@misc{cryptoeprint:2015:340,
author = {Santanu Sarkar},
title = {A New Distinguisher on Grain v1 for 106 rounds},
howpublished = {Cryptology ePrint Archive, Report 2015/340},
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.