@misc{cryptoeprint:2013:241,
author = {Joachim Strömbergson and Simon Josefsson},
title = {The Perils of Repeating Patterns: Observation of Some Weak Keys in RC4},
howpublished = {Cryptology ePrint Archive, Report 2013/241},
year = {2013},
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.