Most collaborative systems are vulnerable to a 51% attack. This typically occurs when a large number of fake accounts are created to gain voting power and to take control of the collaborative systems. The attacker can then completely hack key components of the collaborative system and make Tournesol recommend horrible contents. The use of such fake accounts is also known as a Sybil attack.
On Tournesol, a 51% attack by a reasonable amount of fake accounts would allow the attacker to set arbitrary scores to any video. This would allow them to essentially choose which videos will be promoted by Tournesol's search engine. In addition to promoting potentially dangerous contents (such as hate or pedophile videos), this would seriously undermine the trustworthiness of any collaborative approach to ethics Science4All-21. This is why Tournesol regards the 51% attack as a very serious threat to the success of the project.
To combat the 51% attack, in addition to relying on our strategyproof and Byzantine-resilient collaborative learning Tournesol model, Tournesol asks contributors to validate emails from trusted email domains. This certification thus serves as a Proof of Personhood.
The case of Bitcoin
Namely, with a 51% attack, an attacker can spend the equivalent of, say, 1 million dollars twice, by promising it to a recipient. Moreover, the attacker can provide a proof to the recipient that the payment has been registered to a block of the Bitcoin Blockchain, to which numerous subsequent blocks have been added. This would convince the recipient that the transactions have been validated by the Bitcoin network. The recipient would then give the attacker the good the attacker bought, like the rights to a real estate property. But then, the 51% attack would allow the attacker to fork the Blockchain before the block that validated the transaction, thereby in essence cancelling the transaction.
Such attacks would seriously undermine the trustworthiness of the Bitcoin, and thus its value.
The 51% attack is the central part of the plot of Season 5, Episode 8 of the TV series Silicon Valley.
In this episode entitled "Fifty-One Percent", the network designed by the Piped Piper company is attacked by a Chinese competitor, which aims to rewrite the code base of Piped Piper's decentralized Internet computer. Piped Piper then aims to add contributors to the network that they control, in order to regain 51% of the voting power. Once successful, they rewrite their code to exclude the competitors' nodes and prevent future 51% attacks.