@misc{cryptoeprint:2010:223,
author = {Thomas Peyrin},
title = {Improved Differential Attacks for ECHO and Grostl},
howpublished = {Cryptology ePrint Archive, Report 2010/223},
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.