@misc{cryptoeprint:2011:623,
author = {Donghoon Chang and Mridul Nandi and Moti Yung},
title = {Indifferentiability of the Hash Algorithm BLAKE},
howpublished = {Cryptology ePrint Archive, Report 2011/623},
year = {2011},
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.