@misc{cryptoeprint:2007:433,
author = {Fabien Coelho},
title = {An (Almost) Constant-Effort Solution-Verification Proof-of-Work Protocol based on Merkle Trees},
howpublished = {Cryptology ePrint Archive, Report 2007/433},
year = {2007},
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.