@misc{cryptoeprint:2015:222,
author = {Elena Andreeva, Andrey Bogdanov, and Bart Mennink},
title = {Towards Understanding the Known-Key Security of Block Ciphers},
howpublished = {Cryptology ePrint Archive, Report 2015/222},
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.