@misc{cryptoeprint:2010:587,
author = {Bartosz Przydatek and Douglas Wikstr{\"o}m},
title = {Group Message Authentication},
howpublished = {Cryptology ePrint Archive, Report 2010/587},
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.