@misc{cryptoeprint:2013:848,
author = {Xi-Jun Lin and Lin Sun},
title = {Weakness of Several Identity-based Tripartite Authenticated Key Agreement Protocols},
howpublished = {Cryptology ePrint Archive, Report 2013/848},
year = {2013},
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.