@misc{cryptoeprint:2012:331,
author = {Zhaohui Tang},
title = {Homomorphic Authentication Codes for Network Coding},
howpublished = {Cryptology ePrint Archive, Report 2012/331},
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.