@@ -21,7 +21,7 @@ return
 	}
 	// API to common + verify signature
 	commonAuth := accountCreationAuthAPIToCommon(&apiAuth)
-	if !commonAuth.VerifySignature() {
+	if !commonAuth.VerifySignature(a.chainID, a.hermezAddress) {
 		retBadReq(errors.New("invalid signature"), c)
 		return
 	}

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"sync"
 
+	ethCommon "github.com/ethereum/go-ethereum/common"
 	"github.com/gin-gonic/gin"
 	"github.com/hermeznetwork/hermez-node/common"
 	"github.com/hermeznetwork/hermez-node/db/historydb"
@@ -31,12 +32,13 @@ }
 
 // API serves HTTP requests to allow external interaction with the Hermez node
 type API struct {
-	h       *historydb.HistoryDB
-	cg      *configAPI
-	s       *statedb.StateDB
-	l2      *l2db.L2DB
-	status  Status
-	chainID uint16
+	h             *historydb.HistoryDB
+	cg            *configAPI
+	s             *statedb.StateDB
+	l2            *l2db.L2DB
+	status        Status
+	chainID       uint16
+	hermezAddress ethCommon.Address
 }
 
 // NewAPI sets the endpoints and the appropriate handlers, but doesn't start the server
@@ -47,7 +49,6 @@ hdb *historydb.HistoryDB,
 	sdb *statedb.StateDB,
 	l2db *l2db.L2DB,
 	config *Config,
-	chainID uint16,
 ) (*API, error) {
 	// Check input
 	// TODO: is stateDB only needed for explorer endpoints or for both?
@@ -65,10 +66,11 @@ RollupConstants:   *newRollupConstants(config.RollupConstants),
 			AuctionConstants:  config.AuctionConstants,
 			WDelayerConstants: config.WDelayerConstants,
 		},
-		s:       sdb,
-		l2:      l2db,
-		status:  Status{},
-		chainID: chainID,
+		s:             sdb,
+		l2:            l2db,
+		status:        Status{},
+		chainID:       config.ChainID,
+		hermezAddress: config.HermezAddress,
 	}
 
 	// Add coordinator endpoints

@@ -221,7 +221,7 @@ l2DB := l2db.NewL2DB(database, 10, 100, 24*time.Hour)
 	test.WipeDB(l2DB.DB()) // this will clean HistoryDB and L2DB
 
 	// Config (smart contract constants)
-	_config := getConfigTest()
+	_config := getConfigTest(chainID)
 	config = configAPI{
 		RollupConstants:   *newRollupConstants(_config.RollupConstants),
 		AuctionConstants:  _config.AuctionConstants,
@@ -238,7 +238,6 @@ hdb,
 		sdb,
 		l2DB,
 		&_config,
-		chainID,
 	)
 	if err != nil {
 		panic(err)
@@ -423,7 +422,7 @@ txs:              testTxs,
 		exits:            testExits,
 		poolTxsToSend:    poolTxsToSend,
 		poolTxsToReceive: poolTxsToReceive,
-		auths:            genTestAuths(test.GenAuths(5)),
+		auths:            genTestAuths(test.GenAuths(5, _config.ChainID, _config.HermezAddress)),
 		router:           router,
 		bids:             testBids,
 		slots: api.genTestSlots(

@@ -4,6 +4,7 @@ import (
 	"math/big"
 	"net/http"
 
+	ethCommon "github.com/ethereum/go-ethereum/common"
 	"github.com/gin-gonic/gin"
 	"github.com/hermeznetwork/hermez-node/common"
 )
@@ -51,6 +52,8 @@ type Config struct {
 	RollupConstants   common.RollupConstants
 	AuctionConstants  common.AuctionConstants
 	WDelayerConstants common.WDelayerConstants
+	ChainID           uint16
+	HermezAddress     ethCommon.Address
 }
 
 type configAPI struct {

@@ -9,7 +9,7 @@ "github.com/hermeznetwork/hermez-node/common"
 	"github.com/stretchr/testify/assert"
 )
 
-func getConfigTest() Config {
+func getConfigTest(chainID uint16) Config {
 	var config Config
 
 	var rollupPublicConstants common.RollupConstants
@@ -39,6 +39,9 @@
 	config.RollupConstants = rollupPublicConstants
 	config.AuctionConstants = auctionConstants
 	config.WDelayerConstants = wdelayerConstants
+
+	config.ChainID = chainID
+	config.HermezAddress = ethCommon.HexToAddress("0xc344E203a046Da13b0B4467EB7B3629D0C99F6E6")
 
 	return config
 }

@@ -1,15 +1,20 @@
 package common
 
 import (
+	"encoding/binary"
 	"time"
 
 	ethCommon "github.com/ethereum/go-ethereum/common"
 	ethCrypto "github.com/ethereum/go-ethereum/crypto"
-	"github.com/hermeznetwork/tracerr"
 	"github.com/iden3/go-iden3-crypto/babyjub"
 )
 
-// AccountCreationAuth authorizations sent by users to the L2DB, to be used for account creations when necessary
+// AccountCreationAuthMsg is the message that is signed to authorize an account
+// creation
+const AccountCreationAuthMsg = "I authorize this babyjubjub key for hermez rollup account creation"
+
+// AccountCreationAuth authorizations sent by users to the L2DB, to be used for
+// account creations when necessary
 type AccountCreationAuth struct {
 	EthAddr   ethCommon.Address     `meddler:"eth_addr"`
 	BJJ       babyjub.PublicKeyComp `meddler:"bjj"`
@@ -18,21 +23,21 @@ Timestamp time.Time             `meddler:"timestamp,utctime"`
 }
 
 // HashToSign builds the hash to be signed using BJJ pub key and the constant message
-func (a *AccountCreationAuth) HashToSign() ([]byte, error) {
+func (a *AccountCreationAuth) HashToSign(chainID uint16,
+	hermezContractAddr ethCommon.Address) ([]byte, error) {
 	// Calculate message to be signed
-	const msg = "I authorize this babyjubjub key for hermez rollup account creation"
-	comp, err := a.BJJ.MarshalText()
-	if err != nil {
-		return nil, tracerr.Wrap(err)
-	}
-	// Hash message (msg || compressed-bjj)
-	return ethCrypto.Keccak256Hash([]byte(msg), comp).Bytes(), nil
+	var chainIDBytes [2]byte
+	binary.BigEndian.PutUint16(chainIDBytes[:], chainID)
+	// to hash: [AccountCreationAuthMsg | compressedBJJ | chainID | hermezContractAddr]
+	return ethCrypto.Keccak256Hash([]byte(AccountCreationAuthMsg), a.BJJ[:], chainIDBytes[:],
+		hermezContractAddr[:]).Bytes(), nil
 }
 
 // VerifySignature ensures that the Signature is done with the specified EthAddr
-func (a *AccountCreationAuth) VerifySignature() bool {
+func (a *AccountCreationAuth) VerifySignature(chainID uint16,
+	hermezContractAddr ethCommon.Address) bool {
 	// Calculate hash to be signed
-	msg, err := a.HashToSign()
+	msg, err := a.HashToSign(chainID, hermezContractAddr)
 	if err != nil {
 		return false
 	}

@@ -0,0 +1,41 @@
+package common
+
+import (
+	"encoding/hex"
+	"testing"
+
+	ethCommon "github.com/ethereum/go-ethereum/common"
+	ethCrypto "github.com/ethereum/go-ethereum/crypto"
+	"github.com/iden3/go-iden3-crypto/babyjub"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestAccountCreationAuth(t *testing.T) {
+	// Ethereum key
+	ethSk, err := ethCrypto.HexToECDSA("fad9c8855b740a0b7ed4c221dbad0f33a83a49cad6b3fe8d5817ac83d38b6a19")
+	require.NoError(t, err)
+	ethAddr := ethCrypto.PubkeyToAddress(ethSk.PublicKey)
+
+	// BabyJubJub key
+	var sk babyjub.PrivateKey
+	_, err = hex.Decode(sk[:], []byte("0001020304050607080900010203040506070809000102030405060708090001"))
+	assert.NoError(t, err)
+
+	chainID := uint16(0)
+	hermezContractAddr := ethCommon.HexToAddress("0xc344E203a046Da13b0B4467EB7B3629D0C99F6E6")
+	a := AccountCreationAuth{
+		EthAddr: ethAddr,
+		BJJ:     sk.Public().Compress(),
+	}
+	msg, err := a.HashToSign(chainID, hermezContractAddr)
+	assert.NoError(t, err)
+	assert.Equal(t, "cb5a7e44329ff430c81fec49fb2ac6741f02d5ec96cbcb618a6991f0a9c80ffd", hex.EncodeToString(msg))
+
+	// sign AccountCreationAuth with eth key
+	sig, err := ethCrypto.Sign(msg, ethSk)
+	assert.NoError(t, err)
+	a.Signature = sig
+
+	assert.True(t, a.VerifySignature(chainID, hermezContractAddr))
+}

@@ -611,8 +611,10 @@
 func TestAuth(t *testing.T) {
 	test.WipeDB(l2DB.DB())
 	const nAuths = 5
+	chainID := uint16(0)
+	hermezContractAddr := ethCommon.HexToAddress("0xc344E203a046Da13b0B4467EB7B3629D0C99F6E6")
 	// Generate authorizations
-	auths := test.GenAuths(nAuths)
+	auths := test.GenAuths(nAuths, chainID, hermezContractAddr)
 	for i := 0; i < len(auths); i++ {
 		// Add to the DB
 		err := l2DB.AddAccountCreationAuth(auths[i])

@@ -245,8 +245,9 @@ &api.Config{
 				RollupConstants:   scConsts.Rollup,
 				AuctionConstants:  scConsts.Auction,
 				WDelayerConstants: scConsts.WDelayer,
+				ChainID:           chainIDU16,
+				HermezAddress:     cfg.SmartContracts.Rollup,
 			},
-			chainIDU16,
 		)
 		if err != nil {
 			return nil, tracerr.Wrap(err)
@@ -301,7 +302,6 @@ hdb *historydb.HistoryDB,
 	sdb *statedb.StateDB,
 	l2db *l2db.L2DB,
 	config *api.Config,
-	chainID uint16,
 ) (*NodeAPI, error) {
 	engine := gin.Default()
 	engine.NoRoute(handleNoRoute)
@@ -313,7 +313,6 @@ hdb,
 		sdb,
 		l2db,
 		config,
-		chainID,
 	)
 	if err != nil {
 		return nil, tracerr.Wrap(err)

@@ -1,6 +1,7 @@
 package test
 
 import (
+	ethCommon "github.com/ethereum/go-ethereum/common"
 	ethCrypto "github.com/ethereum/go-ethereum/crypto"
 	"github.com/hermeznetwork/hermez-node/common"
 	"github.com/iden3/go-iden3-crypto/babyjub"
@@ -66,7 +67,7 @@ return nil
 }
 
 // GenAuths generates account creation authorizations
-func GenAuths(nAuths int) []*common.AccountCreationAuth {
+func GenAuths(nAuths int, chainID uint16, hermezContractAddr ethCommon.Address) []*common.AccountCreationAuth {
 	auths := []*common.AccountCreationAuth{}
 	for i := 0; i < nAuths; i++ {
 		// Generate keys
@@ -81,7 +82,7 @@ EthAddr: ethCrypto.PubkeyToAddress(ethPrivK.PublicKey),
 			BJJ:     bjjPrivK.Public().Compress(),
 		}
 		// Sign
-		h, err := auth.HashToSign()
+		h, err := auth.HashToSign(chainID, hermezContractAddr)
 		if err != nil {
 			panic(err)
 		}

@@ -3,13 +3,17 @@
 import (
 	"testing"
 
+	ethCommon "github.com/ethereum/go-ethereum/common"
 	"github.com/stretchr/testify/assert"
 )
 
 func TestGenAuths(t *testing.T) {
+	chainID := uint16(0)
+	hermezContractAddr := ethCommon.HexToAddress("0xc344E203a046Da13b0B4467EB7B3629D0C99F6E6")
+
 	const nAuths = 5
-	auths := GenAuths(nAuths)
+	auths := GenAuths(nAuths, chainID, hermezContractAddr)
 	for _, auth := range auths {
-		assert.True(t, auth.VerifySignature())
+		assert.True(t, auth.VerifySignature(chainID, hermezContractAddr))
 	}
 }