@misc{cryptoeprint:2015:656,
author = {Ruhul Amin and G.P. Biswas},
title = {Cryptanalysis of a Markov Chain Based User Authentication Scheme},
howpublished = {Cryptology ePrint Archive, Report 2015/656},
year = {2015},
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.