(Post originally appeared on the [tor-talk] mailing list)

tl;dr

Where will Tor’s bandwidth come from in 20 years? Will solo volunteers still exist, or will all the bandwidth come from Tor-friendly organizations?

Tor incentive schemes are interesting. There are many proposed schemes but their crypto needs to be reviewed and lots of code/spec needs to be written before they can be deployed.

This document describes the idea of a Flattr-like crowdfunding model for tor relays.

Intro (skip if you know why Tor incentive schemes might be useful)

One of the goals of Tor is to increase its reach and get tens of millions of Tor users. This makes sense from an anonymity point of view, since an increase on the number of users is also an increase on Tor’s anonymity set.

One of the problems of scaling Tor to tens of millions of users is that Tor’s bandwidth capacity is finite. The current total relay bandwidth is about 4GB/s, and it’s donated by kind volunteers and various organizations. As the number of users increases, Tor’s bandwidth must also increase.

Lately the bandwidth coming out of Tor-friendly organizations (like torservers.net, universities, etc.) seems to increase. Currently, there is 50% chance of exiting from an org-controlled exit node, as can be seen in slide 30 of http://freehaven.net/~arma/slides-dimacs13.pdf

If this trend continues, Tor might end up looking like the Bitcoin network – where a number of organizations (mining pools) drive the network. Unfortunately, in contrast with the coin minting of Bitcoin, there is no incentives for organizations to contribute to the Tor network.

At the moment, organizations and solo volunteers pay out of their own pocket (or accept donations) to maintain their Tor relays. There must be ways to help reimburse their costs.

Incentive schemes (feel free to skip if you know this stuff)

Incentive schemes for anonymous networks have been extensively researched and there are multiple papers for systems that apply specifically to Tor [2].

Most of those systems require to modify the code of the Tor network, to be able to give “contribution tokens” to relay operators. Then those tokens can be exchanged to get “premium Tor service” or other goods.

These systems have a few issues that make them hard to implement and deploy:

• Baking anything inside Tor is a lengthy procedure. Secure designs and code must be written, time must pass for the new code to be deployed and used by the majority of the network, etc.
• These schemes might cause anonymity issues, since the set of people who have “contribution tokens” is smaller than the set of Tor users. The proposed incentive schemes try to fix these issues; for example, LIRA solves it by creating a lottery system (yes, on top of Tor) that rewards “contribution tokens” to random relays.
• Alternative currencies, like “contribution tokens”, are not easy to get right. Baking them inside the Tor network is not a trivial task

While these proposed “complex” schemes might be The Right Thing for the long-term, we might be able to create an incentive scheme based on already existing technologies; like the bandwidth authorities and Bitcoin.

Flattor: A simple incentive scheme

Flattor is (fictional) software (or a website) that given a Bitcoin wallet and a number of Bitcoins that the user is willing to spend, splits those Bitcoins in chunks and sends them to contributing relay operators.

The idea assumes that there is some way to find the Bitcoin address of relay operators. There is no standard way to do so, but operators who are interested in getting reimbursed can put their address in the Contact field of their relay.

Flattor uses the bandwidth estimations of Tor to find the contribution factor of each relay. We will assume that these estimates are accurate, since the security of Tor depends on them anyway.

As a simplified example, if the Tor network has 4 relays with bandwidth contribution 0.05, 0.05, 0.3 and 0.6 respectively, and the user is willing to spend 1 bitcoin, Flattor will send 0.05, 0.05, 0.3 and 0.6 bitcoins to each relay operator respectively.

Of course, this gets more complicated as the number of relays increases, or when only a subset of the relays have a registered bitcoin address, etc.

Why this might be a good idea

It’s simple to implement and easy to understand, it doesn’t require any Tor code to be written and it can even be started as an unofficial project.

Furthermore, it doesn’t cause anonymity issues, there are no middlemen, and it doesn’t centralize bandwidth to a single relay operator.

Why this might be a bad idea

(This concern is based on a discussion with gmaxwell.)

Incentivising Tor relay operators with money is not a good way to run an anonymity network

Currently, (we want to believe that) the Tor network is run by a bunch of cypherpunks that are contributing bandwidth because they believe in the Cause.

If relay operators start getting money for their bandwidth, we might end up with relay operators that are just in for the money. It might then be easier for a three-letter org to persuade those relay operators to snoop on their users (by giving them double the money they are currently getting).

While I agree that this concern is legitimate, I would say that it’s pretty far off at the moment: I doubt that anything like Flattor will ever generate a considerable income for anyone. Still, it’s something that we should have in mind.

(Furthermore, since the Bitcoin blockchain is public you can see how much money each relay operator has gotten so far. Maybe there should be some kind of limit on the number of money each operator should get per time period.)

Final thoughts

There are many details that must be sorted out before Flattr can be implemented. There are also multiple improvements that can be applied on top of the simple model described above. Also, there are ethical issues that spawn up when real money is given to relay operators.

My plan was to expand on all these issues in this paragraph, but it seems like I’ve already spent too many hours writing this document.

I’m not planning to implement this system before I hear some opinions from other people. To be honest, I’m not even sure if such an incentive scheme is a good idea, but posting bad ideas to mailing lists is what the Internet is for, right?

Thoughts?