@misc{cryptoeprint:2012:615,
author = {Boaz Tsaban},
title = {Polynomial time solutions of computational problems in noncommutative-algebraic cryptography},
howpublished = {Cryptology ePrint Archive, Report 2012/615},
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.