]byte, error)
+	SignText(account Account, text []byte) ([]byte, error)
 
 	// SignTx requests the wallet to sign the given transaction.
 	//
@@ -113,12 +127,12 @@ type Wallet interface {
 	// the account in a keystore).
 	SignTx(account Account, tx *types.Transaction, chainID *big.Int) (*types.Transaction, error)
 
-	// SignHashWithPassphrase requests the wallet to sign the given hash with the
+	// SignTextWithPassphrase requests the wallet to sign the given text with the
 	// given passphrase as extra authentication information.
 	//
 	// It looks up the account specified either solely via its address contained within,
 	// or optionally with the aid of any location metadata from the embedded URL field.
-	SignHashWithPassphrase(account Account, passphrase string, hash []byte) ([]byte, error)
+	SignTextWithPassphrase(account Account, passphrase string, hash []byte) ([]byte, error)
 
 	// SignTxWithPassphrase requests the wallet to sign the given transaction, with the
 	// given passphrase as extra authentication information.
@@ -148,6 +162,19 @@ type Backend interface {
 	Subscribe(sink chan<- WalletEvent) event.Subscription
 }
 
+// TextHash is a helper function that calculates a hash for the given message that can be
+// safely used to calculate a signature from.
+//
+// The hash is calulcated as
+//   keccak256("\x19Ethereum Signed Message:\n"${message length}${message}).
+//
+// This gives context to the signed message and prevents signing of transactions.
+func TextHash(data []byte) []byte {
+	hash := sha3.NewLegacyKeccak256()
+	fmt.Fprintf(hash, "\x19Ethereum Signed Message:\n%d%s", len(data), data)
+	return hash.Sum(nil)
+}
+
 // WalletEventType represents the different event types that can be fired by
 // the wallet subscription subsystem.
 type WalletEventType int
diff --git a/accounts/accounts_test.go b/accounts/accounts_test.go
new file mode 100644
index 000000000..a49e3954e
--- /dev/null
+++ b/accounts/accounts_test.go
@@ -0,0 +1,32 @@
+// Copyright 2019 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 accounts
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/ethereum/go-ethereum/common/hexutil"
+)
+
+func TestTextHash(t *testing.T) {
+	hash := TextHash([]byte("Hello Joe"))
+	want := hexutil.MustDecode("0xa080337ae51c4e064c189e113edd0ba391df9206e2f49db658bb32cf2911730b")
+	if !bytes.Equal(hash, want) {
+		t.Fatalf("wrong hash: %x", hash)
+	}
+}
diff --git a/accounts/external/backend.go b/accounts/external/backend.go
new file mode 100644
index 000000000..35b9c276d
--- /dev/null
+++ b/accounts/external/backend.go
@@ -0,0 +1,220 @@
+// Copyright 2018 The go-ethereum Authors
+// This file is part of go-ethereum.
+//
+// go-ethereum is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// go-ethereum 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 General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with go-ethereum. If not, see <http://www.gnu.org/licenses/>.
+
+package external
+
+import (
+	"fmt"
+	"math/big"
+	"sync"
+
+	"github.com/ethereum/go-ethereum"
+	"github.com/ethereum/go-ethereum/accounts"
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/ethereum/go-ethereum/event"
+	"github.com/ethereum/go-ethereum/internal/ethapi"
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/rpc"
+	"github.com/ethereum/go-ethereum/signer/core"
+)
+
+type ExternalBackend struct {
+	signers []accounts.Wallet
+}
+
+func (eb *ExternalBackend) Wallets() []accounts.Wallet {
+	return eb.signers
+}
+
+func NewExternalBackend(endpoint string) (*ExternalBackend, error) {
+	signer, err := NewExternalSigner(endpoint)
+	if err != nil {
+		return nil, err
+	}
+	return &ExternalBackend{
+		signers: []accounts.Wallet{signer},
+	}, nil
+}
+
+func (eb *ExternalBackend) Subscribe(sink chan<- accounts.WalletEvent) event.Subscription {
+	return event.NewSubscription(func(quit <-chan struct{}) error {
+		<-quit
+		return nil
+	})
+}
+
+// ExternalSigner provides an API to interact with an external signer (clef)
+// It proxies request to the external signer while forwarding relevant
+// request headers
+type ExternalSigner struct {
+	client   *rpc.Client
+	endpoint string
+	status   string
+	cacheMu  sync.RWMutex
+	cache    []accounts.Account
+}
+
+func NewExternalSigner(endpoint string) (*ExternalSigner, error) {
+	client, err := rpc.Dial(endpoint)
+	if err != nil {
+		return nil, err
+	}
+	extsigner := &ExternalSigner{
+		client:   client,
+		endpoint: endpoint,
+	}
+	// Check if reachable
+	version, err := extsigner.pingVersion()
+	if err != nil {
+		return nil, err
+	}
+	extsigner.status = fmt.Sprintf("ok [version=%v]", version)
+	return extsigner, nil
+}
+
+func (api *ExternalSigner) URL() accounts.URL {
+	return accounts.URL{
+		Scheme: "extapi",
+		Path:   api.endpoint,
+	}
+}
+
+func (api *ExternalSigner) Status() (string, error) {
+	return api.status, nil
+}
+
+func (api *ExternalSigner) Open(passphrase string) error {
+	return fmt.Errorf("operation not supported on external signers")
+}
+
+func (api *ExternalSigner) Close() error {
+	return fmt.Errorf("operation not supported on external signers")
+}
+
+func (api *ExternalSigner) Accounts() []accounts.Account {
+	var accnts []accounts.Account
+	res, err := api.listAccounts()
+	if err != nil {
+		log.Error("account listing failed", "error", err)
+		return accnts
+	}
+	for _, addr := range res {
+		accnts = append(accnts, accounts.Account{
+			URL: accounts.URL{
+				Scheme: "extapi",
+				Path:   api.endpoint,
+			},
+			Address: addr,
+		})
+	}
+	api.cacheMu.Lock()
+	api.cache = accnts
+	api.cacheMu.Unlock()
+	return accnts
+}
+
+func (api *ExternalSigner) Contains(account accounts.Account) bool {
+	api.cacheMu.RLock()
+	defer api.cacheMu.RUnlock()
+	for _, a := range api.cache {
+		if a.Address == account.Address && (account.URL == (accounts.URL{}) || account.URL == api.URL()) {
+			return true
+		}
+	}
+	return false
+}
+
+func (api *ExternalSigner) Derive(path accounts.DerivationPath, pin bool) (accounts.Account, error) {
+	return accounts.Account{}, fmt.Errorf("operation not supported on external signers")
+}
+
+func (api *ExternalSigner) SelfDerive(base accounts.DerivationPath, chain ethereum.ChainStateReader) {
+	log.Error("operation SelfDerive not supported on external signers")
+}
+
+func (api *ExternalSigner) signHash(account accounts.Account, hash []byte) ([]byte, error) {
+	return []byte{}, fmt.Errorf("operation not supported on external signers")
+}
+
+// SignData signs keccak256(data). The mimetype parameter describes the type of data being signed
+func (api *ExternalSigner) SignData(account accounts.Account, mimeType string, data []byte) ([]byte, error) {
+	// TODO! Replace this with a call to clef SignData with correct mime-type for Clique, once we
+	// have that in place
+	return api.signHash(account, crypto.Keccak256(data))
+}
+
+func (api *ExternalSigner) SignText(account accounts.Account, text []byte) ([]byte, error) {
+	return api.signHash(account, accounts.TextHash(text))
+}
+
+func (api *ExternalSigner) SignTx(account accounts.Account, tx *types.Transaction, chainID *big.Int) (*types.Transaction, error) {
+	res := ethapi.SignTransactionResult{}
+	to := common.NewMixedcaseAddress(*tx.To())
+	data := hexutil.Bytes(tx.Data())
+	args := &core.SendTxArgs{
+		Data:     &data,
+		Nonce:    hexutil.Uint64(tx.Nonce()),
+		Value:    hexutil.Big(*tx.Value()),
+		Gas:      hexutil.Uint64(tx.Gas()),
+		GasPrice: hexutil.Big(*tx.GasPrice()),
+		To:       &to,
+		From:     common.NewMixedcaseAddress(account.Address),
+	}
+
+	if err := api.client.Call(&res, "account_signTransaction", args); err != nil {
+		return nil, err
+	}
+	return res.Tx, nil
+}
+
+func (api *ExternalSigner) SignTextWithPassphrase(account accounts.Account, passphrase string, text []byte) ([]byte, error) {
+	return []byte{}, fmt.Errorf("operation not supported on external signers")
+}
+
+func (api *ExternalSigner) SignTxWithPassphrase(account accounts.Account, passphrase string, tx *types.Transaction, chainID *big.Int) (*types.Transaction, error) {
+	return nil, fmt.Errorf("operation not supported on external signers")
+}
+
+func (api *ExternalSigner) listAccounts() ([]common.Address, error) {
+	var res []common.Address
+	if err := api.client.Call(&res, "account_list"); err != nil {
+		return nil, err
+	}
+	return res, nil
+}
+
+func (api *ExternalSigner) signCliqueBlock(a common.Address, rlpBlock hexutil.Bytes) (hexutil.Bytes, error) {
+	var sig hexutil.Bytes
+	if err := api.client.Call(&sig, "account_signData", "application/clique", a, rlpBlock); err != nil {
+		return nil, err
+	}
+	if sig[64] != 27 && sig[64] != 28 {
+		return nil, fmt.Errorf("invalid Ethereum signature (V is not 27 or 28)")
+	}
+	sig[64] -= 27 // Transform V from 27/28 to 0/1 for Clique use
+	return sig, nil
+}
+
+func (api *ExternalSigner) pingVersion() (string, error) {
+	var v string
+	if err := api.client.Call(&v, "account_version"); err != nil {
+		return "", err
+	}
+	return v, nil
+}
diff --git a/accounts/keystore/wallet.go b/accounts/keystore/wallet.go
index 2f774cc94..0490f39ff 100644
--- a/accounts/keystore/wallet.go
+++ b/accounts/keystore/wallet.go
@@ -22,6 +22,7 @@ import (
 	ethereum "github.com/ethereum/go-ethereum"
 	"github.com/ethereum/go-ethereum/accounts"
 	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/crypto"
 )
 
 // keystoreWallet implements the accounts.Wallet interface for the original
@@ -78,11 +79,11 @@ func (w *keystoreWallet) Derive(path accounts.DerivationPath, pin bool) (account
 // there is no notion of hierarchical account derivation for plain keystore accounts.
 func (w *keystoreWallet) SelfDerive(base accounts.DerivationPath, chain ethereum.ChainStateReader) {}
 
-// SignHash implements accounts.Wallet, attempting to sign the given hash with
+// signHash attempts to sign the given hash with
 // the given account. If the wallet does not wrap this particular account, an
 // error is returned to avoid account leakage (even though in theory we may be
 // able to sign via our shared keystore backend).
-func (w *keystoreWallet) SignHash(account accounts.Account, hash []byte) ([]byte, error) {
+func (w *keystoreWallet) signHash(account accounts.Account, hash []byte) ([]byte, error) {
 	// Make sure the requested account is contained within
 	if !w.Contains(account) {
 		return nil, accounts.ErrUnknownAccount
@@ -91,6 +92,15 @@ func (w *keystoreWallet) SignHash(account accounts.Account, hash []byte) ([]byte
 	return w.keystore.SignHash(account, hash)
 }
 
+// SignData signs keccak256(data). The mimetype parameter describes the type of data being signed
+func (w *keystoreWallet) SignData(account accounts.Account, mimeType string, data []byte) ([]byte, error) {
+	return w.signHash(account, crypto.Keccak256(data))
+}
+
+func (w *keystoreWallet) SignText(account accounts.Account, text []byte) ([]byte, error) {
+	return w.signHash(account, accounts.TextHash(text))
+}
+
 // SignTx implements accounts.Wallet, attempting to sign the given transaction
 // with the given account. If the wallet does not wrap this particular account,
 // an error is returned to avoid account leakage (even though in theory we may
@@ -106,13 +116,13 @@ func (w *keystoreWallet) SignTx(account accounts.Account, tx *types.Transaction,
 
 // SignHashWithPassphrase implements accounts.Wallet, attempting to sign the
 // given hash with the given account using passphrase as extra authentication.
-func (w *keystoreWallet) SignHashWithPassphrase(account accounts.Account, passphrase string, hash []byte) ([]byte, error) {
+func (w *keystoreWallet) SignTextWithPassphrase(account accounts.Account, passphrase string, text []byte) ([]byte, error) {
 	// Make sure the requested account is contained within
 	if !w.Contains(account) {
 		return nil, accounts.ErrUnknownAccount
 	}
 	// Account seems valid, request the keystore to sign
-	return w.keystore.SignHashWithPassphrase(account, passphrase, hash)
+	return w.keystore.SignHashWithPassphrase(account, passphrase, accounts.TextHash(text))
 }
 
 // SignTxWithPassphrase implements accounts.Wallet, attempting to sign the given
diff --git a/accounts/usbwallet/wallet.go b/accounts/usbwallet/wallet.go
index 6cef6e0fb..a99dcd0f5 100644
--- a/accounts/usbwallet/wallet.go
+++ b/accounts/usbwallet/wallet.go
@@ -29,6 +29,7 @@ import (
 	"github.com/ethereum/go-ethereum/accounts"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/karalabe/hid"
 )
@@ -495,12 +496,21 @@ func (w *wallet) SelfDerive(base accounts.DerivationPath, chain ethereum.ChainSt
 	w.deriveChain = chain
 }
 
-// SignHash implements accounts.Wallet, however signing arbitrary data is not
+// signHash implements accounts.Wallet, however signing arbitrary data is not
 // supported for hardware wallets, so this method will always return an error.
-func (w *wallet) SignHash(account accounts.Account, hash []byte) ([]byte, error) {
+func (w *wallet) signHash(account accounts.Account, hash []byte) ([]byte, error) {
 	return nil, accounts.ErrNotSupported
 }
 
+// SignData signs keccak256(data). The mimetype parameter describes the type of data being signed
+func (w *wallet) SignData(account accounts.Account, mimeType string, data []byte) ([]byte, error) {
+	return w.signHash(account, crypto.Keccak256(data))
+}
+
+func (w *wallet) SignText(account accounts.Account, text []byte) ([]byte, error) {
+	return w.signHash(account, accounts.TextHash(text))
+}
+
 // SignTx implements accounts.Wallet. It sends the transaction over to the Ledger
 // wallet to request a confirmation from the user. It returns either the signed
 // transaction or a failure if the user denied the transaction.
@@ -550,8 +560,8 @@ func (w *wallet) SignTx(account accounts.Account, tx *types.Transaction, chainID
 // SignHashWithPassphrase implements accounts.Wallet, however signing arbitrary
 // data is not supported for Ledger wallets, so this method will always return
 // an error.
-func (w *wallet) SignHashWithPassphrase(account accounts.Account, passphrase string, hash []byte) ([]byte, error) {
-	return w.SignHash(account, hash)
+func (w *wallet) SignTextWithPassphrase(account accounts.Account, passphrase string, text []byte) ([]byte, error) {
+	return w.SignText(account, accounts.TextHash(text))
 }
 
 // SignTxWithPassphrase implements accounts.Wallet, attempting to sign the given
diff --git a/cmd/clef/main.go b/cmd/clef/main.go
index 519d63b3c..e2b85288d 100644
--- a/cmd/clef/main.go
+++ b/cmd/clef/main.go
@@ -49,12 +49,6 @@ import (
 	"gopkg.in/urfave/cli.v1"
 )
 
-// ExternalAPIVersion -- see extapi_changelog.md
-const ExternalAPIVersion = "4.0.0"
-
-// InternalAPIVersion -- see intapi_changelog.md
-const InternalAPIVersion = "3.0.0"
-
 const legalWarning = `
 WARNING! 
 
@@ -479,8 +473,8 @@ func signer(c *cli.Context) error {
 	}
 	ui.OnSignerStartup(core.StartupInfo{
 		Info: map[string]interface{}{
-			"extapi_version": ExternalAPIVersion,
-			"intapi_version": InternalAPIVersion,
+			"extapi_version": core.ExternalAPIVersion,
+			"intapi_version": core.InternalAPIVersion,
 			"extapi_http":    extapiURL,
 			"extapi_ipc":     ipcapiURL,
 		},
diff --git a/cmd/geth/main.go b/cmd/geth/main.go
index 0a6616312..dca02c82e 100644
--- a/cmd/geth/main.go
+++ b/cmd/geth/main.go
@@ -62,6 +62,7 @@ var (
 		utils.BootnodesV5Flag,
 		utils.DataDirFlag,
 		utils.KeyStoreDirFlag,
+		utils.ExternalSignerFlag,
 		utils.NoUSBFlag,
 		utils.DashboardEnabledFlag,
 		utils.DashboardAddrFlag,
@@ -293,13 +294,14 @@ func startNode(ctx *cli.Context, stack *node.Node) {
 	utils.StartNode(stack)
 
 	// Unlock any account specifically requested
-	ks := stack.AccountManager().Backends(keystore.KeyStoreType)[0].(*keystore.KeyStore)
-
-	passwords := utils.MakePasswordList(ctx)
-	unlocks := strings.Split(ctx.GlobalString(utils.UnlockedAccountFlag.Name), ",")
-	for i, account := range unlocks {
-		if trimmed := strings.TrimSpace(account); trimmed != "" {
-			unlockAccount(ctx, ks, trimmed, i, passwords)
+	if keystores := stack.AccountManager().Backends(keystore.KeyStoreType); len(keystores) > 0 {
+		ks := keystores[0].(*keystore.KeyStore)
+		passwords := utils.MakePasswordList(ctx)
+		unlocks := strings.Split(ctx.GlobalString(utils.UnlockedAccountFlag.Name), ",")
+		for i, account := range unlocks {
+			if trimmed := strings.TrimSpace(account); trimmed != "" {
+				unlockAccount(ctx, ks, trimmed, i, passwords)
+			}
 		}
 	}
 	// Register wallet event handlers to open and auto-derive wallets
diff --git a/cmd/geth/usage.go b/cmd/geth/usage.go
index 1579134f9..0b5c6fd90 100644
--- a/cmd/geth/usage.go
+++ b/cmd/geth/usage.go
@@ -145,6 +145,7 @@ var AppHelpFlagGroups = []flagGroup{
 		Flags: []cli.Flag{
 			utils.UnlockedAccountFlag,
 			utils.PasswordFileFlag,
+			utils.ExternalSignerFlag,
 		},
 	},
 	{
diff --git a/cmd/utils/flags.go b/cmd/utils/flags.go
index a3602182a..bf20abe81 100644
--- a/cmd/utils/flags.go
+++ b/cmd/utils/flags.go
@@ -427,7 +427,11 @@ var (
 		Usage: "Password file to use for non-interactive password input",
 		Value: "",
 	}
-
+	ExternalSignerFlag = cli.StringFlag{
+		Name:  "signer",
+		Usage: "External signer (url or path to ipc file)",
+		Value: "",
+	}
 	VMEnableDebugFlag = cli.BoolFlag{
 		Name:  "vmdebug",
 		Usage: "Record information useful for VM and contract debugging",
@@ -990,11 +994,15 @@ func setEtherbase(ctx *cli.Context, ks *keystore.KeyStore, cfg *eth.Config) {
 	}
 	// Convert the etherbase into an address and configure it
 	if etherbase != "" {
-		account, err := MakeAddress(ks, etherbase)
-		if err != nil {
-			Fatalf("Invalid miner etherbase: %v", err)
+		if ks != nil {
+			account, err := MakeAddress(ks, etherbase)
+			if err != nil {
+				Fatalf("Invalid miner etherbase: %v", err)
+			}
+			cfg.Etherbase = account.Address
+		} else {
+			Fatalf("No etherbase configured")
 		}
-		cfg.Etherbase = account.Address
 	}
 }
 
@@ -1093,6 +1101,10 @@ func SetNodeConfig(ctx *cli.Context, cfg *node.Config) {
 	setNodeUserIdent(ctx, cfg)
 	setDataDir(ctx, cfg)
 
+	if ctx.GlobalIsSet(ExternalSignerFlag.Name) {
+		cfg.ExternalSigner = ctx.GlobalString(ExternalSignerFlag.Name)
+	}
+
 	if ctx.GlobalIsSet(KeyStoreDirFlag.Name) {
 		cfg.KeyStoreDir = ctx.GlobalString(KeyStoreDirFlag.Name)
 	}
@@ -1274,8 +1286,12 @@ func SetEthConfig(ctx *cli.Context, stack *node.Node, cfg *eth.Config) {
 	// Avoid conflicting network flags
 	checkExclusive(ctx, DeveloperFlag, TestnetFlag, RinkebyFlag, GoerliFlag)
 	checkExclusive(ctx, LightServFlag, SyncModeFlag, "light")
-
-	ks := stack.AccountManager().Backends(keystore.KeyStoreType)[0].(*keystore.KeyStore)
+	// Can't use both ephemeral unlocked and external signer
+	checkExclusive(ctx, DeveloperFlag, ExternalSignerFlag)
+	var ks *keystore.KeyStore
+	if keystores := stack.AccountManager().Backends(keystore.KeyStoreType); len(keystores) > 0 {
+		ks = keystores[0].(*keystore.KeyStore)
+	}
 	setEtherbase(ctx, ks, cfg)
 	setGPO(ctx, &cfg.GPO)
 	setTxPool(ctx, &cfg.TxPool)
diff --git a/consensus/clique/clique.go b/consensus/clique/clique.go
index c79c30cae..c0f78ce65 100644
--- a/consensus/clique/clique.go
+++ b/consensus/clique/clique.go
@@ -20,6 +20,7 @@ package clique
 import (
 	"bytes"
 	"errors"
+	"io"
 	"math/big"
 	"math/rand"
 	"sync"
@@ -136,40 +137,9 @@ var (
 	errRecentlySigned = errors.New("recently signed")
 )
 
-// SignerFn is a signer callback function to request a hash to be signed by a
+// SignerFn is a signer callback function to request a header 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.NewLegacyKeccak256()
-
-	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
-}
+type SignerFn func(accounts.Account, string, []byte) ([]byte, error)
 
 // ecrecover extracts the Ethereum account address from a signed header.
 func ecrecover(header *types.Header, sigcache *lru.ARCCache) (common.Address, error) {
@@ -185,7 +155,7 @@ func ecrecover(header *types.Header, sigcache *lru.ARCCache) (common.Address, er
 	signature := header.Extra[len(header.Extra)-extraSeal:]
 
 	// Recover the public key and the Ethereum address
-	pubkey, err := crypto.Ecrecover(sigHash(header).Bytes(), signature)
+	pubkey, err := crypto.Ecrecover(SealHash(header).Bytes(), signature)
 	if err != nil {
 		return common.Address{}, err
 	}
@@ -646,7 +616,7 @@ func (c *Clique) Seal(chain consensus.ChainReader, block *types.Block, results c
 		log.Trace("Out-of-turn signing requested", "wiggle", common.PrettyDuration(wiggle))
 	}
 	// Sign all the things!
-	sighash, err := signFn(accounts.Account{Address: signer}, sigHash(header).Bytes())
+	sighash, err := signFn(accounts.Account{Address: signer}, "application/x-clique-header", CliqueRLP(header))
 	if err != nil {
 		return err
 	}
@@ -663,7 +633,7 @@ func (c *Clique) Seal(chain consensus.ChainReader, block *types.Block, results c
 		select {
 		case results <- block.WithSeal(header):
 		default:
-			log.Warn("Sealing result is not read by miner", "sealhash", c.SealHash(header))
+			log.Warn("Sealing result is not read by miner", "sealhash", SealHash(header))
 		}
 	}()
 
@@ -693,7 +663,7 @@ func CalcDifficulty(snap *Snapshot, signer common.Address) *big.Int {
 
 // SealHash returns the hash of a block prior to it being sealed.
 func (c *Clique) SealHash(header *types.Header) common.Hash {
-	return sigHash(header)
+	return SealHash(header)
 }
 
 // Close implements consensus.Engine. It's a noop for clique as there are no background threads.
@@ -711,3 +681,47 @@ func (c *Clique) APIs(chain consensus.ChainReader) []rpc.API {
 		Public:    false,
 	}}
 }
+
+// SealHash returns the hash of a block prior to it being sealed.
+func SealHash(header *types.Header) (hash common.Hash) {
+	hasher := sha3.NewLegacyKeccak256()
+	encodeSigHeader(hasher, header)
+	hasher.Sum(hash[:0])
+	return hash
+}
+
+// CliqueRLP returns the rlp bytes which needs to be signed for the proof-of-authority
+// sealing. The RLP to sign consists 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 CliqueRLP(header *types.Header) []byte {
+	b := new(bytes.Buffer)
+	encodeSigHeader(b, header)
+	return b.Bytes()
+}
+
+func encodeSigHeader(w io.Writer, header *types.Header) {
+	err := rlp.Encode(w, []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,
+	})
+	if err != nil {
+		panic("can't encode: " + err.Error())
+	}
+}
diff --git a/consensus/clique/snapshot_test.go b/consensus/clique/snapshot_test.go
index 41dae1426..b920312a8 100644
--- a/consensus/clique/snapshot_test.go
+++ b/consensus/clique/snapshot_test.go
@@ -80,7 +80,7 @@ func (ap *testerAccountPool) sign(header *types.Header, signer string) {
 		ap.accounts[signer], _ = crypto.GenerateKey()
 	}
 	// Sign the header and embed the signature in extra data
-	sig, _ := crypto.Sign(sigHash(header).Bytes(), ap.accounts[signer])
+	sig, _ := crypto.Sign(SealHash(header).Bytes(), ap.accounts[signer])
 	copy(header.Extra[len(header.Extra)-extraSeal:], sig)
 }
 
diff --git a/eth/backend.go b/eth/backend.go
index 2a9d56c5c..3ec6749b6 100644
--- a/eth/backend.go
+++ b/eth/backend.go
@@ -434,7 +434,7 @@ func (s *Ethereum) StartMining(threads int) error {
 				log.Error("Etherbase account unavailable locally", "err", err)
 				return fmt.Errorf("signer missing: %v", err)
 			}
-			clique.Authorize(eb, wallet.SignHash)
+			clique.Authorize(eb, wallet.SignData)
 		}
 		// If mining is started, we can disable the transaction rejection mechanism
 		// introduced to speed sync times.
diff --git a/internal/ethapi/api.go b/internal/ethapi/api.go
index 26dc1e8a0..0aeec8ad1 100644
--- a/internal/ethapi/api.go
+++ b/internal/ethapi/api.go
@@ -217,7 +217,7 @@ func NewPrivateAccountAPI(b Backend, nonceLock *AddrLocker) *PrivateAccountAPI {
 	}
 }
 
-// ListAccounts will return a list of addresses for accounts this node manages.
+// listAccounts will return a list of addresses for accounts this node manages.
 func (s *PrivateAccountAPI) ListAccounts() []common.Address {
 	addresses := make([]common.Address, 0) // return [] instead of nil if empty
 	for _, wallet := range s.am.Wallets() {
@@ -409,18 +409,6 @@ func (s *PrivateAccountAPI) SignTransaction(ctx context.Context, args SendTxArgs
 	return &SignTransactionResult{data, signed}, nil
 }
 
-// signHash is a helper function that calculates a hash for the given message that can be
-// safely used to calculate a signature from.
-//
-// The hash is calulcated as
-//   keccak256("\x19Ethereum Signed Message:\n"${message length}${message}).
-//
-// This gives context to the signed message and prevents signing of transactions.
-func signHash(data []byte) []byte {
-	msg := fmt.Sprintf("\x19Ethereum Signed Message:\n%d%s", len(data), data)
-	return crypto.Keccak256([]byte(msg))
-}
-
 // Sign calculates an Ethereum ECDSA signature for:
 // keccack256("\x19Ethereum Signed Message:\n" + len(message) + message))
 //
@@ -439,7 +427,7 @@ func (s *PrivateAccountAPI) Sign(ctx context.Context, data hexutil.Bytes, addr c
 		return nil, err
 	}
 	// Assemble sign the data with the wallet
-	signature, err := wallet.SignHashWithPassphrase(account, passwd, signHash(data))
+	signature, err := wallet.SignTextWithPassphrase(account, passwd, data)
 	if err != nil {
 		log.Warn("Failed data sign attempt", "address", addr, "err", err)
 		return nil, err
@@ -467,7 +455,7 @@ func (s *PrivateAccountAPI) EcRecover(ctx context.Context, data, sig hexutil.Byt
 	}
 	sig[64] -= 27 // Transform yellow paper V from 27/28 to 0/1
 
-	rpk, err := crypto.SigToPub(signHash(data), sig)
+	rpk, err := crypto.SigToPub(accounts.TextHash(data), sig)
 	if err != nil {
 		return common.Address{}, err
 	}
@@ -1357,7 +1345,7 @@ func (s *PublicTransactionPoolAPI) Sign(addr common.Address, data hexutil.Bytes)
 		return nil, err
 	}
 	// Sign the requested hash with the wallet
-	signature, err := wallet.SignHash(account, signHash(data))
+	signature, err := wallet.SignText(account, data)
 	if err == nil {
 		signature[64] += 27 // Transform V from 0/1 to 27/28 according to the yellow paper
 	}
diff --git a/node/config.go b/node/config.go
index 99f325840..ff0725aab 100644
--- a/node/config.go
+++ b/node/config.go
@@ -27,6 +27,7 @@ import (
 	"sync"
 
 	"github.com/ethereum/go-ethereum/accounts"
+	"github.com/ethereum/go-ethereum/accounts/external"
 	"github.com/ethereum/go-ethereum/accounts/keystore"
 	"github.com/ethereum/go-ethereum/accounts/usbwallet"
 	"github.com/ethereum/go-ethereum/common"
@@ -80,6 +81,9 @@ type Config struct {
 	// is created by New and destroyed when the node is stopped.
 	KeyStoreDir string `toml:",omitempty"`
 
+	// ExternalSigner specifies an external URI for a clef-type signer
+	ExternalSigner string `toml:"omitempty"`
+
 	// UseLightweightKDF lowers the memory and CPU requirements of the key store
 	// scrypt KDF at the expense of security.
 	UseLightweightKDF bool `toml:",omitempty"`
@@ -462,23 +466,37 @@ func makeAccountManager(conf *Config) (*accounts.Manager, string, error) {
 		return nil, "", err
 	}
 	// Assemble the account manager and supported backends
-	backends := []accounts.Backend{
-		keystore.NewKeyStore(keydir, scryptN, scryptP),
-	}
-	if !conf.NoUSB {
-		// Start a USB hub for Ledger hardware wallets
-		if ledgerhub, err := usbwallet.NewLedgerHub(); err != nil {
-			log.Warn(fmt.Sprintf("Failed to start Ledger hub, disabling: %v", err))
+	backends := []accounts.Backend{}
+	if len(conf.ExternalSigner) > 0 {
+		log.Info("Using external signer", "url", conf.ExternalSigner)
+		if extapi, err := external.NewExternalBackend(conf.ExternalSigner); err == nil {
+			backends = append(backends, extapi)
 		} else {
-			backends = append(backends, ledgerhub)
+			log.Info("Error configuring external signer", "error", err)
 		}
-		// Start a USB hub for Trezor hardware wallets
-		if trezorhub, err := usbwallet.NewTrezorHub(); err != nil {
-			log.Warn(fmt.Sprintf("Failed to start Trezor hub, disabling: %v", err))
-		} else {
-			backends = append(backends, trezorhub)
+	}
+	if len(backends) == 0 {
+		// For now, we're using EITHER external signer OR local signers.
+		// If/when we implement some form of lockfile for USB and keystore wallets,
+		// we can have both, but it's very confusing for the user to see the same
+		// accounts in both externally and locally, plus very racey.
+		backends = append(backends, keystore.NewKeyStore(keydir, scryptN, scryptP))
+		if !conf.NoUSB {
+			// Start a USB hub for Ledger hardware wallets
+			if ledgerhub, err := usbwallet.NewLedgerHub(); err != nil {
+				log.Warn(fmt.Sprintf("Failed to start Ledger hub, disabling: %v", err))
+			} else {
+				backends = append(backends, ledgerhub)
+			}
+			// Start a USB hub for Trezor hardware wallets
+			if trezorhub, err := usbwallet.NewTrezorHub(); err != nil {
+				log.Warn(fmt.Sprintf("Failed to start Trezor hub, disabling: %v", err))
+			} else {
+				backends = append(backends, trezorhub)
+			}
 		}
 	}
+
 	return accounts.NewManager(backends...), ephemeral, nil
 }
 
diff --git a/signer/core/api.go b/signer/core/api.go
index e9a335785..e112df9c7 100644
--- a/signer/core/api.go
+++ b/signer/core/api.go
@@ -36,8 +36,14 @@ import (
 	"github.com/ethereum/go-ethereum/rlp"
 )
 
-// numberOfAccountsToDerive For hardware wallets, the number of accounts to derive
-const numberOfAccountsToDerive = 10
+const (
+	// numberOfAccountsToDerive For hardware wallets, the number of accounts to derive
+	numberOfAccountsToDerive = 10
+	// ExternalAPIVersion -- see extapi_changelog.md
+	ExternalAPIVersion = "4.0.0"
+	// InternalAPIVersion -- see intapi_changelog.md
+	InternalAPIVersion = "3.0.0"
+)
 
 // ExternalAPI defines the external API through which signing requests are made.
 type ExternalAPI interface {
@@ -55,6 +61,7 @@ type ExternalAPI interface {
 	// Should be moved to Internal API, in next phase when we have
 	// bi-directional communication
 	//Import(ctx context.Context, keyJSON json.RawMessage) (Account, error)
+	Version(ctx context.Context) (string, error)
 }
 
 // SignerUI specifies what method a UI needs to implement to be able to be used as a UI for the signer
@@ -539,7 +546,7 @@ func (api *SignerAPI) Sign(ctx context.Context, addr common.MixedcaseAddress, da
 		return nil, err
 	}
 	// Assemble sign the data with the wallet
-	signature, err := wallet.SignHashWithPassphrase(account, res.Password, sighash)
+	signature, err := wallet.SignTextWithPassphrase(account, res.Password, data)
 	if err != nil {
 		api.UI.ShowError(err.Error())
 		return nil, err
@@ -610,3 +617,9 @@ func (api *SignerAPI) Import(ctx context.Context, keyJSON json.RawMessage) (Acco
 	}
 	return Account{Typ: "Account", URL: acc.URL, Address: acc.Address}, nil
 }
+
+// Returns the external api version. This method does not require user acceptance. Available methods are
+// available via enumeration anyway, and this info does not contain user-specific data
+func (api *SignerAPI) Version(ctx context.Context) (string, error) {
+	return ExternalAPIVersion, nil
+}
diff --git a/signer/core/api_test.go b/signer/core/api_test.go
index a8aa23896..114470cf9 100644
--- a/signer/core/api_test.go
+++ b/signer/core/api_test.go
@@ -29,7 +29,6 @@ import (
 	"time"
 
 	"github.com/ethereum/go-ethereum/accounts/keystore"
-	"github.com/ethereum/go-ethereum/cmd/utils"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/common/hexutil"
 	"github.com/ethereum/go-ethereum/core/types"
@@ -135,7 +134,7 @@ func setup(t *testing.T) (*SignerAPI, chan string) {
 
 	db, err := NewAbiDBFromFile("../../cmd/clef/4byte.json")
 	if err != nil {
-		utils.Fatalf(err.Error())
+		t.Fatal(err.Error())
 	}
 	var (
 		ui  = &HeadlessUI{controller}
diff --git a/signer/core/auditlog.go b/signer/core/auditlog.go
index 1f9c90918..0cb6c9c47 100644
--- a/signer/core/auditlog.go
+++ b/signer/core/auditlog.go
@@ -80,14 +80,13 @@ func (l *AuditLogger) Export(ctx context.Context, addr common.Address) (json.Raw
 	return j, e
 }
 
-//func (l *AuditLogger) Import(ctx context.Context, keyJSON json.RawMessage) (Account, error) {
-//	// Don't actually log the json contents
-//	l.log.Info("Import", "type", "request", "metadata", MetadataFromContext(ctx).String(),
-//		"keyJSON size", len(keyJSON))
-//	a, e := l.api.Import(ctx, keyJSON)
-//	l.log.Info("Import", "type", "response", "addr", a.String(), "error", e)
-//	return a, e
-//}
+func (l *AuditLogger) Version(ctx context.Context) (string, error) {
+	l.log.Info("Version", "type", "request", "metadata", MetadataFromContext(ctx).String())
+	data, err := l.api.Version(ctx)
+	l.log.Info("Version", "type", "response", "data", data, "error", err)
+	return data, err
+
+}
 
 func NewAuditLogger(path string, api ExternalAPI) (*AuditLogger, error) {
 	l := log.New("api", "signer")