Cryptology ePrint Archive: BibTeX Citation of Report 2011/331

@misc{cryptoeprint:2011:331,
    author = {Masoumeh Safkhani and Pedro Peris-Lopez and  Julio Cesar Hernandez-Castro and  Nasour Bagheri and Majid Naderi},
    title = {Cryptanalysis of Cho \textit{et al.}'s Protocol, A Hash-Based Mutual Authentication Protocol for RFID Systems},
    howpublished = {Cryptology ePrint Archive, Report 2011/331},
    year = {2011},
    note = {\url{http://eprint.iacr.org/}},
}

You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.