@misc{cryptoeprint:2003:183,
author = {Craig Gentry},
title = {Certificate-Based Encryption and the Certificate Revocation Problem},
howpublished = {Cryptology ePrint Archive, Report 2003/183},
year = {2003},
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.