Verify X (Twitter) webhook signatures in Go

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"io"
	"net/http"
	"strconv"
	"sync"
	"time"
)

const fiveMinutesMs = int64(5 * 60 * 1000)

var (
	seenNonces = sync.Map{}
)

func verifyWebhook(payload []byte, headers http.Header, secret string) bool {
	timestamp := headers.Get("X-Xquik-Timestamp")
	nonce := headers.Get("X-Xquik-Nonce")
	signature := headers.Get("X-Xquik-Signature")
	if timestamp == "" || nonce == "" || signature == "" {
		return false
	}

	ts, err := strconv.ParseInt(timestamp, 10, 64)
	if err != nil {
		return false
	}
	nowMs := time.Now().UnixMilli()
	diff := nowMs - ts
	if diff < 0 {
		diff = -diff
	}
	if diff > fiveMinutesMs {
		return false
	}

	signingString := fmt.Sprintf("%s.%s.%s", timestamp, nonce, string(payload))
	mac := hmac.New(sha256.New, []byte(secret))
	mac.Write([]byte(signingString))
	expected := "sha256=" + hex.EncodeToString(mac.Sum(nil))
	if !hmac.Equal([]byte(expected), []byte(signature)) {
		return false
	}

	seenNonces.Range(func(key, value any) bool {
		expiresAt, ok := value.(int64)
		if ok && expiresAt <= nowMs {
			seenNonces.Delete(key)
		}
		return true
	})
	_, replayed := seenNonces.LoadOrStore(nonce, nowMs+fiveMinutesMs)
	return !replayed
}

func webhookHandler(w http.ResponseWriter, r *http.Request) {
	payload, err := io.ReadAll(r.Body)
	if err != nil {
		http.Error(w, "Bad request", http.StatusBadRequest)
		return
	}
	if !verifyWebhook(payload, r.Header, webhookSecret) {
		http.Error(w, "Invalid signature", http.StatusUnauthorized)
		return
	}
	// Process event...
	fmt.Fprint(w, "OK")
}