@misc{cryptoeprint:2007:422,
author = {Man Ho Au and Siu-Ming Yiu},
title = {Structural Identity-Based Encryption},
howpublished = {Cryptology ePrint Archive, Report 2007/422},
year = {2007},
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.