Two user authentication schemes for multi-server environments have been proposed by Tsai and Wang et al., respectively. However, there are some flaws existing in both schemes. Therefore, a new scheme for improving these drawbacks is proposed in this paper. The proposed scheme has the following benefits: (1) it complies with all the requirements for multi-server environments; (2) it can withstand all the well-known attacks at the present time; (3) it is equipped with a more secure key agreement procedure; and (4) it is quite efficient in terms of the cost of computation and transmission. In addition, the analysis and comparisons show that the proposed scheme outperforms the other related schemes in various aspects.
