An Escrow-Free Hierarchical Identity-Based Signature Model for Cloud Storage

Hierarchical identity-based cryptography is an efficient technology to address the security issues in cloud storage. However, the inherent key escrow problem primarily hinders the widespread adoption of this cryptosystem in practice. To address the key escrow problem, this paper proposes an escrow-free hierarchical identity-based signature model, in which a user signs messages with a user-selected secret and PKG signing factor apart from the private key. For proving the full security, we formulate three security games with respect to our signature model. We instantiate the escrow-free model into a specific scheme based on the SHER-IBS scheme and prove that our scheme is secure against adaptive chosen ID and message attacks.