@misc{cryptoeprint:2010:548,
author = {Shay Gueron, Simon Johnson, Jesse Walker},
title = {SHA-512/256},
howpublished = {Cryptology ePrint Archive, Report 2010/548},
year = {2010},
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.