@misc{cryptoeprint:2008:170,
author = {Simon R. Blackburn},
title = {Cryptanalysing the Critical Group: Efficiently Solving Biggs's Discrete Logarithm Problem},
howpublished = {Cryptology ePrint Archive, Report 2008/170},
year = {2008},
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.