package main

import (
	"crypto/ecdsa"
	"fmt"
	"log"

	"golang.org/x/crypto/sha3"

	"github.com/ethereum/go-ethereum/accounts"
	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/common/hexutil"
	"github.com/ethereum/go-ethereum/crypto"
)

const message = "Hello Ethereum world"

func main() {
	privateKey, address, err := generateAddressAndPrivateKey()
	if err != nil {
		log.Fatalf("failed to generate address and private key: %v", err)
	}

	signature, err := signMessage(privateKey, message)
	if err != nil {
		log.Fatalf("failed to sign message: %v", err)
	}

	err = verifySignature(address, signature, message)
	if err != nil {
		log.Fatalf("failed to verify signature: %v", err)
	}

	fmt.Printf("Successfully signed and verified message '%s' with Ethereum address %s\n", message, address)
}

func signMessage(privateKey *ecdsa.PrivateKey, message string) (string, error) {
	messageHash := accounts.TextHash([]byte(message))

	signature, err := crypto.Sign(messageHash, privateKey)
	if err != nil {
		return "", err
	}

	signature[crypto.RecoveryIDOffset] += 27

	return hexutil.Encode(signature), nil
}

func verifySignature(fromAddress, signatureHex, message string) error {
	signature, err := hexutil.Decode(signatureHex)
	if err != nil {
		return err
	}

	signature[crypto.RecoveryIDOffset] -= 27 // Transform yellow paper V from 27/28 to 0/1

	messageHash := accounts.TextHash([]byte(message))

	pubKey, err := crypto.SigToPub(messageHash, signature)
	if err != nil {
		return err
	}

	if common.HexToAddress(fromAddress) != crypto.PubkeyToAddress(*pubKey) {
		return fmt.Errorf("failed to verify signature")
	}

	return nil
}

func generateAddressAndPrivateKey() (*ecdsa.PrivateKey, string, error) {
	privateKey, err := crypto.GenerateKey()
	if err != nil {
		return nil, "", err
	}

	publicKey := privateKey.Public().(*ecdsa.PublicKey)
	publicKeyBytes := crypto.FromECDSAPub(publicKey)

	hash := sha3.NewLegacyKeccak256()
	hash.Write(publicKeyBytes[1:])
	address := hexutil.Encode(hash.Sum(nil)[12:])

	return privateKey, address, nil
}