Credence: thwarting p2p pollution?

David Kaspar — Thu, 04 Aug 2005 08:26:19 +0000

Certain peer to peer networks suffer from a high ratio of pollution: files that are fake, damaged, infected with viruses or similar. First attempts to lower the pollution were simple voting systems where users voted on whether a specific file was good or bad.
I haven’t had much faith in such voting systems because it seemed too easy to spam the systems with fake votes. Surely enough, various companies have already been identified who pollute the peer to peer networks with fake files and rate the files as good.
Credence is an academic project that has a different approach to the problem: it implements a voter correlation scheme.
Users that vote similarly become grouped together and results on a particular object (e.g. a p2p file) are then correlated by the voting group you belong to.
This means that malicious users will quickly form their own voting communities and have little effect on groups of honest voters.
There is a second benefit from this strategy. Votes from malicious users will not be disregarded but will ultimately start being counted as direct opposites. If a dishonest voter votes positively on an object, the honest user will count that as a negative vote for that object. The dishonest voting circles will not even be aware of what the current status of an object is to other voting communities.
One possible attack on Credence would be by a user that votes honestly on a large amount of certain files but votes dishonestly on one or a few specific target files. It is however very complex trying to analyze how many honest votes need to be placed for each dishonest vote while retaining it’s own credence.
Another common vulnerability of voting systems is the possibility for a single node to assume many different identities (also known as The Sybil Attack). Credence attempts to minimise the risk for a Sybil attack by requiring a large download for each new identity.
For honest users it will not be a problem to download one single large file. For dishonest individuals it will be very costly to download a large amount of files in order to set up fake identities.
Now if it only was possible to combine this robust voting system with the anonymous P2P networking protocol I wrote about earlier. This would make it much more reliable and risk free to share let’s say political opinions..

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Bittorrent – The new P2P in town

David Kaspar — Fri, 05 Dec 2003 17:04:09 +0000

The idea is brilliant; tap in to your users upload capacity to distribute your product across the Internet more efficiently.
If you have a killer product that your customers are downloading, the distribution (bandwidth) costs will increase proportionally with the amount of customers. This may be ok if the product is commercial and you can transfer the distribution costs to the cost of the product.
If on the other hand it is a free-ware product, lets say an open source application, you may find it hard to finance the popularity of the project.

As mentioned above, Bittorrent uses the (free) upload capacity of your users to distribute the product. In effect, the users that have downloaded parts of the product will start sending those parts to other users and the load on your server will decrease.
How well does the network perform?

Bittorrent routinely supports about 1000 downloaders of files around a gigabyte in size. After current tracker scaling work, the next scaling bottleneck is expected to be in the hundreds of thousands of simultaneous downloaders. – (Bittorrent FAQ)

One characteristic of the Bittorrent network is that popular files that are being downloaded by many users are easy to get and with a high transfer rate (in contrast to other P2P networks). Older files that are not being downloaded by other users are not hard to get.
To get started, download a client that supports the Bittorrent protocol. I have tried various clients and have two favorites. As a lightweight client, I prefer TheSHAD0W’s EXPERIMENTAL BITTORRENT CLIENT.
I have had better results with the more feature packed Shareaza. It supports all of the following networks: Gnutella 1and 2, eDonkey and Bittorrent. The original client can be found here: Bittorrent software.
Any resource that can be downloaded will have a small file called a torrent that holds crucial information about the resource. Among other things it will tell you the name and hash of the resource making it very hard to fake resources and ensuring correct downloads. The torrent will also tell you the tracker for this resource.
Trackers serve as a central point for sharing information about the various users downloading the resource and what parts each user has downloaded. This means a file is never transferred via the tracker, only information about transfers is. If the tracker is down or is not available for some other reason, you will not be able to download the resource.
etree.org is a community for sharing the live concert recordings of trade friendly artists. Other large source of torrents is suprnova.org.
BitTorrent, its logo and its web site are all copyright © 2001-2003 Bram Cohen

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Credence: thwarting p2p pollution?

Bittorrent – The new P2P in town