本文介绍以太坊（Ethereum）的转账，依据web3j库实现。
概念介绍
DSA一种公开密钥算法，它不能用作加密，只用作数字签名。参考
ECDSA椭圆曲线数字签名算法（ECDSA）是使用椭圆曲线密码（ECC）对数字签名算法（DSA）的加密。生成的r、s签名值参考
一、解码钱包
也就是是根据用户密码从钱包读出keystore信息。这基本就是钱包生成的逆向流程。
1.将用户密码根据scrypt算法重新生成derivedKey.如下图红框所示，跟create()相互对照
2.根据derivedKey调用performCipherOperation()解密方法得到私钥。如下图蓝框所示，跟create()相互对照
3.将私钥传给ECKeyPair::create()便可重新得到公钥。具体调用Sign::publicKeyFromPrivate():BigInteger感兴趣的可以追进去看看。
4.根据ECKeyPair生成Credentials类，这个类主要包含ECKeyPair和钱包地址。
这个地方需要注意的是，钱包地址是重新根据公钥生成的，而不是从文件里读取出来。
大伙想一下这样做有什么好处？（安全呗，这不是p话么，放文件里被篡改了咋办。）
decrypt.jpg
具体代码
//Wallet.java内解码钱包
public static ECKeyPair decrypt(String password, WalletFile walletFile)throws CipherException {
validate(walletFile);
WalletFile.Crypto crypto = walletFile.getCrypto();
byte[] mac = Numeric.hexStringToByteArray(crypto.getMac());
byte[] iv = Numeric.hexStringToByteArray(crypto.getCipherparams().getIv());
byte[] cipherText = Numeric.hexStringToByteArray(crypto.getCiphertext());
byte[] derivedKey;
//获得scrypt加密的相关参数，并解码用户密码。
WalletFile.KdfParams kdfParams = crypto.getKdfparams();
if (kdfParams instanceof WalletFile.ScryptKdfParams) {
WalletFile.ScryptKdfParams scryptKdfParams =
(WalletFile.ScryptKdfParams) crypto.getKdfparams();
int dklen = scryptKdfParams.getDklen();
int n = scryptKdfParams.getN();
int p = scryptKdfParams.getP();
int r = scryptKdfParams.getR();
byte[] salt = Numeric.hexStringToByteArray(scryptKdfParams.getSalt());
derivedKey = generateDerivedScryptKey(
password.getBytes(Charset.forName("UTF-8")), salt, n, r, p, dklen);
} else if (kdfParams instanceof WalletFile.Aes128CtrKdfParams) {
WalletFile.Aes128CtrKdfParams aes128CtrKdfParams =
(WalletFile.Aes128CtrKdfParams) crypto.getKdfparams();
int c = aes128CtrKdfParams.getC();
String prf = aes128CtrKdfParams.getPrf();
byte[] salt = Numeric.hexStringToByteArray(aes128CtrKdfParams.getSalt());
derivedKey = generateAes128CtrDerivedKey(
password.getBytes(Charset.forName("UTF-8")), salt, c, prf);
} else {
throw new CipherException("Unable to deserialize params: " + crypto.getKdf());
}
byte[] derivedMac = generateMac(derivedKey, cipherText);
if (!Arrays.equals(derivedMac, mac)) {
throw new CipherException("Invalid password provided");
}
byte[] encryptKey = Arrays.copyOfRange(derivedKey, 0, 16);
//根据用户密码生成的encryptKey解码cipherText获得私钥
byte[] privateKey = performCipherOperation(Cipher.DECRYPT_MODE, iv, encryptKey, cipherText);
return ECKeyPair.create(privateKey);
}
//Credentials.java内，根据ECKeyPair参数重新获取地址并保存到当前类内。
public static Credentials create(ECKeyPair ecKeyPair) {
String address = Numeric.prependHexPrefix(Keys.getAddress(ecKeyPair));
return new Credentials(ecKeyPair, address);
}
经过上面的步骤我们便解码了钱包，后面就可以根据这些信息执行转账功能了。
二、获取Nonce
nonce：整数类型，会随着账户下的交易不断累加。作用是“防止交易的重播攻击”。
我们通过调用ethscan的相关接口查询到上次交易的nonce值，此值从0开始。
三、代码处理
开始之前先介绍一个r、s、v的概念，其中r、s便是ECDSA签名值。v是chainid.
1.根据nonce以及gasPrice、gasLimit等初始化RawTransaction类
也就是交易描述文件。
RawTransaction.createTransaction()
2.根据描述文件生成byte文件。TransactionEncoder.signMessage()
此文件为在网络上传输的文件。此步骤会根据ECDSA进行数字签名以及加密。
3.调用api?action=eth_sendRawTransaction将描述文件发送到相关服务器。
4.服务器将此文件广播到ETH公链。
接口调用代码，具体见Github内TransactionService.kt类
class TransactionService : IntentService("Transaction Service") {
private var builder: NotificationCompat.Builder? = null
internal val mNotificationId = 153
override fun onHandleIntent(intent: Intent?) {
sendNotification()
try {
val fromAddress = intent!!.getStringExtra("FROM_ADDRESS")
val toAddress = intent.getStringExtra("TO_ADDRESS")
val amount = intent.getStringExtra("AMOUNT")
val gas_price = intent.getStringExtra("GAS_PRICE")
val gas_limit = intent.getStringExtra("GAS_LIMIT")
val data = intent.getStringExtra("DATA")
val password = intent.getStringExtra("PASSWORD")
val keys = WalletStorage.getInstance(applicationContext).getFullWallet(applicationContext, password, fromAddress)
EtherscanAPI.INSTANCE.getNonceForAddress(fromAddress)
.subscribe(
object : SingleObserver<NonceForAddress> {
override fun onSuccess(t: NonceForAddress) {
if (t.result.length < 2) return
val nonce = BigInteger(t.result.substring(2), 16)
val tx = RawTransaction.createTransaction(
nonce,
BigInteger(gas_price),
BigInteger(gas_limit),
toAddress,
BigDecimal(amount).multiply(ExchangeCalculator.ONE_ETHER).toBigInteger(),
data
)
Log.d("Aaron",
"Nonce: " + tx.nonce + "\n" +
"gasPrice: " + tx.gasPrice + "\n" +
"gasLimit: " + tx.gasLimit + "\n" +
"To: " + tx.to + "\n" +
"Amount: " + tx.value + "\n" +
"Data: " + tx.data
)
val signed = TransactionEncoder.signMessage(tx, 1.toByte(), keys)
forwardTX(signed)
}
override fun onSubscribe(d: Disposable) {
}
override fun onError(e: Throwable) {
error("Can't connect to network, retry it later")
}
}
)
} catch (e: Exception) {
error("Invalid Wallet Password!")
e.printStackTrace()
}
}
@Throws(IOException::class)
private fun forwardTX(signed: ByteArray) {
EtherscanAPI.INSTANCE.forwardTransaction("0x" + Hex.toHexString(signed))
.subscribe(
object : SingleObserver<ForwardTX> {
override fun onSuccess(t: ForwardTX) {
if (!TextUtils.isEmpty(t.result)) {
suc(t.result)
} else {
var errormsg = t.error.message
if (errormsg.indexOf(".") > 0)
errormsg = errormsg.substring(0, errormsg.indexOf("."))
error(errormsg) // f.E Insufficient funds
}
}
override fun onSubscribe(d: Disposable) {
}
override fun onError(e: Throwable) {
error("Can't connect to network, retry it later")
}
}
)
}
private fun suc(hash: String) {
builder!!
.setContentTitle(getString(R.string.notification_transfersuc))
.setProgress(100, 100, false)
.setOngoing(false)
.setAutoCancel(true)
.setContentText("")
val main = Intent(this, MainActivity::class.java)
main.putExtra("STARTAT", 2)
main.putExtra("TXHASH", hash)
val contentIntent = PendingIntent.getActivity(this, 0,
main, PendingIntent.FLAG_UPDATE_CURRENT)
builder!!.setContentIntent(contentIntent)
val mNotifyMgr = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
mNotifyMgr.notify(mNotificationId, builder!!.build())
}
private fun error(err: String) {
builder!!
.setContentTitle(getString(R.string.notification_transferfail))
.setProgress(100, 100, false)
.setOngoing(false)
.setAutoCancel(true)
.setContentText(err)
val main = Intent(this, MainActivity::class.java)
main.putExtra("STARTAT", 2)
val contentIntent = PendingIntent.getActivity(this, 0,
main, PendingIntent.FLAG_UPDATE_CURRENT)
builder!!.setContentIntent(contentIntent)
val mNotifyMgr = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
mNotifyMgr.notify(mNotificationId, builder!!.build())
}
private fun sendNotification() {
builder = NotificationCompat.Builder(this)
.setSmallIcon(R.drawable.ic_notification)
.setColor(0x2d435c)
.setTicker(getString(R.string.notification_transferingticker))
.setContentTitle(getString(R.string.notification_transfering_title))
.setContentText(getString(R.string.notification_might_take_a_minute))
.setOngoing(true)
.setProgress(0, 0, true)
val mNotifyMgr = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
mNotifyMgr.notify(mNotificationId, builder!!.build())
}
}
