Add offline wallet guide and movefunds utility (#252)

A guide explaining how to spend from a cold wallet while offline
was added along with a utility, movefunds, enabling easier signing
of offline transactions.

cmd/movefunds/config.json

@@ -0,0 +1,6 @@
+{
+    "txfee": 10000000, 
+    "sendtoaddress": "Ssgymtv6sjYPVEE6mwkbYJz1HzPicKpLCwc",
+    "network": "simnet",
+    "dcrctlargs": "--wallet"
+}

cmd/movefunds/createtx.go

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2016 The Decred developers
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+package main
+
+import (
+	"github.com/decred/dcrd/chaincfg"
+	"github.com/decred/dcrd/txscript"
+	"github.com/decred/dcrd/wire"
+	"github.com/decred/dcrutil"
+)
+
+// makeTx generates a transaction spending outputs to a single address.
+func makeTx(params *chaincfg.Params,
+	inputs []*extendedOutPoint,
+	addr dcrutil.Address,
+	txFee int64) (*wire.MsgTx, error) {
+	mtx := wire.NewMsgTx()
+
+	allInAmts := int64(0)
+	for _, input := range inputs {
+		txIn := wire.NewTxIn(input.op, []byte{})
+		mtx.AddTxIn(txIn)
+		allInAmts += input.amt
+	}
+
+	pkScript, err := txscript.PayToAddrScript(addr)
+	if err != nil {
+		return nil, err
+	}
+	txOut := wire.NewTxOut(allInAmts-txFee, pkScript)
+	txOut.Version = txscript.DefaultScriptVersion
+	mtx.AddTxOut(txOut)
+
+	return mtx, nil
+
+}

cmd/movefunds/feeest.go

@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2016 The Decred developers
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+package main
+
+const (
+	// All transactions have 4 bytes for version, 4 bytes of locktime,
+	// 4 bytes of expiry, and 2 varints for the number of inputs and
+	// outputs, and 1 varint for the witnesses.
+	txOverheadEstimate = 4 + 4 + 4 + 1 + 1 + 1
+
+	// A worst case signature script to redeem a P2PKH output for a
+	// compressed pubkey has 73 bytes of the possible DER signature
+	// (with no leading 0 bytes for R and S), 65 bytes of serialized pubkey,
+	// and data push opcodes for both, plus one byte for the hash type flag
+	// appended to the end of the signature.
+	sigScriptEstimate = 1 + 73 + 1 + 65 + 1
+
+	// A best case tx input serialization cost is 32 bytes of sha, 4 bytes
+	// of output index, 1 byte for tree, 4 bytes of sequence, 12 bytes for
+	// fraud proof, one byte for both the txin signature size (0) and the
+	// witness signature script size, and the estimated signature script
+	// size.
+	txInEstimate = 32 + 4 + 1 + 12 + 4 + 1 + 1 + sigScriptEstimate
+
+	// A P2PKH pkScript contains the following bytes:
+	//  - OP_DUP
+	//  - OP_HASH160
+	//  - OP_DATA_20 + 20 bytes of pubkey hash
+	//  - OP_EQUALVERIFY
+	//  - OP_CHECKSIG
+	pkScriptEstimate = 1 + 1 + 1 + 20 + 1 + 1
+
+	// txOutEstimate is a best case tx output serialization cost is 8 bytes
+	// of value, two bytes of version, one byte of varint, and the pkScript
+	// size.
+	txOutEstimate = 8 + 2 + 1 + pkScriptEstimate
+)
+
+var (
+	// maxTxSize is the maximum size of a transaction we can
+	// build with the wallet.
+	maxTxSize int
+)
+
+func estimateTxSize(numInputs, numOutputs int) int {
+	return txOverheadEstimate + txInEstimate*numInputs + txOutEstimate*numOutputs
+}

cmd/movefunds/main.go

@@ -0,0 +1,217 @@
+/*
+ * Copyright (c) 2016 The Decred developers
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+package main
+
+import (
+	"bytes"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"math"
+	"os"
+	"sort"
+
+	"github.com/decred/dcrd/chaincfg"
+	"github.com/decred/dcrd/chaincfg/chainhash"
+	"github.com/decred/dcrd/dcrjson"
+	"github.com/decred/dcrd/wire"
+	"github.com/decred/dcrutil"
+)
+
+// params is the global representing the chain parameters. It is assigned
+// in main.
+var params *chaincfg.Params
+
+// configJSON is a configuration file used for transaction generation.
+type configJSON struct {
+	TxFee         int64  `json:"txfee"`
+	SendToAddress string `json:"sendtoaddress"`
+	Network       string `json:"network"`
+	DcrctlArgs    string `json:"dcrctlargs"`
+}
+
+// extendedOutPoint is a UTXO with an amount.
+type extendedOutPoint struct {
+	op       *wire.OutPoint
+	amt      int64
+	pkScript []byte
+}
+
+// extendedOutPoints is an extendedOutPoint used for sorting by UTXO amount.
+type extendedOutPoints struct {
+	eops []*extendedOutPoint
+}
+
+func (e extendedOutPoints) Len() int { return len(e.eops) }
+func (e extendedOutPoints) Less(i, j int) bool {
+	return e.eops[i].amt < e.eops[j].amt
+}
+func (e extendedOutPoints) Swap(i, j int) {
+	e.eops[i], e.eops[j] = e.eops[j], e.eops[i]
+}
+
+// convertJSONUnspentToOutPoints converts a JSON raw dump from listunspent to
+// a set of UTXOs.
+func convertJSONUnspentToOutPoints(
+	utxos []dcrjson.ListUnspentResult) []*extendedOutPoint {
+	var eops []*extendedOutPoint
+	for _, utxo := range utxos {
+		if utxo.TxType == 1 && utxo.Vout == 0 {
+			continue
+		}
+
+		op := new(wire.OutPoint)
+		hash, _ := chainhash.NewHashFromStr(utxo.TxID)
+		op.Hash = *hash
+		op.Index = uint32(utxo.Vout)
+		op.Tree = int8(utxo.Tree)
+
+		pks, err := hex.DecodeString(utxo.ScriptPubKey)
+		if err != nil {
+			fmt.Println("failure decoding pkscript from unspent list")
+			os.Exit(1)
+		}
+
+		eop := new(extendedOutPoint)
+		eop.op = op
+		amtCast, _ := dcrutil.NewAmount(utxo.Amount)
+		eop.amt = int64(amtCast)
+		eop.pkScript = pks
+
+		eops = append(eops, eop)
+	}
+
+	return eops
+}
+
+func main() {
+	// 1. Load the UTXOs ----------------------------------------------------------
+	unspentFile, err := os.Open("unspent.json")
+	if err != nil {
+		fmt.Println("error opening unspent file unspent.json", err.Error())
+	}
+
+	var utxos []dcrjson.ListUnspentResult
+
+	jsonParser := json.NewDecoder(unspentFile)
+	if err = jsonParser.Decode(&utxos); err != nil {
+		fmt.Println("error parsing unspent file", err.Error())
+	}
+
+	// Sort the inputs so that the largest one is first.
+	inputs := extendedOutPoints{convertJSONUnspentToOutPoints(utxos)}
+	sort.Sort(sort.Reverse(inputs))
+
+	// 2. Load the config ---------------------------------------------------------
+	configFile, err := os.Open("config.json")
+	if err != nil {
+		fmt.Println("error opening config file config.json", err.Error())
+	}
+
+	cfg := new(configJSON)
+
+	jsonParser = json.NewDecoder(configFile)
+	if err = jsonParser.Decode(cfg); err != nil {
+		fmt.Println("error parsing config file", err.Error())
+	}
+
+	// 3. Check the config and parse ----------------------------------------------
+	switch cfg.Network {
+	case "testnet":
+		params = &chaincfg.TestNetParams
+	case "mainnet":
+		params = &chaincfg.MainNetParams
+	case "simnet":
+		params = &chaincfg.SimNetParams
+	default:
+		fmt.Println("Failed to parse a correct network")
+		return
+	}
+
+	maxTxSize = params.MaximumBlockSize - 75000
+
+	sendToAddress, err := dcrutil.DecodeAddress(cfg.SendToAddress, params)
+	if err != nil {
+		fmt.Println("Failed to parse tx address: ", err.Error())
+	}
+
+	// 4. Create the transaction --------------------------------------------------
+	// First get how much we're sending.
+	allInAmts := int64(0)
+	var utxosToUse []*extendedOutPoint
+	for _, utxo := range inputs.eops {
+		utxosToUse = append(utxosToUse, utxo)
+		allInAmts += utxo.amt
+	}
+
+	// Convert to KB.
+	sz := float64(estimateTxSize(len(utxosToUse), 1)) / 1000
+	feeEst := int64(math.Ceil(sz * float64(cfg.TxFee)))
+
+	tx, err := makeTx(params, utxosToUse, sendToAddress, feeEst)
+	if err != nil {
+		fmt.Println("Couldn't produce tx: ", err.Error())
+		return
+	}
+
+	if tx.SerializeSize() > maxTxSize {
+		fmt.Printf("tx too big: got %v, max %v", tx.SerializeSize(),
+			maxTxSize)
+		return
+	}
+
+	// 5. Write the transactions to files in raw form with the proper command
+	// required to sign them.
+	txB, err := tx.Bytes()
+	if err != nil {
+		fmt.Println("Failed to serialize tx: ", err.Error())
+		return
+	}
+
+	// The command to sign the transaction.
+	var buf bytes.Buffer
+	buf.WriteString("dcrctl ")
+	buf.WriteString(cfg.DcrctlArgs)
+	buf.WriteString(" signrawtransaction ")
+	buf.WriteString(hex.EncodeToString(txB))
+	buf.WriteString(" '[")
+	last := len(utxosToUse) - 1
+	for i, utxo := range utxosToUse {
+		buf.WriteString("{\"txid\":\"")
+		buf.WriteString(utxo.op.Hash.String())
+		buf.WriteString("\",\"vout\":")
+		buf.WriteString(fmt.Sprintf("%v", utxo.op.Index))
+		buf.WriteString(",\"tree\":")
+		buf.WriteString(fmt.Sprintf("%v", utxo.op.Tree))
+		buf.WriteString(",\"scriptpubkey\":\"")
+		buf.WriteString(hex.EncodeToString(utxo.pkScript))
+		buf.WriteString("\",\"redeemscript\":\"\"}")
+		if i != last {
+			buf.WriteString(",")
+		}
+	}
+	buf.WriteString("]' ")
+	buf.WriteString("| jq -r .hex")
+	err = ioutil.WriteFile("sign.sh", []byte(buf.String()), 0755)
+	if err != nil {
+		fmt.Println("Failed to write signing script: ", err.Error())
+		return
+	}
+
+	fmt.Println("Successfully wrote transaction to sign script.")
+}

docs/README.md

@@ -1,3 +1,5 @@
 ### Guides
 
 [Rebuilding all transaction history with forced rescans](https://github.com/decred/dcrwallet/tree/master/docs/force_rescans.md)
+
+[Spending funds offline using cold wallets](https://github.com/decred/dcrwallet/tree/master/docs/offline_wallets.md)