How to Construct Identity-Based Signatures without the Key Escrow Problem

The inherent key escrow problem is one of the main reasons for the slow adoption of identity-based cryptography. The existing solution for mitigating the key escrow problem is by adopting multiple Private Key Generators (PKGs). Recently, there was a proposal that attempted to reduce the trust of the PKG by allowing a malicious PKG to be caught if he reveals the user's identity-based secret key illegally. Nonetheless, the proposal does not consider that the PKG can simply decrypt the ciphertext instead of revealing the secret key itself (in the case of identity-based encryption schemes). The aim of this paper is to present an escrow-free identity-based signature (IBS) scheme, in which the malicious PKG will be caught if it releases a signature on behalf of the user but signed by itself. We present a formal model to capture such a scheme and provide a concrete construction.

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