@misc{cryptoeprint:2004:031,
author = {Igor Semaev},
title = {Summation polynomials and the discrete logarithm problem on elliptic curves},
howpublished = {Cryptology ePrint Archive, Report 2004/031},
year = {2004},
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.