@misc{cryptoeprint:2006:085,
author = {Xavier Boyen and Brent Waters},
title = {Anonymous Hierarchical Identity-Based Encryption (Without Random Oracles)},
howpublished = {Cryptology ePrint Archive, Report 2006/085},
year = {2006},
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.