A certificateless signature scheme and a certificateless public auditing scheme with authority trust level 3+

Many certificateless cryptosystems have been proposed for cloud security applications. These applications have to face the inherent issues of dealing with low authority trust levels, especially when the cloud server takes charge of doing the role of the key generation center (KGC), that is the authority to trust. This paper focuses on popular authority trust problems in certificateless signatures and proposes a public cloud auditing scheme with high trust level. In current cloud security applications based on certificateless cryptography, the level of trust can at most detect and prove that the authority is guilty but can never punish it for its malicious behavior; precisely, in those settings where malicious servers have to be punished, an external arbitrator becomes necessary. We develop a novel notion of enhanced authority trust level, that we call 3+, where even if KGCs can impersonate any entity, still there is no way to avoid an immediate penalty for its malicious behavior. First, we construct a certificateless signature scheme with authority trust level 3+, then we prove its security in the random oracle model, illustrating some benefits in applications made for clouds. In particular, we propose a certificateless homomorphic authenticable signature scheme and a cloud public auditing scheme. Our proposed trust level 3+ sensibly boosts the trustworthiness and acceptability of such cloud computation environments by its ordinary customers.