@misc{cryptoeprint:2007:298,
author = {David A. McGrew and Scott R. Fluhrer},
title = {The Security of the Extended Codebook (XCB) Mode of Operation},
howpublished = {Cryptology ePrint Archive, Report 2007/298},
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.