@misc{cryptoeprint:1996:006,
author = {Andris Ambainis},
title = {Upper bound on the communication complexity of private information retrieval},
howpublished = {Cryptology ePrint Archive, Report 1996/006},
year = {1996},
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.