Recent findings from Perl et al. indicate that the num- ber of trusted CAs could be reduced considerably: based on scans of all publicly reachable web servers listening on port 443, they observed that 34% of the trusted CAs have not issued a single certificate yet [17]. They suggest to remove the root certificates of these CAs from the browser
trust stores. However, this approach may have undesirable consequences for closed user groups that use non-public HTTPS sites that use a certificate of one of the deleted CAs. These users will see a warning message when they access their secure sites, which weakens the effectiveness of such warnings [18].
These two measures, limiting the scope of CAs as well as removing unnecessary certificates from the browser trust store, may help to decrease the likelihood of an adver-sary obtaining a fake certificate in practice. However, they cannot prevent MitM attacks reliably and have not been integrated into Laribus. Laribus is supposed to be a com-plementary certificate validation service that provides an additional layer of security. Therefore, it should be oblivi- ous of the trust relationships within the existing PKI trust model.