One of the main reason zero-confirmation transactions can’t be performed on the blockchain to date is because of a 51% attack in where a malicious miner can upload a malicious block through having obtained the majority of the hashing power on the network.
that is wrong and i think you have misunderstood the meaning of 51% attack. first of all there is no "malicious block", it is all legitimate blocks and that happens only because in PoW we follow the chain with the most work.
secondly zero-confirmation transactions have nothing to do with 51% attack because you are talking about a transaction that is
not yet confirmed so how can 51% attack affect it when it is not yet even in a block!!!
Ummm there are malicious blocks, this is due to the fact that each miner works on their own block. So a miner with the majority of the hash rate could upload their own malicous block on the network in which would then call for a fork.
![Huh](https://bitcointalk.org/Smileys/default/huh.gif)
and exactly in PoW we follow the chain with the most work which means that if a malicious user has the most work nodes will follow that chain.
![Huh](https://bitcointalk.org/Smileys/default/huh.gif)
why do I run into these types of replies on bitcointalk I don't get it.
Secondly zero confirmation transactions have everything to do with a 51% attack because it is the main reason why confirmations are required...
a 51% attack can affect it because since each miner chooses which block to upload the user cant be assured that their their transaction is valid until 6 blocks have been built on top of theirs ensuring that the block their transaction is in is the one the community is following.
![Huh](https://bitcointalk.org/Smileys/default/huh.gif)
so if a user isn't assured that their transaction can't be on the blockchain then of course they cant rely on zero confirmation transactions, it is common sense
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that's why we are focusing on ensuring users that their transaction will be included in the block from the start thus allowing for zero confirmation transactions.