Maxwell N. Krohn, Michael J. Freedman, David Mazières
IEEE Symposium on Security and Privacy
Oakland, California, May 2004.

The quality of peer-to-peer content distribution can suffer when malicious participants intentionally corrupt content. Some systems using simple block-by-block downloading can verify blocks with traditional cryptographic signatures and hashes, but these techniques do not apply well to more elegant systems that use rateless erasure codes for efficient multicast transfers. This paper presents a practical scheme, based on homomorphic hashing, that enables a downloader to perform on-the-fly verification of erasure-encoded blocks.

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Older extended-abstract version:
1st IRIS Student Workshop on Peer-to-Peer Systems   Cambridge, MA, August 2003.   [ps, pdf]