@misc{cryptoeprint:2005:110,
author = {Ayan Mahalanobis},
title = {Diffie-Hellman key exchange protocol and non-abelian nilpotent groups.},
howpublished = {Cryptology ePrint Archive, Report 2005/110},
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.