Webhook signature verification with HMAC (timing-safe compare)

                  package webhooks

import (
  "bytes"
  "crypto/hmac"
  "crypto/sha256"
  "encoding/hex"
  "errors"
  "io"
  "net/http"
)

func Verify(secret []byte, r *http.Request) ([]byte, error) {
  sigHex := r.Header.Get("X-Signature-SHA256")
  if sigHex == "" {
    return nil, errors.New("missing signature")
  }
  sig, err := hex.DecodeString(sigHex)
  if err != nil {
    return nil, errors.New("bad signature encoding")
  }

  body, err := io.ReadAll(r.Body)
  if err != nil {
    return nil, err
  }
  _ = r.Body.Close()
  r.Body = io.NopCloser(bytes.NewReader(body))

  mac := hmac.New(sha256.New, secret)
  mac.Write(body)
  expected := mac.Sum(nil)
  if !hmac.Equal(sig, expected) {
    return nil, errors.New("invalid signature")
  }
  return body, nil
}