[NOD-374] Create transaction signer tool (#435)

* [NOD-374 ] Create transaction signer tool

* [NOD-374] Rename variables

* [NOD-374] Add network selection; Move error handling to control-flow function

* [NOD-374] Rename variables

* [NOD-374] Add new line after if blocks

* [NOD-374] Fix formatting error (gofmt)

cmd/txsigner/config.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"fmt"
+	"github.com/daglabs/btcd/dagconfig"
+	"github.com/jessevdk/go-flags"
+	"os"
+)
+
+var activeNetParams = &dagconfig.MainNetParams
+
+type config struct {
+	Transaction    string `long:"transaction" short:"t" description:"Unsigned transaction in HEX format" required:"true"`
+	PrivateKey     string `long:"private-key" short:"p" description:"Private key" required:"true"`
+	TestNet        bool   `long:"testnet" description:"Use the test network"`
+	RegressionTest bool   `long:"regtest" description:"Use the regression test network"`
+	SimNet         bool   `long:"simnet" description:"Use the simulation test network"`
+	DevNet         bool   `long:"devnet" description:"Use the development test network"`
+}
+
+func parseCommandLine() (*config, error) {
+	cfg := &config{}
+	parser := flags.NewParser(cfg, flags.PrintErrors|flags.HelpFlag)
+	_, err := parser.Parse()
+	// Multiple networks can't be selected simultaneously.
+	funcName := "loadConfig"
+	numNets := 0
+	// Count number of network flags passed; assign active network params
+	// while we're at it
+	if cfg.TestNet {
+		numNets++
+		activeNetParams = &dagconfig.TestNetParams
+	}
+	if cfg.RegressionTest {
+		numNets++
+		activeNetParams = &dagconfig.RegressionNetParams
+	}
+	if cfg.SimNet {
+		numNets++
+		activeNetParams = &dagconfig.SimNetParams
+	}
+	if cfg.DevNet {
+		numNets++
+		activeNetParams = &dagconfig.DevNetParams
+	}
+	if numNets > 1 {
+		str := "%s: The testnet, regtest, simnet and devent params can't be " +
+			"used together -- choose one of the four"
+		err := fmt.Errorf(str, funcName)
+		fmt.Fprintln(os.Stderr, err)
+		parser.WriteHelp(os.Stderr)
+		return nil, err
+	}
+
+	return cfg, err
+}

cmd/txsigner/txsigner.go

@@ -0,0 +1,88 @@
+package main
+
+import (
+	"bytes"
+	"encoding/hex"
+	"fmt"
+	"github.com/daglabs/btcd/btcec"
+	"github.com/daglabs/btcd/txscript"
+	"github.com/daglabs/btcd/util"
+	"github.com/daglabs/btcd/wire"
+	"os"
+)
+
+func main() {
+	cfg, err := parseCommandLine()
+	if err != nil {
+		printErrorAndExit(err, "Failed to parse arguments")
+	}
+
+	privateKey, err := parsePrivateKey(cfg.PrivateKey)
+	if err != nil {
+		printErrorAndExit(err, "Failed to decode private key")
+	}
+
+	transaction, err := parseTransaction(cfg.Transaction)
+	if err != nil {
+		printErrorAndExit(err, "Failed to decode transaction")
+	}
+
+	scriptPubKey, err := createScriptPubKey(privateKey.PubKey())
+	if err != nil {
+		printErrorAndExit(err, "Failed to create scriptPubKey")
+	}
+
+	err = signTransaction(transaction, privateKey, scriptPubKey)
+	if err != nil {
+		printErrorAndExit(err, "Failed to sign transaction")
+	}
+
+	serializedTransaction, err := serializeTransaction(transaction)
+	if err != nil {
+		printErrorAndExit(err, "Failed to serialize transaction")
+	}
+
+	fmt.Printf("Signed Transaction (hex): %s\n\n", serializedTransaction)
+}
+
+func parsePrivateKey(privateKeyHex string) (*btcec.PrivateKey, error) {
+	privateKeyBytes, err := hex.DecodeString(privateKeyHex)
+	privateKey, _ := btcec.PrivKeyFromBytes(btcec.S256(), privateKeyBytes)
+	return privateKey, err
+}
+
+func parseTransaction(transactionHex string) (*wire.MsgTx, error) {
+	serializedTx, err := hex.DecodeString(transactionHex)
+	var transaction wire.MsgTx
+	err = transaction.Deserialize(bytes.NewReader(serializedTx))
+	return &transaction, err
+}
+
+func createScriptPubKey(publicKey *btcec.PublicKey) ([]byte, error) {
+	p2pkhAddress, err := util.NewAddressPubKeyHashFromPublicKey(publicKey.SerializeCompressed(), activeNetParams.Prefix)
+	scriptPubKey, err := txscript.PayToAddrScript(p2pkhAddress)
+	return scriptPubKey, err
+}
+
+func signTransaction(transaction *wire.MsgTx, privateKey *btcec.PrivateKey, scriptPubKey []byte) error {
+	for i, transactionInput := range transaction.TxIn {
+		signatureScript, err := txscript.SignatureScript(transaction, i, scriptPubKey, txscript.SigHashAll, privateKey, true)
+		if err != nil {
+			return err
+		}
+		transactionInput.SignatureScript = signatureScript
+	}
+	return nil
+}
+
+func serializeTransaction(transaction *wire.MsgTx) (string, error) {
+	buf := bytes.NewBuffer(make([]byte, 0, transaction.SerializeSize()))
+	err := transaction.Serialize(buf)
+	serializedTransaction := hex.EncodeToString(buf.Bytes())
+	return serializedTransaction, err
+}
+
+func printErrorAndExit(err error, message string) {
+	fmt.Fprintf(os.Stderr, "%s: %s", message, err)
+	os.Exit(1)
+}