@misc{cryptoeprint:2005:197,
author = {Qiang Tang and Liqun Chen},
title = {Weaknesses in two group Diffie-Hellman key exchange protocols},
howpublished = {Cryptology ePrint Archive, Report 2005/197},
year = {2005},
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.