Comments on Four Multi-Server Authentication Protocols Using Smartcard
A two-party password authentication protocol for client-server architecture is usually not sufficient when network size increases, especially for an open network such as Internet. Thus, researchers have proposed several multi-server authentication protocols to cope with this problem. In 2003, Li et al. proposed a multi-server protocol based on the ElGamal digital signature and geometric transformations on an Euclidean plane. The researchers have proposed several nice multi-server authentication protocols. They claim that their protocols are secure and can withstand various attacks. However, after reviewing their schemes, the authors found that they although are perfect whereas flawed. Due to this observation, in this paper, they list the weakness found in these recent literatures.