KireinaHoro · October 23, 2019 16:05 · Oct 23, 2019
diff --git a/hashcash.go b/hashcash.go
@@ -0,0 +1,117 @@
+package main
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"math/big"
+	"os"
+	"strconv"
+)
+
+const Id = "1700012980"
+
+type Input struct {
+	Block          int
+	Version        string
+	HashPrevBlock  string `json:"hashPrevBlock"`
+	HashMerkleRoot string `json:"hashMerkleRoot"`
+	Time           string
+	Bits           string
+}
+
+type Output struct {
+	Block int
+	Nonce string
+	Id    string
+}
+
+func reverse(a []byte) {
+	for i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 {
+		a[i], a[j] = a[j], a[i]
+	}
+}
+
+func reverseString(a string) string {
+	runes := []rune(a)
+	for i, j := 0, len(runes)-1; i < j; i, j = i+1, j-1 {
+		runes[i], runes[j] = runes[j], runes[i]
+	}
+	return string(runes)
+}
+
+func decodeToLittleEndian(str string) []byte {
+	ret, err := hex.DecodeString(str[2:])
+	if err != nil {
+		panic(err)
+	}
+	reverse(ret)
+	return ret
+}
+
+func constructHeader(input Input) []byte {
+	var ret []byte
+	ret = append(ret, decodeToLittleEndian(input.Version)...)
+	ret = append(ret, decodeToLittleEndian(input.HashPrevBlock)...)
+	ret = append(ret, decodeToLittleEndian(input.HashMerkleRoot)...)
+	ret = append(ret, decodeToLittleEndian(input.Time)...)
+	ret = append(ret, decodeToLittleEndian(input.Bits)...)
+	return ret
+}
+
+func hashOnce(headerWithoutNonce []byte, nonce uint32) []byte {
+	bs := make([]byte, 4)
+	binary.LittleEndian.PutUint32(bs, nonce)
+	fullBlock := append(headerWithoutNonce, bs...)
+	firstPass := sha256.Sum256(fullBlock)
+	secondPass := sha256.Sum256(firstPass[:])
+	return secondPass[:]
+}
+
+func findNonceWithHint(headerWithoutNonce []byte, hint uint32, input Input) *Output {
+	bits, _ := strconv.ParseUint(input.Bits[2:], 16, 32)
+	mantissa := bits & 0x00ffffff
+	exponent := (bits & 0xff000000) >> 24
+	target := big.NewInt(int64(mantissa))
+	target.Mul(target, new(big.Int).Exp(big.NewInt(256), big.NewInt(int64(exponent)-3), nil))
+	targetStr := fmt.Sprintf("%064x", target)
+	fmt.Printf("Target: 0x%s\n", targetStr)
+
+	for nonce, i := hint, 0; nonce > 0; nonce, i = nonce+1, i+1 {
+		if i%1000 == 0 {
+			fmt.Printf("%d...", i)
+			if i%10000 == 0 {
+				fmt.Print("\n")
+			}
+		}
+		hashStr := reverseString(hex.EncodeToString(hashOnce(headerWithoutNonce, nonce)))
+		if hashStr < targetStr {
+			fmt.Println("===== We have a block now =====")
+			return &Output{
+				Block: input.Block,
+				Nonce: fmt.Sprintf("0x%08x", nonce),
+				Id:    Id,
+			}
+		}
+	}
+	return nil
+}
+
+func runRequest() string {
+	var input Input
+	err := json.NewDecoder(os.Stdin).Decode(&input)
+	if err != nil {
+		panic(err)
+	}
+	ret, err := json.Marshal(findNonceWithHint(constructHeader(input), 0, input))
+	if err != nil {
+		panic(err)
+	}
+	return string(ret)
+}
+
+func main() {
+	fmt.Println(runRequest())
+}
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