@misc{cryptoeprint:2015:414,
author = {Mridul Nandi},
title = {On the Optimality of Non-Linear Computations of Length-Preserving Encryption Schemes},
howpublished = {Cryptology ePrint Archive, Report 2015/414},
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.