Is Cardano 51% attack resistant?

To answer this question, we’ll start by defining what a 51% attack is. It refers to an attack on a blockchain by a group of users that control more than 50% of the network’s hashing/computing power for a PoW system or stake for PoS system. The main concern of this attack is that by controlling the majority of the resources on the network, an adversary can interfere with the process of recording new blocks.

No PoW or PoS system is resistant to an adversary holding 51% of the resource (hashing power or stake respectively), and Cardano as a PoS based system is no different. It is, however, resistant against 34% attacks.

Cardano uses the Ouroboros protocol, which is provably secure under the assumption that there is an honest majority of stake. This is in line with all other cryptocurrencies, whether proof of stake or proof of work - if an adversary were to acquire more than 51% of the resource that determines block creation (hashing power for PoW, stake for PoS) they would be able to take control of the system.

You can find more information on Ouroboros (papers and videos) here:


Hi Maki. From where do you get those 34%? I didn’t find a reference to it in the linked paper.

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The fact that Ouroboros requires majority to be corrupted to perform adversarial actions means that 34% of adversarial stake is not enough. Hence - 34% resistant.


Thanks Vantuz, I think I understand now.

34% was just an example of amount of stake adversary can own and which will not allow him to execute successful attack. He would need to have more then 50% to succeed.

Yes, it’s just that there’s a whole separate family of consensus protocols called “33% resistant” which cannot resist 34% of adversarial control, e.g.: Algorand, Hyperledger, RSCoin, Casper (in asynchronous setting), Omniledger, etc.

Ouroboros is “50% resistant” (in synchronous setting) which automatically puts it into “34% resistant” group too :slight_smile: So it was more like a note from Maki, that no one can do more than 1/2 yet, but at least it’s not 1/3 :smiley:


Thanks @vantuz-subhuman, now I finally see from where is those 34% coming from :smiley:

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