@misc{cryptoeprint:2008:156,
author = {Kristina Altmann and Tibor Jager and Andy Rupp},
title = {On Black-Box Ring Extraction and Integer Factorization},
howpublished = {Cryptology ePrint Archive, Report 2008/156},
year = {2008},
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.