plugeth/signer/core/api.go
John Chase ab0e0f3517
signer/core: show helptext when clef rejects signing (#27538)
Co-authored-by: Martin Holst Swende <martin@swende.se>
2023-07-06 03:50:44 -04:00

671 lines
24 KiB
Go

// Copyright 2018 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 core
import (
"bytes"
"context"
"encoding/json"
"errors"
"fmt"
"math/big"
"os"
"reflect"
"github.com/ethereum/go-ethereum/accounts"
"github.com/ethereum/go-ethereum/accounts/keystore"
"github.com/ethereum/go-ethereum/accounts/scwallet"
"github.com/ethereum/go-ethereum/accounts/usbwallet"
"github.com/ethereum/go-ethereum/common"
"github.com/ethereum/go-ethereum/common/hexutil"
"github.com/ethereum/go-ethereum/common/math"
"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/apitypes"
"github.com/ethereum/go-ethereum/signer/storage"
)
const (
// numberOfAccountsToDerive For hardware wallets, the number of accounts to derive
numberOfAccountsToDerive = 10
// ExternalAPIVersion -- see extapi_changelog.md
ExternalAPIVersion = "6.1.0"
// InternalAPIVersion -- see intapi_changelog.md
InternalAPIVersion = "7.0.1"
)
// ExternalAPI defines the external API through which signing requests are made.
type ExternalAPI interface {
// List available accounts
List(ctx context.Context) ([]common.Address, error)
// New request to create a new account
New(ctx context.Context) (common.Address, error)
// SignTransaction request to sign the specified transaction
SignTransaction(ctx context.Context, args apitypes.SendTxArgs, methodSelector *string) (*ethapi.SignTransactionResult, error)
// SignData - request to sign the given data (plus prefix)
SignData(ctx context.Context, contentType string, addr common.MixedcaseAddress, data interface{}) (hexutil.Bytes, error)
// SignTypedData - request to sign the given structured data (plus prefix)
SignTypedData(ctx context.Context, addr common.MixedcaseAddress, data apitypes.TypedData) (hexutil.Bytes, error)
// EcRecover - recover public key from given message and signature
EcRecover(ctx context.Context, data hexutil.Bytes, sig hexutil.Bytes) (common.Address, error)
// Version info about the APIs
Version(ctx context.Context) (string, error)
// SignGnosisSafeTransaction signs/confirms a gnosis-safe multisig transaction
SignGnosisSafeTx(ctx context.Context, signerAddress common.MixedcaseAddress, gnosisTx GnosisSafeTx, methodSelector *string) (*GnosisSafeTx, error)
}
// UIClientAPI specifies what method a UI needs to implement to be able to be used as a
// UI for the signer
type UIClientAPI interface {
// ApproveTx prompt the user for confirmation to request to sign Transaction
ApproveTx(request *SignTxRequest) (SignTxResponse, error)
// ApproveSignData prompt the user for confirmation to request to sign data
ApproveSignData(request *SignDataRequest) (SignDataResponse, error)
// ApproveListing prompt the user for confirmation to list accounts
// the list of accounts to list can be modified by the UI
ApproveListing(request *ListRequest) (ListResponse, error)
// ApproveNewAccount prompt the user for confirmation to create new Account, and reveal to caller
ApproveNewAccount(request *NewAccountRequest) (NewAccountResponse, error)
// ShowError displays error message to user
ShowError(message string)
// ShowInfo displays info message to user
ShowInfo(message string)
// OnApprovedTx notifies the UI about a transaction having been successfully signed.
// This method can be used by a UI to keep track of e.g. how much has been sent to a particular recipient.
OnApprovedTx(tx ethapi.SignTransactionResult)
// OnSignerStartup is invoked when the signer boots, and tells the UI info about external API location and version
// information
OnSignerStartup(info StartupInfo)
// OnInputRequired is invoked when clef requires user input, for example master password or
// pin-code for unlocking hardware wallets
OnInputRequired(info UserInputRequest) (UserInputResponse, error)
// RegisterUIServer tells the UI to use the given UIServerAPI for ui->clef communication
RegisterUIServer(api *UIServerAPI)
}
// Validator defines the methods required to validate a transaction against some
// sanity defaults as well as any underlying 4byte method database.
//
// Use fourbyte.Database as an implementation. It is separated out of this package
// to allow pieces of the signer package to be used without having to load the
// 7MB embedded 4byte dump.
type Validator interface {
// ValidateTransaction does a number of checks on the supplied transaction, and
// returns either a list of warnings, or an error (indicating that the transaction
// should be immediately rejected).
ValidateTransaction(selector *string, tx *apitypes.SendTxArgs) (*apitypes.ValidationMessages, error)
}
// SignerAPI defines the actual implementation of ExternalAPI
type SignerAPI struct {
chainID *big.Int
am *accounts.Manager
UI UIClientAPI
validator Validator
rejectMode bool
credentials storage.Storage
}
// Metadata about a request
type Metadata struct {
Remote string `json:"remote"`
Local string `json:"local"`
Scheme string `json:"scheme"`
UserAgent string `json:"User-Agent"`
Origin string `json:"Origin"`
}
func StartClefAccountManager(ksLocation string, nousb, lightKDF bool, scpath string) *accounts.Manager {
var (
backends []accounts.Backend
n, p = keystore.StandardScryptN, keystore.StandardScryptP
)
if lightKDF {
n, p = keystore.LightScryptN, keystore.LightScryptP
}
// support password based accounts
if len(ksLocation) > 0 {
backends = append(backends, keystore.NewKeyStore(ksLocation, n, p))
}
if !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)
log.Debug("Ledger support enabled")
}
// Start a USB hub for Trezor hardware wallets (HID version)
if trezorhub, err := usbwallet.NewTrezorHubWithHID(); err != nil {
log.Warn(fmt.Sprintf("Failed to start HID Trezor hub, disabling: %v", err))
} else {
backends = append(backends, trezorhub)
log.Debug("Trezor support enabled via HID")
}
// Start a USB hub for Trezor hardware wallets (WebUSB version)
if trezorhub, err := usbwallet.NewTrezorHubWithWebUSB(); err != nil {
log.Warn(fmt.Sprintf("Failed to start WebUSB Trezor hub, disabling: %v", err))
} else {
backends = append(backends, trezorhub)
log.Debug("Trezor support enabled via WebUSB")
}
}
// Start a smart card hub
if len(scpath) > 0 {
// Sanity check that the smartcard path is valid
fi, err := os.Stat(scpath)
if err != nil {
log.Info("Smartcard socket file missing, disabling", "err", err)
} else {
if fi.Mode()&os.ModeType != os.ModeSocket {
log.Error("Invalid smartcard socket file type", "path", scpath, "type", fi.Mode().String())
} else {
if schub, err := scwallet.NewHub(scpath, scwallet.Scheme, ksLocation); err != nil {
log.Warn(fmt.Sprintf("Failed to start smart card hub, disabling: %v", err))
} else {
backends = append(backends, schub)
}
}
}
}
// Clef doesn't allow insecure http account unlock.
return accounts.NewManager(&accounts.Config{InsecureUnlockAllowed: false}, backends...)
}
// MetadataFromContext extracts Metadata from a given context.Context
func MetadataFromContext(ctx context.Context) Metadata {
info := rpc.PeerInfoFromContext(ctx)
m := Metadata{"NA", "NA", "NA", "", ""} // batman
if info.Transport != "" {
if info.Transport == "http" {
m.Scheme = info.HTTP.Version
}
m.Scheme = info.Transport
}
if info.RemoteAddr != "" {
m.Remote = info.RemoteAddr
}
if info.HTTP.Host != "" {
m.Local = info.HTTP.Host
}
m.Origin = info.HTTP.Origin
m.UserAgent = info.HTTP.UserAgent
return m
}
// String implements Stringer interface
func (m Metadata) String() string {
s, err := json.Marshal(m)
if err == nil {
return string(s)
}
return err.Error()
}
// types for the requests/response types between signer and UI
type (
// SignTxRequest contains info about a Transaction to sign
SignTxRequest struct {
Transaction apitypes.SendTxArgs `json:"transaction"`
Callinfo []apitypes.ValidationInfo `json:"call_info"`
Meta Metadata `json:"meta"`
}
// SignTxResponse result from SignTxRequest
SignTxResponse struct {
//The UI may make changes to the TX
Transaction apitypes.SendTxArgs `json:"transaction"`
Approved bool `json:"approved"`
}
SignDataRequest struct {
ContentType string `json:"content_type"`
Address common.MixedcaseAddress `json:"address"`
Rawdata []byte `json:"raw_data"`
Messages []*apitypes.NameValueType `json:"messages"`
Callinfo []apitypes.ValidationInfo `json:"call_info"`
Hash hexutil.Bytes `json:"hash"`
Meta Metadata `json:"meta"`
}
SignDataResponse struct {
Approved bool `json:"approved"`
}
NewAccountRequest struct {
Meta Metadata `json:"meta"`
}
NewAccountResponse struct {
Approved bool `json:"approved"`
}
ListRequest struct {
Accounts []accounts.Account `json:"accounts"`
Meta Metadata `json:"meta"`
}
ListResponse struct {
Accounts []accounts.Account `json:"accounts"`
}
Message struct {
Text string `json:"text"`
}
StartupInfo struct {
Info map[string]interface{} `json:"info"`
}
UserInputRequest struct {
Title string `json:"title"`
Prompt string `json:"prompt"`
IsPassword bool `json:"isPassword"`
}
UserInputResponse struct {
Text string `json:"text"`
}
)
var ErrRequestDenied = errors.New("request denied")
// NewSignerAPI creates a new API that can be used for Account management.
// ksLocation specifies the directory where to store the password protected private
// key that is generated when a new Account is created.
// noUSB disables USB support that is required to support hardware devices such as
// ledger and trezor.
func NewSignerAPI(am *accounts.Manager, chainID int64, noUSB bool, ui UIClientAPI, validator Validator, advancedMode bool, credentials storage.Storage) *SignerAPI {
if advancedMode {
log.Info("Clef is in advanced mode: will warn instead of reject")
}
signer := &SignerAPI{big.NewInt(chainID), am, ui, validator, !advancedMode, credentials}
if !noUSB {
signer.startUSBListener()
}
return signer
}
func (api *SignerAPI) openTrezor(url accounts.URL) {
resp, err := api.UI.OnInputRequired(UserInputRequest{
Prompt: "Pin required to open Trezor wallet\n" +
"Look at the device for number positions\n\n" +
"7 | 8 | 9\n" +
"--+---+--\n" +
"4 | 5 | 6\n" +
"--+---+--\n" +
"1 | 2 | 3\n\n",
IsPassword: true,
Title: "Trezor unlock",
})
if err != nil {
log.Warn("failed getting trezor pin", "err", err)
return
}
// We're using the URL instead of the pointer to the
// Wallet -- perhaps it is not actually present anymore
w, err := api.am.Wallet(url.String())
if err != nil {
log.Warn("wallet unavailable", "url", url)
return
}
err = w.Open(resp.Text)
if err != nil {
log.Warn("failed to open wallet", "wallet", url, "err", err)
return
}
}
// startUSBListener starts a listener for USB events, for hardware wallet interaction
func (api *SignerAPI) startUSBListener() {
eventCh := make(chan accounts.WalletEvent, 16)
am := api.am
am.Subscribe(eventCh)
// Open any wallets already attached
for _, wallet := range am.Wallets() {
if err := wallet.Open(""); err != nil {
log.Warn("Failed to open wallet", "url", wallet.URL(), "err", err)
if err == usbwallet.ErrTrezorPINNeeded {
go api.openTrezor(wallet.URL())
}
}
}
go api.derivationLoop(eventCh)
}
// derivationLoop listens for wallet events
func (api *SignerAPI) derivationLoop(events chan accounts.WalletEvent) {
// Listen for wallet event till termination
for event := range events {
switch event.Kind {
case accounts.WalletArrived:
if err := event.Wallet.Open(""); err != nil {
log.Warn("New wallet appeared, failed to open", "url", event.Wallet.URL(), "err", err)
if err == usbwallet.ErrTrezorPINNeeded {
go api.openTrezor(event.Wallet.URL())
}
}
case accounts.WalletOpened:
status, _ := event.Wallet.Status()
log.Info("New wallet appeared", "url", event.Wallet.URL(), "status", status)
var derive = func(limit int, next func() accounts.DerivationPath) {
// Derive first N accounts, hardcoded for now
for i := 0; i < limit; i++ {
path := next()
if acc, err := event.Wallet.Derive(path, true); err != nil {
log.Warn("Account derivation failed", "error", err)
} else {
log.Info("Derived account", "address", acc.Address, "path", path)
}
}
}
log.Info("Deriving default paths")
derive(numberOfAccountsToDerive, accounts.DefaultIterator(accounts.DefaultBaseDerivationPath))
if event.Wallet.URL().Scheme == "ledger" {
log.Info("Deriving ledger legacy paths")
derive(numberOfAccountsToDerive, accounts.DefaultIterator(accounts.LegacyLedgerBaseDerivationPath))
log.Info("Deriving ledger live paths")
// For ledger live, since it's based off the same (DefaultBaseDerivationPath)
// as one we've already used, we need to step it forward one step to avoid
// hitting the same path again
nextFn := accounts.LedgerLiveIterator(accounts.DefaultBaseDerivationPath)
nextFn()
derive(numberOfAccountsToDerive, nextFn)
}
case accounts.WalletDropped:
log.Info("Old wallet dropped", "url", event.Wallet.URL())
event.Wallet.Close()
}
}
}
// List returns the set of wallet this signer manages. Each wallet can contain
// multiple accounts.
func (api *SignerAPI) List(ctx context.Context) ([]common.Address, error) {
var accs = make([]accounts.Account, 0)
// accs is initialized as empty list, not nil. We use 'nil' to signal
// rejection, as opposed to an empty list.
for _, wallet := range api.am.Wallets() {
accs = append(accs, wallet.Accounts()...)
}
result, err := api.UI.ApproveListing(&ListRequest{Accounts: accs, Meta: MetadataFromContext(ctx)})
if err != nil {
return nil, err
}
if result.Accounts == nil {
return nil, ErrRequestDenied
}
addresses := make([]common.Address, 0)
for _, acc := range result.Accounts {
addresses = append(addresses, acc.Address)
}
return addresses, nil
}
// New creates a new password protected Account. The private key is protected with
// the given password. Users are responsible to backup the private key that is stored
// in the keystore location that was specified when this API was created.
func (api *SignerAPI) New(ctx context.Context) (common.Address, error) {
if be := api.am.Backends(keystore.KeyStoreType); len(be) == 0 {
return common.Address{}, errors.New("password based accounts not supported")
}
if resp, err := api.UI.ApproveNewAccount(&NewAccountRequest{MetadataFromContext(ctx)}); err != nil {
return common.Address{}, err
} else if !resp.Approved {
return common.Address{}, ErrRequestDenied
}
return api.newAccount()
}
// newAccount is the internal method to create a new account. It should be used
// _after_ user-approval has been obtained
func (api *SignerAPI) newAccount() (common.Address, error) {
be := api.am.Backends(keystore.KeyStoreType)
if len(be) == 0 {
return common.Address{}, errors.New("password based accounts not supported")
}
// Three retries to get a valid password
for i := 0; i < 3; i++ {
resp, err := api.UI.OnInputRequired(UserInputRequest{
"New account password",
fmt.Sprintf("Please enter a password for the new account to be created (attempt %d of 3)", i),
true})
if err != nil {
log.Warn("error obtaining password", "attempt", i, "error", err)
continue
}
if pwErr := ValidatePasswordFormat(resp.Text); pwErr != nil {
api.UI.ShowError(fmt.Sprintf("Account creation attempt #%d failed due to password requirements: %v", i+1, pwErr))
} else {
// No error
acc, err := be[0].(*keystore.KeyStore).NewAccount(resp.Text)
log.Info("Your new key was generated", "address", acc.Address)
log.Warn("Please backup your key file!", "path", acc.URL.Path)
log.Warn("Please remember your password!")
return acc.Address, err
}
}
// Otherwise fail, with generic error message
return common.Address{}, errors.New("account creation failed")
}
// logDiff logs the difference between the incoming (original) transaction and the one returned from the signer.
// it also returns 'true' if the transaction was modified, to make it possible to configure the signer not to allow
// UI-modifications to requests
func logDiff(original *SignTxRequest, new *SignTxResponse) bool {
var intPtrModified = func(a, b *hexutil.Big) bool {
aBig := (*big.Int)(a)
bBig := (*big.Int)(b)
if aBig != nil && bBig != nil {
return aBig.Cmp(bBig) != 0
}
// One or both of them are nil
return a != b
}
modified := false
if f0, f1 := original.Transaction.From, new.Transaction.From; !reflect.DeepEqual(f0, f1) {
log.Info("Sender-account changed by UI", "was", f0, "is", f1)
modified = true
}
if t0, t1 := original.Transaction.To, new.Transaction.To; !reflect.DeepEqual(t0, t1) {
log.Info("Recipient-account changed by UI", "was", t0, "is", t1)
modified = true
}
if g0, g1 := original.Transaction.Gas, new.Transaction.Gas; g0 != g1 {
modified = true
log.Info("Gas changed by UI", "was", g0, "is", g1)
}
if a, b := original.Transaction.GasPrice, new.Transaction.GasPrice; intPtrModified(a, b) {
log.Info("GasPrice changed by UI", "was", a, "is", b)
modified = true
}
if a, b := original.Transaction.MaxPriorityFeePerGas, new.Transaction.MaxPriorityFeePerGas; intPtrModified(a, b) {
log.Info("maxPriorityFeePerGas changed by UI", "was", a, "is", b)
modified = true
}
if a, b := original.Transaction.MaxFeePerGas, new.Transaction.MaxFeePerGas; intPtrModified(a, b) {
log.Info("maxFeePerGas changed by UI", "was", a, "is", b)
modified = true
}
if v0, v1 := big.Int(original.Transaction.Value), big.Int(new.Transaction.Value); v0.Cmp(&v1) != 0 {
modified = true
log.Info("Value changed by UI", "was", v0, "is", v1)
}
if d0, d1 := original.Transaction.Data, new.Transaction.Data; d0 != d1 {
d0s := ""
d1s := ""
if d0 != nil {
d0s = hexutil.Encode(*d0)
}
if d1 != nil {
d1s = hexutil.Encode(*d1)
}
if d1s != d0s {
modified = true
log.Info("Data changed by UI", "was", d0s, "is", d1s)
}
}
if n0, n1 := original.Transaction.Nonce, new.Transaction.Nonce; n0 != n1 {
modified = true
log.Info("Nonce changed by UI", "was", n0, "is", n1)
}
return modified
}
func (api *SignerAPI) lookupPassword(address common.Address) (string, error) {
return api.credentials.Get(address.Hex())
}
func (api *SignerAPI) lookupOrQueryPassword(address common.Address, title, prompt string) (string, error) {
// Look up the password and return if available
if pw, err := api.lookupPassword(address); err == nil {
return pw, nil
}
// Password unavailable, request it from the user
pwResp, err := api.UI.OnInputRequired(UserInputRequest{title, prompt, true})
if err != nil {
log.Warn("error obtaining password", "error", err)
// We'll not forward the error here, in case the error contains info about the response from the UI,
// which could leak the password if it was malformed json or something
return "", errors.New("internal error")
}
return pwResp.Text, nil
}
// SignTransaction signs the given Transaction and returns it both as json and rlp-encoded form
func (api *SignerAPI) SignTransaction(ctx context.Context, args apitypes.SendTxArgs, methodSelector *string) (*ethapi.SignTransactionResult, error) {
var (
err error
result SignTxResponse
)
msgs, err := api.validator.ValidateTransaction(methodSelector, &args)
if err != nil {
return nil, err
}
// If we are in 'rejectMode', then reject rather than show the user warnings
if api.rejectMode {
if err := msgs.GetWarnings(); err != nil {
log.Info("Signing aborted due to warnings. In order to continue despite warnings, please use the flag '--advanced'.")
return nil, err
}
}
if args.ChainID != nil {
requestedChainId := (*big.Int)(args.ChainID)
if api.chainID.Cmp(requestedChainId) != 0 {
log.Error("Signing request with wrong chain id", "requested", requestedChainId, "configured", api.chainID)
return nil, fmt.Errorf("requested chainid %d does not match the configuration of the signer",
requestedChainId)
}
}
req := SignTxRequest{
Transaction: args,
Meta: MetadataFromContext(ctx),
Callinfo: msgs.Messages,
}
// Process approval
result, err = api.UI.ApproveTx(&req)
if err != nil {
return nil, err
}
if !result.Approved {
return nil, ErrRequestDenied
}
// Log changes made by the UI to the signing-request
logDiff(&req, &result)
var (
acc accounts.Account
wallet accounts.Wallet
)
acc = accounts.Account{Address: result.Transaction.From.Address()}
wallet, err = api.am.Find(acc)
if err != nil {
return nil, err
}
// Convert fields into a real transaction
var unsignedTx = result.Transaction.ToTransaction()
// Get the password for the transaction
pw, err := api.lookupOrQueryPassword(acc.Address, "Account password",
fmt.Sprintf("Please enter the password for account %s", acc.Address.String()))
if err != nil {
return nil, err
}
// The one to sign is the one that was returned from the UI
signedTx, err := wallet.SignTxWithPassphrase(acc, pw, unsignedTx, api.chainID)
if err != nil {
api.UI.ShowError(err.Error())
return nil, err
}
data, err := signedTx.MarshalBinary()
if err != nil {
return nil, err
}
response := ethapi.SignTransactionResult{Raw: data, Tx: signedTx}
// Finally, send the signed tx to the UI
api.UI.OnApprovedTx(response)
// ...and to the external caller
return &response, nil
}
func (api *SignerAPI) SignGnosisSafeTx(ctx context.Context, signerAddress common.MixedcaseAddress, gnosisTx GnosisSafeTx, methodSelector *string) (*GnosisSafeTx, error) {
// Do the usual validations, but on the last-stage transaction
args := gnosisTx.ArgsForValidation()
msgs, err := api.validator.ValidateTransaction(methodSelector, args)
if err != nil {
return nil, err
}
// If we are in 'rejectMode', then reject rather than show the user warnings
if api.rejectMode {
if err := msgs.GetWarnings(); err != nil {
log.Info("Signing aborted due to warnings. In order to continue despite warnings, please use the flag '--advanced'.")
return nil, err
}
}
typedData := gnosisTx.ToTypedData()
// might aswell error early.
// we are expected to sign. If our calculated hash does not match what they want,
// The gnosis safetx input contains a 'safeTxHash' which is the expected safeTxHash that
sighash, _, err := apitypes.TypedDataAndHash(typedData)
if err != nil {
return nil, err
}
if !bytes.Equal(sighash, gnosisTx.InputExpHash.Bytes()) {
// It might be the case that the json is missing chain id.
if gnosisTx.ChainId == nil {
gnosisTx.ChainId = (*math.HexOrDecimal256)(api.chainID)
typedData = gnosisTx.ToTypedData()
sighash, _, _ = apitypes.TypedDataAndHash(typedData)
if !bytes.Equal(sighash, gnosisTx.InputExpHash.Bytes()) {
return nil, fmt.Errorf("mismatched safeTxHash; have %#x want %#x", sighash, gnosisTx.InputExpHash[:])
}
}
}
signature, preimage, err := api.signTypedData(ctx, signerAddress, typedData, msgs)
if err != nil {
return nil, err
}
checkSummedSender, _ := common.NewMixedcaseAddressFromString(signerAddress.Address().Hex())
gnosisTx.Signature = signature
gnosisTx.SafeTxHash = common.BytesToHash(preimage)
gnosisTx.Sender = *checkSummedSender // Must be checksummed to be accepted by relay
return &gnosisTx, 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
}