@misc{cryptoeprint:2011:125,
author = {Masao KASAHARA},
title = {A Construction of A New Class of Knapsack-Type Public Key Cryptosystem, K(III)$\Sigma$PKC},
howpublished = {Cryptology ePrint Archive, Report 2011/125},
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.