Notes on Two Flawed Attacks on Undeniable Signature Schemes

In Eurocrypt 2005, Kurosawa and Heng proposed a set of 3-move witness indistinguishable protocols to be incorporated in the confirmation and disavowal protocols of the FDH variant of undeniable signature schemes. Their 3-move protocols gave rise to the development of many other variants of undeniable signature schemes. In 2010, Zhou, Zhang and Li showed a weakness in Kurosawa and Heng´s proof system which enables a malicious prover to impersonate the signer. Libert and Quisquater proposed the first identity-based undeniable signature scheme. In 2007, Li et al. proposed a universal forgery attack on their scheme. In this paper, we show that both of the attacks which were proposed on Kurosawa and Heng´s proof system and Libert and Quisquater´s scheme have fatal errors, and therefore, both claims are false.