Kevin Walsh and Emin Gün Sirer, of Cornell University, have a new paper on Credence, a system for detecting unwanted files in P2P networks. It’s a kind of reputation system for files, designed to detect in advance that certain files are not what they claim to be. One use of this technology is to detect spoofed files inserted into P2P nets by copyright owners.

Credence is really a reputation system for files. Users cast votes, which are simple thumbs-up or thumbs-down verdicts on particular files, saying whether a file is what it claims to be. Every vote is digitally signed by the user who cast it, so that recipients can verify the authenticity of votes they are given. If you’re not sure whether a file is genuine, you can ask people to send you votes, and then you can combine the votes in a special way to yield a quality score for the file.

P2P systems are open, and they generally don’t register their users (or at least they don’t prevent fraudulent or repeated registrations) so users cannot reliably be identified by their real names. Instead, users make up pseudonyms for themselves. Suppose Alice wants to join the system. She makes a pseudonym for herself, by generating a cryptographic key-pair, consisting of a private key and a public key. Alice keeps the private key secret, and uses it to put digital signatures on the votes she casts. Along with each vote, she distributes a copy of the public key, which anybody can use to verify the digital signature. The public key also serves as a “name” for Alice’s pseudonym. The key attribute of this system is that if Bob wants to forge a vote, that is, to create a vote that appears to have come from Alice’s pseudonym, he must somehow determine Alice’s private key, which is essentially impossible if Alice does her cryptography correctly. In short, the cryptography ensures that anybody can make a pseudonym, but only the creator of a pseudonym can cast votes on its behalf.

This only solves half of the problem, because an adversary can create as many pseudonyms as he likes, and have them cast false votes (i.e, votes in favor of the validity of files that are actually invalid, or against the validity of files that are actually valid). So you can’t just add up all of the votes you receive; you need some way to tell whose votes to trust and whose to ignore. Here Credence uses a simple rule – trust people who tend to vote the same way that you do. Suppose Alice knows that files A, B, and C are valid, and that files X, Y, and Z are not valid. If some pseudonym “Bob” has votes in favor of A, B, and C, and against X, Y, and Z, then Alice concludes that “Bob” tends to vote accurately. If another pseudonym “Charlie” votes the opposite way on those six files, then Alice concludes that votes from “Charlie” tend to be the opposite of the truth. So if she sees some new file that “Bob” says is valid and “Charlie” says is invalid, Alice will conclude that the file is valid. Each party’s vote on the new file gets a weight, equal to the correlation between that party’s votes and Alice’s votes on other files. (The paper hints at further mechanisms that assign trust to people whose votes correlate with those of other people Alice trusts.)

This scheme presents would-be adversaries with a dilemma. If Alice’s votes are truthful, then if you want to mislead Alice about one file, you have to earn her trust by telling her the truth about other files. You can tell occasional lies, but on the whole you have to be a truth-teller. (You can achieve the same effect by lying about almost everything, and telling the truth about just one file. Then Alice will conclude that you are a habitual liar, and will count your votes with negative weight, giving credence to the opposite of what you say. Again, you have to provide Alice with many useful-to-her votes in order to trick her once.)

It looks like this method will work, if it can be implemented efficiently in a real network. The real question, I think, is whether it will scale up to enormous P2P networks containing huge numbers of files. Here I have serious doubts. The paper’s authors don’t claim that users have to know about all of the votes cast in the system. But they’re not entirely clear on how individual users can efficiently get the votes they need to make good decisions, if the network is very large. In the long run, I don’t think this scaling problem is insurmountable; but more research is required to solve it.