Cryptology ePrint Archive: BibTeX Citation of Report 2007/221

@misc{cryptoeprint:2007:221,
    author = {Michel Abdalla and Eike Kiltz and Gregory Neven},
    title = {Generalized Key Delegation for Hierarchical Identity-Based Encryption},
    howpublished = {Cryptology ePrint Archive, Report 2007/221},
    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.