Cryptology ePrint Archive: BibTeX Citation of Report 2012/720

@misc{cryptoeprint:2012:720,
    author = {Antoine Joux},
    title = {Faster index calculus for the medium prime case. Application to 1175-bit and 1425-bit finite fields},
    howpublished = {Cryptology ePrint Archive, Report 2012/720},
    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.