@misc{cryptoeprint:2010:406,
author = {Julien Francq and Céline Thuillet},
title = {Unfolding Method for Shabal on Virtex-5 FPGAs: Concrete Results.pdf},
howpublished = {Cryptology ePrint Archive, Report 2010/406},
year = {2010},
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.