@misc{cryptoeprint:2010:254,
author = {Elias Yarrkov},
title = {Cryptanalysis of XXTEA},
howpublished = {Cryptology ePrint Archive, Report 2010/254},
year = {2010},
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.