@misc{cryptoeprint:2012:057,
author = {Somindu C. Ramanna and Palash Sarkar},
title = {Anonymous Constant-Size Ciphertext HIBE From Asymmetric Pairings},
howpublished = {Cryptology ePrint Archive, Report 2012/057},
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.