add ExtendedKey.ChildForPurpose method to derive wallet or chat keys (#1312)

* add ExtendedKey.ChildForPurpose method to derive wallet or chat keys

* Update extkeys/hdkey.go

Co-Authored-By: gravityblast <andrea@gravityblast.com>

* Update extkeys/hdkey_test.go

Co-Authored-By: gravityblast <andrea@gravityblast.com>

* Update extkeys/hdkey_test.go

Co-Authored-By: gravityblast <andrea@gravityblast.com>

* Update extkeys/hdkey_test.go

Co-Authored-By: gravityblast <andrea@gravityblast.com>

* Update extkeys/hdkey_test.go

Co-Authored-By: gravityblast <andrea@gravityblast.com>

* refactoring and comments

* set EIP1581KeyTypeChat type to uint32

* set 0x00 instead of zero to avoid lint errors

extkeys/hdkey.go

@@ -41,6 +41,13 @@ import (
 
 // TODO make sure we're doing this ^^^^ !!!!!!
 
+type KeyPurpose int
+
+const (
+	KeyPurposeWallet KeyPurpose = iota + 1
+	KeyPurposeChat
+)
+
 const (
 	// HardenedKeyStart defines a starting point for hardened key.
 	// Each extended key has 2^31 normal child keys and 2^31 hardened child keys.
@@ -79,6 +86,7 @@ const (
 // errors
 var (
 	ErrInvalidKey                 = errors.New("key is invalid")
+	ErrInvalidKeyPurpose          = errors.New("key purpose is invalid")
 	ErrInvalidSeed                = errors.New("seed is invalid")
 	ErrInvalidSeedLen             = fmt.Errorf("the recommended size of seed is %d-%d bits", MinSeedBytes, MaxSeedBytes)
 	ErrDerivingHardenedFromPublic = errors.New("cannot derive a hardened key from public key")
@@ -95,6 +103,25 @@ var (
 
 	// PublicKeyVersion is version for public key
 	PublicKeyVersion, _ = hex.DecodeString("0488B21E")
+
+	// EthBIP44ParentPath is BIP44 keys parent's derivation path
+	EthBIP44ParentPath = []uint32{
+		HardenedKeyStart + 44,          // purpose
+		HardenedKeyStart + CoinTypeETH, // cointype set to ETH
+		HardenedKeyStart + 0,           // account
+		0,                              // 0 - public, 1 - private
+	}
+
+	// EIP1581KeyTypeChat is used as chat key_type in the derivation of EIP1581 keys
+	EIP1581KeyTypeChat uint32 = 0x00
+
+	// EthEIP1581ChatParentPath is EIP-1581 chat keys parent's derivation path
+	EthEIP1581ChatParentPath = []uint32{
+		HardenedKeyStart + 43,                 // purpose
+		HardenedKeyStart + CoinTypeETH,        // cointype set to ETH
+		HardenedKeyStart + 1581,               // EIP-1581 subpurpose
+		HardenedKeyStart + EIP1581KeyTypeChat, // key_type (chat)
+	}
 )
 
 // ExtendedKey represents BIP44-compliant HD key
@@ -236,9 +263,29 @@ func (k *ExtendedKey) Child(i uint32) (*ExtendedKey, error) {
 	return child, nil
 }
 
+// ChildForPurpose derives the child key at index i using a derivation path based on the purpose.
+func (k *ExtendedKey) ChildForPurpose(p KeyPurpose, i uint32) (*ExtendedKey, error) {
+	switch p {
+	case KeyPurposeWallet:
+		return k.EthBIP44Child(i)
+	case KeyPurposeChat:
+		return k.EthEIP1581ChatChild(i)
+	default:
+		return nil, ErrInvalidKeyPurpose
+	}
+}
+
 // BIP44Child returns Status CKD#i (where i is child index).
 // BIP44 format is used: m / purpose' / coin_type' / account' / change / address_index
+// BIP44Child is depracated in favour of EthBIP44Child
+// Param coinType is deprecated; we override it to always use CoinTypeETH.
 func (k *ExtendedKey) BIP44Child(coinType, i uint32) (*ExtendedKey, error) {
+	return k.EthBIP44Child(i)
+}
+
+// BIP44Child returns Status CKD#i (where i is child index).
+// BIP44 format is used: m / purpose' / coin_type' / account' / change / address_index
+func (k *ExtendedKey) EthBIP44Child(i uint32) (*ExtendedKey, error) {
 	if !k.IsPrivate {
 		return nil, ErrInvalidMasterKey
 	}
@@ -248,13 +295,28 @@ func (k *ExtendedKey) BIP44Child(coinType, i uint32) (*ExtendedKey, error) {
 	}
 
 	// m/44'/60'/0'/0/index
-	extKey, err := k.Derive([]uint32{
-		HardenedKeyStart + 44,       // purpose
-		HardenedKeyStart + coinType, // cointype
-		HardenedKeyStart + 0,        // account
-		0,                           // 0 - public, 1 - private
-		i,                           // index
-	})
+	extKey, err := k.Derive(append(EthBIP44ParentPath, i))
+	if err != nil {
+		return nil, err
+	}
+
+	return extKey, nil
+}
+
+// EthEIP1581ChatChild returns the whisper key #i (where i is child index).
+// EthEIP1581ChatChild format is used is the one defined in the EIP-1581:
+// m / 43' / coin_type' / 1581' / key_type / index
+func (k *ExtendedKey) EthEIP1581ChatChild(i uint32) (*ExtendedKey, error) {
+	if !k.IsPrivate {
+		return nil, ErrInvalidMasterKey
+	}
+
+	if k.Depth != 0 {
+		return nil, ErrInvalidMasterKey
+	}
+
+	// m/43'/60'/1581'/0/index
+	extKey, err := k.Derive(append(EthEIP1581ChatParentPath, i))
 	if err != nil {
 		return nil, err
 	}

extkeys/hdkey_test.go

@@ -577,6 +577,48 @@ func TestBIP44ChildDerivation(t *testing.T) {
 	t.Logf("Account 1 key: %s", accounKey2.String())
 }
 
+func TestChildForPurpose(t *testing.T) {
+	masterKey, err := NewKeyFromString(masterPrivKey1)
+	if err != nil {
+		t.Error("NewKeyFromString: cannot create master extended key")
+	}
+
+	bip44Child, err := masterKey.EthBIP44Child(0)
+	if err != nil {
+		t.Error("Error deriving BIP44-compliant key")
+	}
+
+	eip1581Child, err := masterKey.EthEIP1581ChatChild(0)
+	if err != nil {
+		t.Error("Error deriving EIP1581-compliant key")
+	}
+
+	walletChild, err := masterKey.ChildForPurpose(KeyPurposeWallet, 0)
+	if err != nil {
+		t.Error("Error deriving BIP44-compliant key")
+	}
+
+	chatChild, err := masterKey.ChildForPurpose(KeyPurposeChat, 0)
+	if err != nil {
+		t.Error("Error deriving EIP1581-compliant key")
+	}
+
+	// Check that ChildForPurpose with KeyPurposeWallet generates a BIP44 key
+	if walletChild.String() != bip44Child.String() {
+		t.Errorf("wrong wallet key. expected to be equal to bip44Child")
+	}
+
+	// Check that ChildForPurpose with KeyPurposeChat generates a EIP1581 key
+	if chatChild.String() != eip1581Child.String() {
+		t.Errorf("wrong chat key. expected to be equal to eip1581Child")
+	}
+
+	// Check that the key generated by ChildForPurpose with KeyPurposeChat is different from the BIP44
+	if walletChild.String() == chatChild.String() {
+		t.Errorf("wrong chat key. expected to be diferrent from the wallet key")
+	}
+}
+
 func TestHDWalletCompatibility(t *testing.T) {
 	password := "TREZOR"
 	mnemonic := NewMnemonic()