@misc{cryptoeprint:2011:009,
author = {Helger Lipmaa},
title = {Progression-Free Sets and Sublinear Pairing-Based Non-Interactive Zero-Knowledge Arguments},
howpublished = {Cryptology ePrint Archive, Report 2011/009},
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.