Most of Public Key Infrastructures (PKIs) are based on the ITU-T X.509, and the top-down hierarchical structure is extensively employed for the PKI community. However, the prominent drawback of the hierarchical PKI structure is that the CAs can be the target of serious attacks such as Distributed Denial-of-Service (DDoS). In this paper, we present two new models, Back-up CA and Mesh PKI, to cope with such Internet attacks. The proposed Back-up CA sets up an alternative path when an original CA is under attack, consequently improving availability and flexibility. Mesh PKI is a collection of CAs dynamically linked by multiple peer-to-peer cross-certifications. The Mesh PKI is very attractive, not only because they are robust to attacks but also because they help to reduce overall certificate validation time and to balance the load across multiple CAs.