A privacy problem on Hu–Huang's proxy key generation protocol

A proxy signature scheme enables an original signer to delegate his signing capability to a proxy signer and then the proxy signer can sign a message on behalf of the original signer. Recently, several ID-based proxy signature schemes have been proposed. ID-based Cryptosystems (ID-Cs) are advantageous than the traditional Public Key Cryptosystems (PKCs), as the key distribution is not required. But, an inherent problem of ID-Cs is the key escrow. Hu and Huang also proposed an ID-based proxy signature scheme with proxy key generation protocol. In Hu–Huang's scheme, whenever the original signer requests the Public Key Generation Center (PKG) to generate a proxy derivation key for a designated proxy signer, it generates a proxy key using the proxy signer's private key without his agreement. Thus, the PKG gets to infringe general privacy laws and regulations. Also, this is an example of PKG's misuse of the user's private key due to ID-C's key escrow problem. To solve these problems we propose an improvement of Hu–Huang's scheme.