@misc{cryptoeprint:2015:239,
author = {Patrick Derbez and Léo Perrin},
title = {Meet-in-the-Middle Attacks and Structural Analysis of Round-Reduced PRINCE},
howpublished = {Cryptology ePrint Archive, Report 2015/239},
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.