Cryptology ePrint Archive: BibTeX Citation of Report 1999/024

@misc{cryptoeprint:1999:024,
    author = {M. Bellare and R. Impagliazzo},
    title = {A tool for obtaining tighter security analyses of pseudorandom function based constructions, with applications to PRP to PRF conversion  },
    howpublished = {Cryptology ePrint Archive, Report 1999/024},
    year = {1999},
    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.