@misc{cryptoeprint:2011:646,
author = {Daniel J. Bernstein and Tanja Lange and Peter Schwabe},
title = {The security impact of a new cryptographic library},
howpublished = {Cryptology ePrint Archive, Report 2011/646},
year = {2011},
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.