at about 6:37 in the video i think Ivan explains that you can’t change block data unless you have 51% or more of the hashrate and even after you are able to make the changes ounce you have said hashrate. You would then have to outpace other miners to catch up and be able to add the modified block to the chain.
Do i have that understanding correct? Could someone help break that down for me if i don’t have that correct. Thank you.
You are correct. That is what we call a 51% attack. Ideally you would want to have more than the 51% of the hash power. Doing this on the bitcoin network would be quite expensive due to the total hashrate the network has. The further you go back the more expensive it would be. Its simply not worth it. Keep in mind that you would need a ton of equipment and you would need a ton of power to make something like that happen. That’s why many people agree that POW (Proof of Work) is the most secure consensus we have right now. It is connected to real world, with energy. Keep up the great work. 
Hello, I have a question which has been bothering me for quite a while now. I’m not a programmer, maybe it’s obvious for somebody who is.
"Is a sybil attack possible on a PRIVATE blockchain?
"Does it make a difference if it’s DPoS/PoS/PoW in fundamental terms?
“Theoretically, is a 51% attack more probable than a sybil attack on a private blockchain?”
“Do those questions make any sense or have I mixed up something?”
Thank you.
Possible yes, but it has a very low possibility to happen, if you control the node’s permissions, you have all the participants identified, so the only way could be by many participants start proposing data that is on their favor, until they can reach some kind of sybil attack, but still it could be worthless and difficult to achieve.
Yes, is the consensus mechanism that you will use to define the playground for your use case, I can extend myself explaining main features of each, but to answer to your question: it makes a difference in the fundamental way that consensus will be reached.
51% attack = I control 51% of the resources to generate new blocks/data/transactions, so I could choose which blocks/data/transactions can be include or not.
Sybil attack = One or many entities agree to control more than 51% of the resources to generate new blocks/data/transactions, so I could choose which blocks/data/transactions can be include or not.
Basically they work in the same way, because we are talking of being able to achieve 51% of the resources of any network in order to control or change some few rules or force it to behave it in a way that is in your favor.
I dont know, I try to answer them
. Do my explanation works for you?
Hope this gives you a clear picture of the subject.
If you have any more questions, please let us know so we can help you!
Carlos Z.
Yes, that was helpful.
I can conclude that, in order to achieve security we need to modify the permissions of the nodes in a way that a 51% percent would be much too expensive to try.
TECHNICALLY - for a successful attack “the mastermind hacker” would need a longer chain of transactions and/or higher value of invested work (PoW) or stake (PoS) so that the blockchain would think that the “hacker chain” is the trustworthy chain. And as far as my understanding goes, this can be prevented with permissions. And it can be prevented by having more % of “healthy nodes” because with every additional healthy node percentage increase, we dilute the % of “hacker” nodes. The cheapest way of hacking the blockchain would be to define an attribute (a factor) which would result in preference of malicious blocks. (for example a code which says: blocks which are submitted via IP address ending with XY are more trustworthy). But for that we would need a hardfork anyway, and the public would see that the code is malicious.
SUMMARY - An attempt is possible and probable, but it would cost a lot of RESOURCES in terms of a) hardware (nodes) and transaction costs (gas or electricity).
We could calculate how much energy and resources do we need if we want to hack Bitcoin the following way:
I believe on a newb level, I kind of understand it.
So basically, if Apple would access all of apple computers sold in the past 5 years and turn them on and connect to the internet, install a “michelangelo virus” which would run in the back it would be possible…? Is there a mathematic solution to this?
Apple sold about 100 million computers in the past 5 years.
Bitcoin blocks 630.000
Yes, you have understood it well!
Basically to hack bitcoin, you will need either to have 51% of nodes at your favor, or 51% of hashpower to create a chain that is longest than the actual one (mining speed higher), both methods can be resume in A LOT of resources. So if it worth it? Is a risk process that involve money to achieve, but probabilities of sucess are so low that you probably will lose all the money that you invest in the “hacking” process.
If Apple want to install a virus on their devices, they can do it easily through a system update, they control the software and the hardware, they can even install some kind of chip inside of one of the hardware’s components and get data from it. Also, even Windows could do that if they want. Just have to push a malicious update into the system and they can spy or do what ever they want with your system, because they own the operating system their using.
If you have any more questions, please let us know so we can help you!
Carlos Z.
