@misc{cryptoeprint:2008:383,
author = {Craig Gentry and Shai Halevi},
title = {Hierarchical Identity Based Encryption with Polynomially Many Levels},
howpublished = {Cryptology ePrint Archive, Report 2008/383},
year = {2008},
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.