@misc{cryptoeprint:2012:691,
author = {Shi Bai and Richard P. Brent and Emmanuel Thom\'e},
title = {Root Optimization of Polynomials in the Number Field Sieve},
howpublished = {Cryptology ePrint Archive, Report 2012/691},
year = {2012},
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.