@misc{cryptoeprint:2012:538,
author = {Myungsun Kim and Abedelaziz Mohaisen and Jung Hee Cheon and Yongdae Kim},
title = {Private Top-k Aggregation Protocols},
howpublished = {Cryptology ePrint Archive, Report 2012/538},
year = {2012},
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.