Generating A Signature For Farcaster Signer in Golang

signer.go

package signer

import (
	"context"
	"encoding/hex"
	"math/big"
	"strings"

	"github.com/ethereum/go-ethereum/accounts"
	"github.com/ethereum/go-ethereum/common/math"
	"github.com/ethereum/go-ethereum/signer/core/apitypes"
	hdwallet "github.com/miguelmota/go-ethereum-hdwallet"
        // can be any env library e.g. viper
	"env"
)


// response from: POST /v2/farcaster/signer
type NeynarSigner struct {
	SignerUUID        string `json:"signer_uuid"`
	PublicKey         string `json:"public_key"`
	Status            string `json:"status"`
	SignerApprovalURL string `json:"signer_approval_url"`
	SignerApprovalFID any    `json:"fid"`
}


func GenerateSignatureForSigner(ctx context.Context, curSigner NeynarSigner, appFid, deadline *big.Int) ([]byte, error) {

	mnemonic := env.GetString("FARCASTER_MNEMONIC")
	wallet, err := hdwallet.NewFromMnemonic(mnemonic)
	if err != nil {
		return nil, err
	}

	account, err := wallet.Derive(accounts.DefaultBaseDerivationPath, true)
	if err != nil {
		return nil, err
	}

	pubBytes, err := hex.DecodeString(strings.TrimPrefix(curSigner.PublicKey, "0x"))
	if err != nil {
		return nil, err
	}

	domain := apitypes.TypedDataDomain{
		Name:              "Farcaster SignedKeyRequestValidator",
		Version:           "1",
		ChainId:           math.NewHexOrDecimal256(10),
		VerifyingContract: "0x00000000fc700472606ed4fa22623acf62c60553",
	}

	typedData := apitypes.TypedData{
		Types: apitypes.Types{
			"EIP712Domain": []apitypes.Type{
				{Name: "name", Type: "string"},
				{Name: "version", Type: "string"},
				{Name: "chainId", Type: "uint256"},
				{Name: "verifyingContract", Type: "address"},
			},
			"SignedKeyRequest": []apitypes.Type{
				{Name: "requestFid", Type: "uint256"},
				{Name: "key", Type: "bytes"},
				{Name: "deadline", Type: "uint256"},
			},
		},
		PrimaryType: "SignedKeyRequest",
		Domain:      domain,
		Message: map[string]interface{}{
			"requestFid": appFid.String(),
			"key":        pubBytes,
			"deadline":   deadline.String(),
		},
	}

	signature, err := signEIP712TypedData(wallet, account, typedData)
	if err != nil {
		return nil, err
	}
	return signature, nil
}

func signEIP712TypedData(wallet *hdwallet.Wallet, account accounts.Account, typedData apitypes.TypedData) ([]byte, error) {

	hash, _, err := apitypes.TypedDataAndHash(typedData)
	if err != nil {
		return nil, err
	}

	// Sign the final hash
	signature, err := wallet.SignHash(account, hash)
	if err != nil {
		return nil, err
	}

	signature[64] += 27

	return signature, nil
}