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I'm wondering since a long time why the developers of Tor do not have already make Tor a truly P2P network where the entire Tor network would be 100% decentralized, only running be voluntary and where nobody could take the network down or interfere with it, even versus a big adversary (eg : the United States)

One example of one attack that I remember because of the fact that tor is not a fully-decentralized net : https://blog.torproject.org/blog/possible-upcoming-attempts-disable-tor-network

Is this at least considered in one future version ?

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The answer would be simple: Decentralization is a difficult problem.

I suggest you read The fifteen year struggle of decentralizing privacy-enhancing technology. In this paper the authors list 4 major problems that needs to be addressed for decentralization:

  1. Incentives in decentralized systems

Most people take up bandwidth more than they donate and proposals to introduce incentives into Tor have failed. . If one would also have to build an incentive system into a decentralized system, there should be a way to manage the ratings of each client without allowing falsification.

  1. NAT traversal

A truly decentralized system requires the participating nodes to have direct connection to each other. But because IPv4 addresses are getting scarce, ISP often tend to put more subscribers in a single network address suing carrier-grade NAT.

  1. Bootstrapping new nodes

If in a decentralized system one client wants to become a bridge or relay, there is no central directory server to tell a new node where to locate neighbours.

  1. Key exchange

With a decentralized model, for a secure communication users will have to share the keys directly with each other making it vulnerable to man-in-the-middle attack.

They also give a comparison of decentralized peer-to-peer overlay networks:

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It is interesting to read their conclusion:

..we conclude that there is no fully decentralized system capable of offering Tor anonymity today. For the first time we document in detail, the amount of wasted effort and pain spent in decentralization. The current generation of technology lead by Tor still has room for improvement, while the next generation is only just appearing on the horizon. The major problems involving decentralization are excruciatingly difficult to overcome. None of the projects have succeeded in making the internet secure and private.

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