@misc{cryptoeprint:2015:495,
author = {Sondre R{\o}njom},
title = {Powers of Subfield Polynomials and Algebraic Attacks on Word-Based Stream Ciphers},
howpublished = {Cryptology ePrint Archive, Report 2015/495},
year = {2015},
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.