From 2ce04f8f595ad4442a54cbeb7fcd026f657d2934 Mon Sep 17 00:00:00 2001
From: Ivan Kuznetsov <me@jeiwan.ru>
Date: Sun, 10 Sep 2017 10:34:39 +0700
Subject: [PATCH] Implement transactions signing and verification

---
 blockchain.go  | 66 ++++++++++++++++++++++++++++++++++++++++++++++++++
 transaction.go | 12 ++++++---
 2 files changed, 74 insertions(+), 4 deletions(-)

diff --git a/blockchain.go b/blockchain.go
index d644fe3..48709ae 100644
--- a/blockchain.go
+++ b/blockchain.go
@@ -1,7 +1,10 @@
 package main
 
 import (
+	"bytes"
+	"crypto/ecdsa"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"log"
 	"os"
@@ -29,6 +32,12 @@ type BlockchainIterator struct {
 func (bc *Blockchain) MineBlock(transactions []*Transaction) {
 	var lastHash []byte
 
+	for _, tx := range transactions {
+		if bc.VerifyTransaction(tx) != true {
+			log.Panic("ERROR: Invalid transaction")
+		}
+	}
+
 	err := bc.db.View(func(tx *bolt.Tx) error {
 		b := tx.Bucket([]byte(blocksBucket))
 		lastHash = b.Get([]byte("l"))
@@ -60,6 +69,63 @@ func (bc *Blockchain) MineBlock(transactions []*Transaction) {
 	})
 }
 
+// FindTransaction finds a transaction by its ID
+func (bc *Blockchain) FindTransaction(ID []byte) (Transaction, error) {
+	bci := bc.Iterator()
+
+	for {
+		block := bci.Next()
+
+		for _, tx := range block.Transactions {
+			if bytes.Compare(tx.ID, ID) == 0 {
+				return *tx, nil
+			}
+		}
+
+		if len(block.PrevBlockHash) == 0 {
+			break
+		}
+	}
+
+	return Transaction{}, errors.New("Transaction is not found")
+}
+
+// SignTransaction signs a Transaction
+func (bc *Blockchain) SignTransaction(tx *Transaction, privKey ecdsa.PrivateKey) {
+	prevTXs := make(map[string]Transaction)
+
+	for _, vin := range tx.Vin {
+		prevTX, err := bc.FindTransaction(vin.Txid)
+		if err != nil {
+			log.Panic(err)
+		}
+		prevTXs[hex.EncodeToString(prevTX.ID)] = prevTX
+	}
+
+	tx.Sign(privKey, prevTXs)
+}
+
+// VerifyTransaction verifies transaction
+func (bc *Blockchain) VerifyTransaction(tx *Transaction) bool {
+	prevTXs := make(map[string]Transaction)
+
+	for _, vin := range tx.Vin {
+		prevTX, err := bc.FindTransaction(vin.Txid)
+		if err != nil {
+			log.Panic(err)
+		}
+		prevTXs[hex.EncodeToString(prevTX.ID)] = prevTX
+	}
+
+	for _, tx := range prevTXs {
+		fmt.Println(tx)
+	}
+	// fmt.Println()
+	// fmt.Println(tx)
+
+	return tx.Verify(prevTXs)
+}
+
 // FindUnspentTransactions returns a list of transactions containing unspent outputs
 func (bc *Blockchain) FindUnspentTransactions(pubKeyHash []byte) []Transaction {
 	var unspentTXs []Transaction
diff --git a/transaction.go b/transaction.go
index 81bbf46..b7463f1 100644
--- a/transaction.go
+++ b/transaction.go
@@ -36,6 +36,7 @@ func (tx Transaction) String() string {
 	lines = append(lines, fmt.Sprintf("Transaction %x:", tx.ID))
 
 	for i, input := range tx.Vin {
+
 		lines = append(lines, fmt.Sprintf("  Input %d:", i))
 		lines = append(lines, fmt.Sprintf("    TXID:   %x", input.Txid))
 		lines = append(lines, fmt.Sprintf("    Out:    %d", input.Vout))
@@ -84,13 +85,13 @@ func (tx *Transaction) TrimmedCopy() Transaction {
 }
 
 // Sign signs each input of a Transaction
-func (tx *Transaction) Sign(privKey ecdsa.PrivateKey, prevTx *Transaction) {
+func (tx *Transaction) Sign(privKey ecdsa.PrivateKey, prevTXs map[string]Transaction) {
 	if tx.IsCoinbase() {
 		return
 	}
 
 	for _, vin := range tx.Vin {
-		if bytes.Compare(vin.Txid, prevTx.ID) != 0 {
+		if prevTXs[hex.EncodeToString(vin.Txid)].ID == nil {
 			log.Panic("ERROR: Previous transaction is not correct")
 		}
 	}
@@ -98,6 +99,7 @@ func (tx *Transaction) Sign(privKey ecdsa.PrivateKey, prevTx *Transaction) {
 	txCopy := tx.TrimmedCopy()
 
 	for inID, vin := range txCopy.Vin {
+		prevTx := prevTXs[hex.EncodeToString(vin.Txid)]
 		txCopy.Vin[inID].ScriptSig = prevTx.Vout[vin.Vout].ScriptPubKey
 		txCopy.SetID()
 		txCopy.Vin[inID].ScriptSig = []byte{}
@@ -113,7 +115,7 @@ func (tx *Transaction) Sign(privKey ecdsa.PrivateKey, prevTx *Transaction) {
 }
 
 // Verify verifies signatures of Transaction inputs
-func (tx *Transaction) Verify(prevTx *Transaction) bool {
+func (tx *Transaction) Verify(prevTXs map[string]Transaction) bool {
 	sigLen := 64
 
 	if tx.IsCoinbase() {
@@ -121,7 +123,7 @@ func (tx *Transaction) Verify(prevTx *Transaction) bool {
 	}
 
 	for _, vin := range tx.Vin {
-		if bytes.Compare(vin.Txid, prevTx.ID) != 0 {
+		if prevTXs[hex.EncodeToString(vin.Txid)].ID == nil {
 			log.Panic("ERROR: Previous transaction is not correct")
 		}
 	}
@@ -130,6 +132,7 @@ func (tx *Transaction) Verify(prevTx *Transaction) bool {
 	curve := elliptic.P256()
 
 	for inID, vin := range tx.Vin {
+		prevTx := prevTXs[hex.EncodeToString(vin.Txid)]
 		txCopy.Vin[inID].ScriptSig = prevTx.Vout[vin.Vout].ScriptPubKey
 		txCopy.SetID()
 		txCopy.Vin[inID].ScriptSig = []byte{}
@@ -249,6 +252,7 @@ func NewUTXOTransaction(from, to string, amount int, bc *Blockchain) *Transactio
 
 	tx := Transaction{nil, inputs, outputs}
 	tx.SetID()
+	bc.SignTransaction(&tx, wallet.PrivateKey)
 
 	return &tx
 }