[bisq-network/projects] Remove the need for Bisq to trust Bitcoin Average by querying exchange APIs directly and calculating our own weighted average for fiat and altcoin prices (#35)

[wiz]

> I'm not familiar with the different feeds and how much work it would be to use all the different data feeds.

Since @cd2357 worked on the Pricenode PR that implemented https://github.com/bisq-network/projects/issues/27, I've asked him to work on the PR for this project as well. I understand he already has a WIP that enables Pricenodes to directly query the 10+ centralized exchange APIs directly without adding any jar deps to Bisq. Since the jar dep library that Bisq already uses to query Exchange APIs has now added support for over 80 exchanges, it's not much work to add these new exchanges: https://github.com/knowm/XChange

> I do see a risk in inconsistent price feeds though using an aggregate of feeds from different exchanges calculated at each node.

While this is possible, in practice I don't think it will be an issue as we have price thresholds to resolve this. In any case, it will require lots of testing and a slow phase-in to the new system.

> We have seen people abusing the tolerance margin introduced to allow for difference in price feed before.

Yes, but AFAIK the only cases of this in the past were caused by BitcoinAverage flaws, which would be resolved by this proposal. For example, BA used LocalBitcoins for SEK prices instead of the largest Bitcoin Exchange in Sweden called Safello, which made Bisq vulnerable to someone trading a small amount of LocalBitcoins to move the Bisq price and buy Bitcoin below market: https://bisq.community/t/btc-sek-is-way-off/6558

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