@misc{cryptoeprint:2011:620,
author = {Elena Andreeva and Atul Luykx and Bart Mennink},
title = {Provable Security of BLAKE with Non-Ideal Compression Function},
howpublished = {Cryptology ePrint Archive, Report 2011/620},
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.