Cryptanalysis and Improvement on an Threshold Key Issuing Protocol

Key issuing protocols deal with overcoming the two inherent problems: key escrow and secure channel requirement of the identity based cryptosystems. An efficient and secure key issuing protocol enables the identity-based cryptosystems to be applicable in the real world. We analyze an threshold key issuing protocol due to Gangishetti et al. and show that the protocol suffers from the attacks of the malicious KGC and the users can deny that they have never received the key. Furthermore, we present an improved protocol, which is undeniable and secure against KGC´s attacks.