Cryptanalysis and Improvement of Two Ring Signature Schemes

The security of the two ring signature schemes proposed by Xu(2004) and Zhang(2004) is analyzed, and its found that both the ring signature schemes are susceptible to group-changing attack. In addition, Zhangs scheme can easily be attacked by multiple-known-signature existential forgery. To guard against the two attacks, the two ring signature schemes are improved, which can make the improved schemes is still secure even in the strongest security model (proposed by Joseph in 2004).