// Verify if request has valid AWS Signature Version '4'.
func isReqAuthenticated(ctx context.Context, r *http.Request, region string, stype serviceType) (s3Error APIErrorCode) {
	if errCode := reqSignatureV4Verify(r, region, stype); errCode != ErrNone {
		return errCode
	}

	clientETag, err := etag.FromContentMD5(r.Header)
	if err != nil {
		return ErrInvalidDigest
	}