2.1 Limiting the scope of CAs
One of the main limitations of the X.509 PKI is the fact that any CA can issue a certificate for any domain name. The large number of CAs that are trusted by default by common browsers constitutes a significant attack surface. In order to decrease the risk, CAs could be restricted to specific top-level domains. Karsten et al. [16] show that in practice most CAs provide online certificates for a small number of top-level domains and that the domains in most top-level domains use certificates from a small set of CAs only. Moreover, some CAs appear to operate for a single organization in a single country only. Browser ven- dors could incorporate this kind of information into their software in order to warn the user when a certificate for a domain is issued from a CA that has not issued certificates for the corresponding top-level domain so far.