Add key header to encrypted keys

* Add key header containing key version, kdf and kdf params
* Store key header as JSON in the key file
* Read in KDF params from key header
* Include key header in MAC calculation and MAC verification

crypto/key.go

@@ -48,19 +48,34 @@ type plainKeyJSON struct {
 	PrivateKey []byte
 }
 
-type cipherJSON struct {
-	MAC        []byte
-	Salt       []byte
-	IV         []byte
-	CipherText []byte
-}
-
 type encryptedKeyJSON struct {
 	Id      []byte
 	Address []byte
 	Crypto  cipherJSON
 }
 
+type cipherJSON struct {
+	MAC        []byte
+	Salt       []byte
+	IV         []byte
+	KeyHeader  keyHeaderJSON
+	CipherText []byte
+}
+
+type keyHeaderJSON struct {
+	Version   string
+	Kdf       string
+	KdfParams scryptParamsJSON // TODO: make more generic?
+}
+
+type scryptParamsJSON struct {
+	N       int
+	R       int
+	P       int
+	DkLen   int
+	SaltLen int
+}
+
 func (k *Key) MarshalJSON() (j []byte, err error) {
 	jStruct := plainKeyJSON{
 		k.Id,